1.
Mercury (planet)
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Mercury is the smallest and innermost planet in the Solar System. Its orbital period around the Sun of 88 days is the shortest of all the planets in the Solar System and it is named after the Roman deity Mercury, the messenger to the gods. Like Venus, Mercury orbits the Sun within Earths orbit as a planet, so it can only be seen visually in the morning or the evening sky. Also, like Venus and the Moon, the displays the complete range of phases as it moves around its orbit relative to Earth. Seen from Earth, this cycle of phases reoccurs approximately every 116 days, although Mercury can appear as a bright star-like object when viewed from Earth, its proximity to the Sun often makes it more difficult to see than Venus. Mercury is tidally or gravitationally locked with the Sun in a 3,2 resonance, as seen relative to the fixed stars, it rotates on its axis exactly three times for every two revolutions it makes around the Sun. As seen from the Sun, in a frame of reference that rotates with the orbital motion, an observer on Mercury would therefore see only one day every two years. Mercurys axis has the smallest tilt of any of the Solar Systems planets, at aphelion, Mercury is about 1.5 times as far from the Sun as it is at perihelion. Mercurys surface appears heavily cratered and is similar in appearance to the Moons, the polar regions are constantly below 180 K. The planet has no natural satellites. Mercury is one of four planets in the Solar System. It is the smallest planet in the Solar System, with a radius of 2,439.7 kilometres. Mercury is also smaller—albeit more massive—than the largest natural satellites in the Solar System, Ganymede, Mercury consists of approximately 70% metallic and 30% silicate material. Mercurys density is the second highest in the Solar System at 5.427 g/cm3, Mercurys density can be used to infer details of its inner structure. Although Earths high density results appreciably from gravitational compression, particularly at the core, Mercury is much smaller, therefore, for it to have such a high density, its core must be large and rich in iron. Geologists estimate that Mercurys core occupies about 55% of its volume, Research published in 2007 suggests that Mercury has a molten core. Surrounding the core is a 500–700 km mantle consisting of silicates, based on data from the Mariner 10 mission and Earth-based observation, Mercurys crust is estimated to be 35 km thick. One distinctive feature of Mercurys surface is the presence of narrow ridges
2.
Venus
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Venus is the second planet from the Sun, orbiting it every 224.7 Earth days. It has the longest rotation period of any planet in the Solar System and it is named after the Roman goddess of love and beauty. It is the second-brightest natural object in the sky after the Moon, reaching an apparent magnitude of −4.6. Because Venus orbits within Earths orbit it is a planet and never appears to venture far from the Sun. Venus is a planet and is sometimes called Earths sister planet because of their similar size, mass, proximity to the Sun. It is radically different from Earth in other respects and it has the densest atmosphere of the four terrestrial planets, consisting of more than 96% carbon dioxide. The atmospheric pressure at the surface is 92 times that of Earth. Venus is by far the hottest planet in the Solar System, with a surface temperature of 735 K. Venus is shrouded by an layer of highly reflective clouds of sulfuric acid. It may have had water oceans in the past, but these would have vaporized as the temperature rose due to a greenhouse effect. The water has probably photodissociated, and the hydrogen has been swept into interplanetary space by the solar wind because of the lack of a planetary magnetic field. Venuss surface is a dry desertscape interspersed with rocks and is periodically resurfaced by volcanism. As one of the brightest objects in the sky, Venus has been a fixture in human culture for as long as records have existed. It has been sacred to gods of many cultures, and has been a prime inspiration for writers and poets as the morning star. Venus was the first planet to have its motions plotted across the sky, as the closest planet to Earth, Venus has been a prime target for early interplanetary exploration. It was the first planet beyond Earth visited by a spacecraft, Venuss thick clouds render observation of its surface impossible in visible light, and the first detailed maps did not emerge until the arrival of the Magellan orbiter in 1991. Plans have been proposed for rovers or more missions. Venus is one of the four planets in the Solar System
3.
Earth
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Earth, otherwise known as the World, or the Globe, is the third planet from the Sun and the only object in the Universe known to harbor life. It is the densest planet in the Solar System and the largest of the four terrestrial planets, according to radiometric dating and other sources of evidence, Earth formed about 4.54 billion years ago. Earths gravity interacts with objects in space, especially the Sun. During one orbit around the Sun, Earth rotates about its axis over 365 times, thus, Earths axis of rotation is tilted, producing seasonal variations on the planets surface. The gravitational interaction between the Earth and Moon causes ocean tides, stabilizes the Earths orientation on its axis, Earths lithosphere is divided into several rigid tectonic plates that migrate across the surface over periods of many millions of years. About 71% of Earths surface is covered with water, mostly by its oceans, the remaining 29% is land consisting of continents and islands that together have many lakes, rivers and other sources of water that contribute to the hydrosphere. The majority of Earths polar regions are covered in ice, including the Antarctic ice sheet, Earths interior remains active with a solid iron inner core, a liquid outer core that generates the Earths magnetic field, and a convecting mantle that drives plate tectonics. Within the first billion years of Earths history, life appeared in the oceans and began to affect the Earths atmosphere and surface, some geological evidence indicates that life may have arisen as much as 4.1 billion years ago. Since then, the combination of Earths distance from the Sun, physical properties, in the history of the Earth, biodiversity has gone through long periods of expansion, occasionally punctuated by mass extinction events. Over 99% of all species that lived on Earth are extinct. Estimates of the number of species on Earth today vary widely, over 7.4 billion humans live on Earth and depend on its biosphere and minerals for their survival. Humans have developed diverse societies and cultures, politically, the world has about 200 sovereign states, the modern English word Earth developed from a wide variety of Middle English forms, which derived from an Old English noun most often spelled eorðe. It has cognates in every Germanic language, and their proto-Germanic root has been reconstructed as *erþō, originally, earth was written in lowercase, and from early Middle English, its definite sense as the globe was expressed as the earth. By early Modern English, many nouns were capitalized, and the became the Earth. More recently, the name is simply given as Earth. House styles now vary, Oxford spelling recognizes the lowercase form as the most common, another convention capitalizes Earth when appearing as a name but writes it in lowercase when preceded by the. It almost always appears in lowercase in colloquial expressions such as what on earth are you doing, the oldest material found in the Solar System is dated to 4. 5672±0.0006 billion years ago. By 4. 54±0.04 Gya the primordial Earth had formed, the formation and evolution of Solar System bodies occurred along with the Sun
4.
Mars
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Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, after Mercury. Named after the Roman god of war, it is referred to as the Red Planet because the iron oxide prevalent on its surface gives it a reddish appearance. Mars is a planet with a thin atmosphere, having surface features reminiscent both of the impact craters of the Moon and the valleys, deserts, and polar ice caps of Earth. The rotational period and seasonal cycles of Mars are likewise similar to those of Earth, Mars is the site of Olympus Mons, the largest volcano and second-highest known mountain in the Solar System, and of Valles Marineris, one of the largest canyons in the Solar System. The smooth Borealis basin in the northern hemisphere covers 40% of the planet, Mars has two moons, Phobos and Deimos, which are small and irregularly shaped. These may be captured asteroids, similar to 5261 Eureka, a Mars trojan, there are ongoing investigations assessing the past habitability potential of Mars, as well as the possibility of extant life. Future astrobiology missions are planned, including the Mars 2020 and ExoMars rovers, liquid water cannot exist on the surface of Mars due to low atmospheric pressure, which is about 6⁄1000 that of the Earths, except at the lowest elevations for short periods. The two polar ice caps appear to be largely of water. The volume of ice in the south polar ice cap, if melted. On November 22,2016, NASA reported finding a large amount of ice in the Utopia Planitia region of Mars. The volume of water detected has been estimated to be equivalent to the volume of water in Lake Superior, Mars can easily be seen from Earth with the naked eye, as can its reddish coloring. Its apparent magnitude reaches −2.91, which is surpassed only by Jupiter, Venus, the Moon, optical ground-based telescopes are typically limited to resolving features about 300 kilometers across when Earth and Mars are closest because of Earths atmosphere. Mars is approximately half the diameter of Earth with an area only slightly less than the total area of Earths dry land. Mars is less dense than Earth, having about 15% of Earths volume and 11% of Earths mass, the red-orange appearance of the Martian surface is caused by iron oxide, or rust. It can look like butterscotch, other common colors include golden, brown, tan. Like Earth, Mars has differentiated into a metallic core overlaid by less dense materials. Current models of its interior imply a core with a radius of about 1,794 ±65 kilometers, consisting primarily of iron and this iron sulfide core is thought to be twice as rich in lighter elements than Earths. The core is surrounded by a mantle that formed many of the tectonic and volcanic features on the planet
5.
Planet
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The term planet is ancient, with ties to history, astrology, science, mythology, and religion. Several planets in the Solar System can be seen with the naked eye and these were regarded by many early cultures as divine, or as emissaries of deities. As scientific knowledge advanced, human perception of the planets changed, in 2006, the International Astronomical Union officially adopted a resolution defining planets within the Solar System. This definition is controversial because it excludes many objects of mass based on where or what they orbit. The planets were thought by Ptolemy to orbit Earth in deferent, at about the same time, by careful analysis of pre-telescopic observation data collected by Tycho Brahe, Johannes Kepler found the planets orbits were not circular but elliptical. As observational tools improved, astronomers saw that, like Earth, the planets rotated around tilted axes, and some shared such features as ice caps and seasons. Since the dawn of the Space Age, close observation by space probes has found that Earth and the planets share characteristics such as volcanism, hurricanes, tectonics. Planets are generally divided into two types, large low-density giant planets, and smaller rocky terrestrials. Under IAU definitions, there are eight planets in the Solar System, in order of increasing distance from the Sun, they are the four terrestrials, Mercury, Venus, Earth, and Mars, then the four giant planets, Jupiter, Saturn, Uranus, and Neptune. Six of the planets are orbited by one or more natural satellites, several thousands of planets around other stars have been discovered in the Milky Way. e. in the habitable zone. On December 20,2011, the Kepler Space Telescope team reported the discovery of the first Earth-sized extrasolar planets, Kepler-20e and Kepler-20f, orbiting a Sun-like star, Kepler-20. A2012 study, analyzing gravitational microlensing data, estimates an average of at least 1.6 bound planets for every star in the Milky Way, around one in five Sun-like stars is thought to have an Earth-sized planet in its habitable zone. The idea of planets has evolved over its history, from the lights of antiquity to the earthly objects of the scientific age. The concept has expanded to include not only in the Solar System. The ambiguities inherent in defining planets have led to much scientific controversy, the five classical planets, being visible to the naked eye, have been known since ancient times and have had a significant impact on mythology, religious cosmology, and ancient astronomy. In ancient times, astronomers noted how certain lights moved across the sky, as opposed to the fixed stars, ancient Greeks called these lights πλάνητες ἀστέρες or simply πλανῆται, from which todays word planet was derived. In ancient Greece, China, Babylon, and indeed all pre-modern civilizations, it was almost universally believed that Earth was the center of the Universe and that all the planets circled Earth. The first civilization known to have a theory of the planets were the Babylonians
6.
Silicate
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A silicate is a compound containing an anionic silicon compound. The great majority of the silicates are oxides, but hexafluorosilicate, orthosilicate is the anion SiO4−4 or its compounds. Related to orthosilicate are families of anions with the formula 2n−, important members are the cyclic and single chain silicates n and the sheet-forming silicates n. Silicates constitute the majority of Earths crust, as well as the terrestrial planets, rocky moons. Sand, Portland cement, and thousands of minerals are examples of silicates, silicate compounds, including the minerals, consist of silicate anions whose charge is balanced by various cations. Myriad silicate anions can exist, and each can form compounds with many different cations, hence this class of compounds is very large. Both minerals and synthetic materials fit in this class, in the vast majority of silicates, including silicate minerals, the Si occupies a tetrahedral environment, being surrounded by 4 oxygen centres. In these structures, the bonds to silicon conform to the octet rule. These tetrahedra sometimes occur as isolated SiO4−4 centres, but most commonly, commonly the silicate anions are chains, double chains, sheets, and three-dimensional frameworks. All these such species have negligible solubility in water at normal conditions, silicates are well characterized as solids, but are less commonly observed in solution. The anion SiO4−4 is the base of silicic acid, Si4. Instead, solutions of silicates are usually observed as mixtures of condensed, the nature of soluble silicates is relevant to understanding biomineralization and the synthesis of aluminosilicates, such as the industrially important catalysts called zeolites. Although the tetrahedron is the coordination geometry for silicon compounds. A well-known example of such a high number is hexafluorosilicate. Octahedral coordination by 6 oxygen centres is observed, at very high pressure, even SiO2 adopts this geometry in the mineral stishovite, a dense polymorph of silica found in the lower mantle of the Earth. This structure is formed by shock during meteorite impacts. In geology and astronomy, the silicate is used to denote types of rock that consist predominantly of silicate minerals. On Earth, a variety of silicate minerals occur in an even wider range of combinations as a result of the processes that form
7.
Rock (geology)
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Rock or stone is a natural substance, a solid aggregate of one or more minerals or mineraloids. For example, granite, a rock, is a combination of the minerals quartz, feldspar. The Earths outer solid layer, the lithosphere, is made of rock, rock has been used by mankind throughout history. The minerals and metals found in rocks have been essential to human civilization, three major groups of rocks are defined, igneous, sedimentary, and metamorphic. The scientific study of rocks is called petrology, which is a component of geology. At a granular level, rocks are composed of grains of minerals, the aggregate minerals forming the rock are held together by chemical bonds. The types and abundance of minerals in a rock are determined by the manner in which the rock was formed, many rocks contain silica, a compound of silicon and oxygen that forms 74. 3% of the Earths crust. This material forms crystals with other compounds in the rock, the proportion of silica in rocks and minerals is a major factor in determining their name and properties. Rocks are geologically classified according to such as mineral and chemical composition, permeability, the texture of the constituent particles. These physical properties are the end result of the processes that formed the rocks, over the course of time, rocks can transform from one type into another, as described by the geological model called the rock cycle. These events produce three general classes of rock, igneous, sedimentary, and metamorphic, the three classes of rocks are subdivided into many groups. However, there are no hard and fast boundaries between allied rocks, hence the definitions adopted in establishing rock nomenclature merely correspond to more or less arbitrary selected points in a continuously graduated series. Igneous rock forms through the cooling and solidification of magma or lava and this magma can be derived from partial melts of pre-existing rocks in either a planets mantle or crust. Typically, the melting of rocks is caused by one or more of three processes, an increase in temperature, a decrease in pressure, or a change in composition, igneous rocks are divided into two main categories, plutonic rock and volcanic. Plutonic or intrusive rocks result when magma cools and crystallizes slowly within the Earths crust, a common example of this type is granite. Volcanic or extrusive rocks result from magma reaching the surface either as lava or fragmental ejecta, the chemical abundance and the rate of cooling of magma typically forms a sequence known as Bowens reaction series. Most major igneous rocks are found along this scale, about 64. 7% of the Earths crust by volume consists of igneous rocks, making it the most plentiful category. Of these, 66% are basalts and gabbros, 16% are granite, only 0. 6% are syenites and 0. 3% peridotites and dunites
8.
Metal
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A metal is a material that is typically hard, opaque, shiny, and has good electrical and thermal conductivity. Metals are generally malleable—that is, they can be hammered or pressed permanently out of shape without breaking or cracking—as well as fusible and ductile, about 91 of the 118 elements in the periodic table are metals, the others are nonmetals or metalloids. Some elements appear in both metallic and non-metallic forms, astrophysicists use the term metal to collectively describe all elements other than hydrogen and helium, the simplest two, in a star. The star fuses smaller atoms, mostly hydrogen and helium, to larger ones over its lifetime. In that sense, the metallicity of an object is the proportion of its matter made up of all chemical elements. Many elements and compounds that are not normally classified as metals become metallic under high pressures, the atoms of metallic substances are typically arranged in one of three common crystal structures, namely body-centered cubic, face-centered cubic, and hexagonal close-packed. In bcc, each atom is positioned at the center of a cube of eight others, in fcc and hcp, each atom is surrounded by twelve others, but the stacking of the layers differs. Some metals adopt different structures depending on the temperature, atoms of metals readily lose their outer shell electrons, resulting in a free flowing cloud of electrons within their otherwise solid arrangement. This provides the ability of metallic substances to easily transmit heat, while this flow of electrons occurs, the solid characteristic of the metal is produced by electrostatic interactions between each atom and the electron cloud. This type of bond is called a metallic bond, Metals are usually inclined to form cations through electron loss, reacting with oxygen in the air to form oxides over various timescales. Examples,4 Na + O2 →2 Na2O2 Ca + O2 →2 CaO4 Al +3 O2 →2 Al2O3, the transition metals are slower to oxidize because they form a passivating layer of oxide that protects the interior. Others, like palladium, platinum and gold, do not react with the atmosphere at all, some metals form a barrier layer of oxide on their surface which cannot be penetrated by further oxygen molecules and thus retain their shiny appearance and good conductivity for many decades. The oxides of metals are generally basic, as opposed to those of nonmetals, exceptions are largely oxides with very high oxidation states such as CrO3, Mn2O7, and OsO4, which have strictly acidic reactions. Painting, anodizing or plating metals are good ways to prevent their corrosion, however, a more reactive metal in the electrochemical series must be chosen for coating, especially when chipping of the coating is expected. Water and the two form an electrochemical cell, and if the coating is less reactive than the coatee. Metals in general have high conductivity, high thermal conductivity. Typically they are malleable and ductile, deforming under stress without cleaving, in terms of optical properties, metals are shiny and lustrous. Sheets of metal beyond a few micrometres in thickness appear opaque, although most metals have higher densities than most nonmetals, there is wide variation in their densities, lithium being the least dense solid element and osmium the densest
9.
Solar System
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The Solar System is the gravitationally bound system comprising the Sun and the objects that orbit it, either directly or indirectly. Of those objects that orbit the Sun directly, the largest eight are the planets, with the remainder being significantly smaller objects, such as dwarf planets, of the objects that orbit the Sun indirectly, the moons, two are larger than the smallest planet, Mercury. The Solar System formed 4.6 billion years ago from the collapse of a giant interstellar molecular cloud. The vast majority of the mass is in the Sun. The four smaller inner planets, Mercury, Venus, Earth and Mars, are terrestrial planets, being composed of rock. The four outer planets are giant planets, being more massive than the terrestrials. All planets have almost circular orbits that lie within a flat disc called the ecliptic. The Solar System also contains smaller objects, the asteroid belt, which lies between the orbits of Mars and Jupiter, mostly contains objects composed, like the terrestrial planets, of rock and metal. Beyond Neptunes orbit lie the Kuiper belt and scattered disc, which are populations of trans-Neptunian objects composed mostly of ices, within these populations are several dozen to possibly tens of thousands of objects large enough that they have been rounded by their own gravity. Such objects are categorized as dwarf planets, identified dwarf planets include the asteroid Ceres and the trans-Neptunian objects Pluto and Eris. In addition to two regions, various other small-body populations, including comets, centaurs and interplanetary dust clouds. Six of the planets, at least four of the dwarf planets, each of the outer planets is encircled by planetary rings of dust and other small objects. The solar wind, a stream of charged particles flowing outwards from the Sun, the heliopause is the point at which pressure from the solar wind is equal to the opposing pressure of the interstellar medium, it extends out to the edge of the scattered disc. The Oort cloud, which is thought to be the source for long-period comets, the Solar System is located in the Orion Arm,26,000 light-years from the center of the Milky Way. For most of history, humanity did not recognize or understand the concept of the Solar System, the invention of the telescope led to the discovery of further planets and moons. The principal component of the Solar System is the Sun, a G2 main-sequence star that contains 99. 86% of the known mass. The Suns four largest orbiting bodies, the giant planets, account for 99% of the mass, with Jupiter. The remaining objects of the Solar System together comprise less than 0. 002% of the Solar Systems total mass, most large objects in orbit around the Sun lie near the plane of Earths orbit, known as the ecliptic
10.
Sun
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The Sun is the star at the center of the Solar System. It is a perfect sphere of hot plasma, with internal convective motion that generates a magnetic field via a dynamo process. It is by far the most important source of energy for life on Earth. Its diameter is about 109 times that of Earth, and its mass is about 330,000 times that of Earth, accounting for about 99. 86% of the total mass of the Solar System. About three quarters of the Suns mass consists of hydrogen, the rest is mostly helium, with smaller quantities of heavier elements, including oxygen, carbon, neon. The Sun is a G-type main-sequence star based on its spectral class and it formed approximately 4.6 billion years ago from the gravitational collapse of matter within a region of a large molecular cloud. Most of this matter gathered in the center, whereas the rest flattened into a disk that became the Solar System. The central mass became so hot and dense that it eventually initiated nuclear fusion in its core and it is thought that almost all stars form by this process. The Sun is roughly middle-aged, it has not changed dramatically for more than four billion years and it is calculated that the Sun will become sufficiently large enough to engulf the current orbits of Mercury, Venus, and probably Earth. The enormous effect of the Sun on Earth has been recognized since prehistoric times, the synodic rotation of Earth and its orbit around the Sun are the basis of the solar calendar, which is the predominant calendar in use today. The English proper name Sun developed from Old English sunne and may be related to south, all Germanic terms for the Sun stem from Proto-Germanic *sunnōn. The English weekday name Sunday stems from Old English and is ultimately a result of a Germanic interpretation of Latin dies solis, the Latin name for the Sun, Sol, is not common in general English language use, the adjectival form is the related word solar. The term sol is used by planetary astronomers to refer to the duration of a solar day on another planet. A mean Earth solar day is approximately 24 hours, whereas a mean Martian sol is 24 hours,39 minutes, and 35.244 seconds. From at least the 4th Dynasty of Ancient Egypt, the Sun was worshipped as the god Ra, portrayed as a falcon-headed divinity surmounted by the solar disk, and surrounded by a serpent. In the New Empire period, the Sun became identified with the dung beetle, in the form of the Sun disc Aten, the Sun had a brief resurgence during the Amarna Period when it again became the preeminent, if not only, divinity for the Pharaoh Akhenaton. The Sun is viewed as a goddess in Germanic paganism, Sól/Sunna, in ancient Roman culture, Sunday was the day of the Sun god. It was adopted as the Sabbath day by Christians who did not have a Jewish background, the symbol of light was a pagan device adopted by Christians, and perhaps the most important one that did not come from Jewish traditions
11.
Latin
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Latin is a classical language belonging to the Italic branch of the Indo-European languages. The Latin alphabet is derived from the Etruscan and Greek alphabets, Latin was originally spoken in Latium, in the Italian Peninsula. Through the power of the Roman Republic, it became the dominant language, Vulgar Latin developed into the Romance languages, such as Italian, Portuguese, Spanish, French, and Romanian. Latin, Italian and French have contributed many words to the English language, Latin and Ancient Greek roots are used in theology, biology, and medicine. By the late Roman Republic, Old Latin had been standardised into Classical Latin, Vulgar Latin was the colloquial form spoken during the same time and attested in inscriptions and the works of comic playwrights like Plautus and Terence. Late Latin is the language from the 3rd century. Later, Early Modern Latin and Modern Latin evolved, Latin was used as the language of international communication, scholarship, and science until well into the 18th century, when it began to be supplanted by vernaculars. Ecclesiastical Latin remains the language of the Holy See and the Roman Rite of the Catholic Church. Today, many students, scholars and members of the Catholic clergy speak Latin fluently and it is taught in primary, secondary and postsecondary educational institutions around the world. The language has been passed down through various forms, some inscriptions have been published in an internationally agreed, monumental, multivolume series, the Corpus Inscriptionum Latinarum. Authors and publishers vary, but the format is about the same, volumes detailing inscriptions with a critical apparatus stating the provenance, the reading and interpretation of these inscriptions is the subject matter of the field of epigraphy. The works of several hundred ancient authors who wrote in Latin have survived in whole or in part and they are in part the subject matter of the field of classics. The Cat in the Hat, and a book of fairy tales, additional resources include phrasebooks and resources for rendering everyday phrases and concepts into Latin, such as Meissners Latin Phrasebook. The Latin influence in English has been significant at all stages of its insular development. From the 16th to the 18th centuries, English writers cobbled together huge numbers of new words from Latin and Greek words, dubbed inkhorn terms, as if they had spilled from a pot of ink. Many of these words were used once by the author and then forgotten, many of the most common polysyllabic English words are of Latin origin through the medium of Old French. Romance words make respectively 59%, 20% and 14% of English, German and those figures can rise dramatically when only non-compound and non-derived words are included. Accordingly, Romance words make roughly 35% of the vocabulary of Dutch, Roman engineering had the same effect on scientific terminology as a whole
12.
Planetary surface
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Land is the term given to non-liquid planetary surfaces. The term landing is used to describe the collision of an object with a surface and is usually at a velocity in which the object can remain intact. In differentiated bodies, the surface is where the crust meets the boundary layer. Anything below this is regarded as being sub-surface or sub-marine, most bodies more massive than Super-Earths, including stars and gas giants, as well as smaller gas dwarfs, transition contiguously between phases including gas, liquid and solids. As such they are regarded as lacking surfaces. Planetary surfaces and surface life are of particular interest to humans as it the primary habitat of the species, human space exploration and space colonization therefore focuses heavily on them. Humans have only directly explored the surface of the Earth and Moon, the vast distances of space and the complexities of makes direct exploration of even Near-Earth objects dangerous and expensive. As such, all other exploration has been indirect via Space probes, indirect observations by flyby or orbit currently provide insufficient information to confirm the composition and properties of planetary surfaces. Much of what is known is from the use of such as astronomical spectroscopy. Lander spacecraft have explored the surfaces of planets Mars and Venus, Mars is the only other planet to have had its surface explored by a mobile surface probe. Titan is the only object of planetary mass to have been explored by lander. Landers have explored several smaller bodies including 433 Eros,25143 Itokawa, Tempel 1, surface conditions, temperatures and terrain vary significantly due to a number of factors including Albedo often generated by the surfaces itself. Measures of surface conditions include surface area, surface gravity, surface temperature, surface stability may be affected by erosion through Aeolian processes, hydrology, subduction, volcanism, sediment or seismic activity. Some surfaces are dynamic while others remain unchanged for millions of years, distance, gravity, atmospheric conditions and unknown factors make exploration is both costly and risky. This necessitates the space probes for early exploration of planetary surfaces, many probes are stationary have a limited study range and generally survive on extraterrestrial surfaces for a short period, however mobile probes have surveyed larger surface areas. The first extraterrestrial planetary surface to be explored was the surface by Luna 2 in 1959. Venera 7 was the first landing of a probe on another planet on December 15,1970, NEAR Shoemaker was the first to soft land on an asteroid -433 Eros in February 2001 while Hayabusa was the first to return samples from 25143 Itokawa on 13 June 2010. Huygens soft landed and returned data from Titan on January 14,2005, there have been many failed attempts, more recently Fobos-Grunt, a sample return mission aimed at exploring the surface of Phobos
13.
Giant planet
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A giant planet is any massive planet. They are usually composed of low-boiling-point materials, rather than rock or other solid matter. There are four giant planets in the Solar System, Jupiter, Saturn, Uranus. Many extrasolar giant planets have been identified orbiting other stars, giant planets are also sometimes called jovian planets, after Jupiter. They are also known as gas giants. However, many astronomers apply the term only to Jupiter and Saturn, classifying Uranus and Neptune. Both names are misleading, all of the giant planets consist primarily of fluids above their critical points. The principal components are hydrogen and helium in the case of Jupiter and Saturn, the defining differences between a very low-mass brown dwarf and a gas giant are debated. One school of thought is based on formation, the other, part of the debate concerns whether brown dwarfs must, by definition, have experienced nuclear fusion at some point in their history. The term gas giant was coined in 1952 by the fiction writer James Blish and was originally used to refer to all giant planets. Arguably it is something of a misnomer, because throughout most of the volume of these planets the pressure is so high that matter is not in gaseous form. Other than solids in the core and the layers of the atmosphere, all matter is above the critical point. Fluid planet would be an accurate term. Jupiter also has metallic hydrogen near its center, but much of its volume is hydrogen, helium, the observable atmospheres of all these planets are quite thin compared to their radii, only extending perhaps one percent of the way to the center. Thus the observable portions are gaseous, in the outer Solar System, hydrogen and helium are referred to as gases, water, methane, and ammonia as ices, and silicates and metals as rock. When deep planetary interiors are considered, it may not be far off to say that, by ice astronomers mean oxygen and carbon, by rock they mean silicon, and by gas they mean hydrogen and helium. The many ways in which Uranus and Neptune differ from Jupiter, with this terminology in mind, some astronomers have started referring to Uranus and Neptune as ice giants to indicate the predominance of the ices in their interior composition. The alternative term jovian planet refers to the Roman god Jupiter—the genitive form of which is Jovis, objects large enough to start deuterium fusion are called brown dwarfs, and these occupy the mass range between that of large giant planets and the lowest-mass stars
14.
Hydrogen
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Hydrogen is a chemical element with chemical symbol H and atomic number 1. With a standard weight of circa 1.008, hydrogen is the lightest element on the periodic table. Its monatomic form is the most abundant chemical substance in the Universe, non-remnant stars are mainly composed of hydrogen in the plasma state. The most common isotope of hydrogen, termed protium, has one proton, the universal emergence of atomic hydrogen first occurred during the recombination epoch. At standard temperature and pressure, hydrogen is a colorless, odorless, tasteless, non-toxic, nonmetallic, since hydrogen readily forms covalent compounds with most nonmetallic elements, most of the hydrogen on Earth exists in molecular forms such as water or organic compounds. Hydrogen plays an important role in acid–base reactions because most acid-base reactions involve the exchange of protons between soluble molecules. In ionic compounds, hydrogen can take the form of a charge when it is known as a hydride. The hydrogen cation is written as though composed of a bare proton, Hydrogen gas was first artificially produced in the early 16th century by the reaction of acids on metals. Industrial production is mainly from steam reforming natural gas, and less often from more energy-intensive methods such as the electrolysis of water. Most hydrogen is used near the site of its production, the two largest uses being fossil fuel processing and ammonia production, mostly for the fertilizer market, Hydrogen is a concern in metallurgy as it can embrittle many metals, complicating the design of pipelines and storage tanks. Hydrogen gas is flammable and will burn in air at a very wide range of concentrations between 4% and 75% by volume. The enthalpy of combustion is −286 kJ/mol,2 H2 + O2 →2 H2O +572 kJ Hydrogen gas forms explosive mixtures with air in concentrations from 4–74%, the explosive reactions may be triggered by spark, heat, or sunlight. The hydrogen autoignition temperature, the temperature of spontaneous ignition in air, is 500 °C, the detection of a burning hydrogen leak may require a flame detector, such leaks can be very dangerous. Hydrogen flames in other conditions are blue, resembling blue natural gas flames, the destruction of the Hindenburg airship was a notorious example of hydrogen combustion and the cause is still debated. The visible orange flames in that incident were the result of a mixture of hydrogen to oxygen combined with carbon compounds from the airship skin. H2 reacts with every oxidizing element, the ground state energy level of the electron in a hydrogen atom is −13.6 eV, which is equivalent to an ultraviolet photon of roughly 91 nm wavelength. The energy levels of hydrogen can be calculated fairly accurately using the Bohr model of the atom, however, the atomic electron and proton are held together by electromagnetic force, while planets and celestial objects are held by gravity. The most complicated treatments allow for the effects of special relativity
15.
Helium
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Helium is a chemical element with symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and its boiling point is the lowest among all the elements. Its abundance is similar to figure in the Sun and in Jupiter. This is due to the high nuclear binding energy of helium-4 with respect to the next three elements after helium. This helium-4 binding energy also accounts for why it is a product of nuclear fusion and radioactive decay. Most helium in the universe is helium-4, and is believed to have formed during the Big Bang. Large amounts of new helium are being created by fusion of hydrogen in stars. Helium is named for the Greek god of the Sun, Helios and it was first detected as an unknown yellow spectral line signature in sunlight during a solar eclipse in 1868 by French astronomer Jules Janssen. Janssen is jointly credited with detecting the element along with Norman Lockyer, Janssen observed during the solar eclipse of 1868 while Lockyer observed from Britain. Lockyer was the first to propose that the line was due to a new element, the formal discovery of the element was made in 1895 by two Swedish chemists, Per Teodor Cleve and Nils Abraham Langlet, who found helium emanating from the uranium ore cleveite. In 1903, large reserves of helium were found in gas fields in parts of the United States. Liquid helium is used in cryogenics, particularly in the cooling of superconducting magnets, a well-known but minor use is as a lifting gas in balloons and airships. As with any gas whose density differs from that of air, inhaling a small volume of helium temporarily changes the timbre, on Earth it is relatively rare—5.2 ppm by volume in the atmosphere. Most terrestrial helium present today is created by the radioactive decay of heavy radioactive elements. Previously, terrestrial helium—a non-renewable resource, because once released into the atmosphere it readily escapes into space—was thought to be in short supply. The first evidence of helium was observed on August 18,1868, the line was detected by French astronomer Jules Janssen during a total solar eclipse in Guntur, India. This line was assumed to be sodium. He concluded that it was caused by an element in the Sun unknown on Earth, Lockyer and English chemist Edward Frankland named the element with the Greek word for the Sun, ἥλιος
16.
Water
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Water is a transparent and nearly colorless chemical substance that is the main constituent of Earths streams, lakes, and oceans, and the fluids of most living organisms. Its chemical formula is H2O, meaning that its molecule contains one oxygen, Water strictly refers to the liquid state of that substance, that prevails at standard ambient temperature and pressure, but it often refers also to its solid state or its gaseous state. It also occurs in nature as snow, glaciers, ice packs and icebergs, clouds, fog, dew, aquifers, Water covers 71% of the Earths surface. It is vital for all forms of life. Only 2. 5% of this water is freshwater, and 98. 8% of that water is in ice and groundwater. Less than 0. 3% of all freshwater is in rivers, lakes, and the atmosphere, a greater quantity of water is found in the earths interior. Water on Earth moves continually through the cycle of evaporation and transpiration, condensation, precipitation. Evaporation and transpiration contribute to the precipitation over land, large amounts of water are also chemically combined or adsorbed in hydrated minerals. Safe drinking water is essential to humans and other even though it provides no calories or organic nutrients. There is a correlation between access to safe water and gross domestic product per capita. However, some observers have estimated that by 2025 more than half of the population will be facing water-based vulnerability. A report, issued in November 2009, suggests that by 2030, in developing regions of the world. Water plays an important role in the world economy, approximately 70% of the freshwater used by humans goes to agriculture. Fishing in salt and fresh water bodies is a source of food for many parts of the world. Much of long-distance trade of commodities and manufactured products is transported by boats through seas, rivers, lakes, large quantities of water, ice, and steam are used for cooling and heating, in industry and homes. Water is an excellent solvent for a variety of chemical substances, as such it is widely used in industrial processes. Water is also central to many sports and other forms of entertainment, such as swimming, pleasure boating, boat racing, surfing, sport fishing, Water is a liquid at the temperatures and pressures that are most adequate for life. Specifically, at atmospheric pressure of 1 bar, water is a liquid between the temperatures of 273.15 K and 373.15 K
17.
Phase (matter)
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In the physical sciences, a phase is a region of space, throughout which all physical properties of a material are essentially uniform. Examples of physical properties include density, index of refraction, magnetization, a simple description is that a phase is a region of material that is chemically uniform, physically distinct, and mechanically separable. In a system consisting of ice and water in a jar, the ice cubes are one phase, the water is a second phase. The glass of the jar is another separate phase, the term phase is sometimes used as a synonym for state of matter, but there can be several immiscible phases of the same state of matter. Also, the phase is sometimes used to refer to a set of equilibrium states demarcated in terms of state variables such as pressure and temperature by a phase boundary on a phase diagram. Distinct phases may be described as different states of such as gas, liquid, solid. Useful mesophases between solid and liquid form other states of matter, distinct phases may also exist within a given state of matter. As shown in the diagram for iron alloys, several phases exist for both the solid and liquid states, phases may also be differentiated based on solubility as in polar or non-polar. A mixture of water and oil will separate into two phases. Water has a low solubility in oil, and oil has a low solubility in water. Solubility is the amount of a solute that can dissolve in a solvent before the solute ceases to dissolve. A mixture can separate into more than two phases and the concept of phase separation extends to solids, i. e. solids can form solid solutions or crystallize into distinct crystal phases. Metal pairs that are mutually soluble can form alloys, whereas metal pairs that are mutually insoluble cannot, as many as eight immiscible liquid phases have been observed. Mutually immiscible liquid phases are formed from water, hydrophobic solvents, perfluorocarbons, silicones, several different metals. Not all organic solvents are completely miscible, e. g. a mixture of ethylene glycol, phases do not need to macroscopically separate spontaneously. Emulsions and colloids are examples of immiscible phase pair combinations that do not physically separate, left to equilibration, many compositions will form a uniform single phase, but depending on the temperature and pressure even a single substance may separate into two or more distinct phases. Within each phase, the properties are uniform but between the two phases properties differ, water in a closed jar with an air space over it forms a two phase system. Most of the water is in the phase, where it is held by the mutual attraction of water molecules
18.
Planetary core
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The planetary core consists of the innermost layer of a planet, which may be composed of solid and liquid layers. Cores of specific planets may be solid or entirely liquid. In the Solar System, core size can range from about 20% to 85% of a planets radius. Gas giants also have cores, though the composition of these are still a matter of debate and range in composition from traditional stony/iron. In 1798, Henry Cavendish calculated the density of the earth to be 5.48 times the density of water. The first detection of Earths core occurred in 1906 by Richard Dixon Oldham upon discovery of the P-wave shadow zone, by 1936 seismologists had determined the size of the overall core as well as the boundary between the fluid outer core and the solid inner core. Planetary systems form from a disk of dust and gas that accrete rapidly into planetesimals around 10 km in diameter. From here gravity takes over to produce Moon to Mars sized planetary embryos, Jupiter and Saturn most likely formed around previously existing rocky and/or icy bodies, rendering these previous primordial planets into gas-giant cores. This is the core accretion model of planet formation. Planetary differentiation is broadly defined as the development from one thing to many things, the hafnium-182/tungsten-182 isotopic system has a half-life of 9 million years, and is approximated as an extinct system after 45 million years. Hafnium is an element and tungsten is siderophile element. The observed Hf/W ratios in iron meteorites constrain metal segregation to under 5 million years, several factors control segregation of a metal core including the crystallization of perovskite. Crystallization of perovskite in a magma ocean is an oxidation process. Impacts between planet-sized bodies in the early Solar System are important aspects in the formation and growth of planets, the giant impact hypothesis states that an impact between a theoretical Mars-sized planet Theia and the early Earth formed the modern Earth and moon. During this impact the majority of the iron from Theia and the Earth became incorporated into the Earths core, core merging between the proto-mars and another differentiated planetoid could have been as fast as 1000 years or as slow as 300,000 years. Earths core contains half the Earths vanadium and chromium, and may contain considerable niobium and tantalum, Earths core is depleted in germanium and gallium. Sulfur is strongly siderophile and only moderately volatile and depleted in the silicate earth, by similar argument, phosphorus may be present up to 0.2 weight %. Hydrogen and carbon, however, are volatile and thus would have been lost during early accretion
19.
Iron
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Iron is a chemical element with symbol Fe and atomic number 26. It is a metal in the first transition series and it is by mass the most common element on Earth, forming much of Earths outer and inner core. It is the fourth most common element in the Earths crust, like the other group 8 elements, ruthenium and osmium, iron exists in a wide range of oxidation states, −2 to +6, although +2 and +3 are the most common. Elemental iron occurs in meteoroids and other low oxygen environments, but is reactive to oxygen, fresh iron surfaces appear lustrous silvery-gray, but oxidize in normal air to give hydrated iron oxides, commonly known as rust. Unlike the metals that form passivating oxide layers, iron oxides occupy more volume than the metal and thus flake off, Iron metal has been used since ancient times, although copper alloys, which have lower melting temperatures, were used even earlier in human history. Pure iron is soft, but is unobtainable by smelting because it is significantly hardened and strengthened by impurities, in particular carbon. A certain proportion of carbon steel, which may be up to 1000 times harder than pure iron. Crude iron metal is produced in blast furnaces, where ore is reduced by coke to pig iron, further refinement with oxygen reduces the carbon content to the correct proportion to make steel. Steels and iron alloys formed with metals are by far the most common industrial metals because they have a great range of desirable properties. Iron chemical compounds have many uses, Iron oxide mixed with aluminium powder can be ignited to create a thermite reaction, used in welding and purifying ores. Iron forms binary compounds with the halogens and the chalcogens, among its organometallic compounds is ferrocene, the first sandwich compound discovered. Iron plays an important role in biology, forming complexes with oxygen in hemoglobin and myoglobin. Iron is also the metal at the site of many important redox enzymes dealing with cellular respiration and oxidation and reduction in plants. A human male of average height has about 4 grams of iron in his body and this iron is distributed throughout the body in hemoglobin, tissues, muscles, bone marrow, blood proteins, enzymes, ferritin, hemosiderin, and transport in plasma. The mechanical properties of iron and its alloys can be evaluated using a variety of tests, including the Brinell test, Rockwell test, the data on iron is so consistent that it is often used to calibrate measurements or to compare tests. An increase in the content will cause a significant increase in the hardness. Maximum hardness of 65 Rc is achieved with a 0. 6% carbon content, because of the softness of iron, it is much easier to work with than its heavier congeners ruthenium and osmium. Because of its significance for planetary cores, the properties of iron at high pressures and temperatures have also been studied extensively
20.
Mantle (geology)
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The mantle is a layer inside a terrestrial planet and some other rocky planetary bodies. For a mantle to form, the body must be large enough to have undergone the process of planetary differentiation by density. The mantle surrounds the planetary core, the mantle is surrounded by the crust. The terrestrial planets, the Moon, two of Jupiters moons and the asteroid Vesta each have a made of silicate rock. Interpretation of spacecraft data suggests that at least two moons of Jupiter, as well as Titan and Triton each have a mantle made of ice or other solid volatile substances. The interior of Earth, similar to the terrestrial planets, is divided into layers of different composition. The mantle is a layer between the crust and the outer core, Earths mantle is a silicate rocky shell with an average thickness of 2,886 kilometres. The mantle makes up about 84% of Earths volume and it is predominantly solid but in geological time it behaves as a very viscous fluid. The mantle encloses the hot core rich in iron and nickel, past episodes of melting and volcanism at the shallower levels of the mantle have produced a thin crust of crystallized melt products near the surface. A thin crust, the part of the lithosphere, surrounds the mantle and is about 5 to 75 km thick. In some places under the ocean the mantle is exposed on the surface of Earth. The mantle is divided into sections which are based upon results from seismology and these layers are the following, the upper mantle, the transition zone, the lower mantle, and anomalous core–mantle boundary with a variable thickness. The uppermost mantle plus overlying crust are relatively rigid and form the lithosphere, below the lithosphere the upper mantle becomes notably more plastic. In some regions below the lithosphere, the shear velocity is reduced. Inge Lehmann discovered a seismic discontinuity at about 220 km depth, although this discontinuity has been found in other studies, the transition zone is an area of great complexity, it physically separates the upper and lower mantle. Very little is known about the lower mantle apart from that it appears to be relatively seismically homogeneous, the D layer at the core–mantle boundary separates the mantle from the core. A principal source of the heat that drives plate tectonics is the decay of uranium, thorium. The mantle differs substantially from the crust in its properties as the direct consequence of the difference in composition
21.
Moon
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The Moon is an astronomical body that orbits planet Earth, being Earths only permanent natural satellite. It is the fifth-largest natural satellite in the Solar System, following Jupiters satellite Io, the Moon is second-densest satellite among those whose densities are known. The average distance of the Moon from the Earth is 384,400 km, the Moon is thought to have formed about 4.51 billion years ago, not long after Earth. It is the second-brightest regularly visible celestial object in Earths sky, after the Sun and its surface is actually dark, although compared to the night sky it appears very bright, with a reflectance just slightly higher than that of worn asphalt. Its prominence in the sky and its cycle of phases have made the Moon an important cultural influence since ancient times on language, calendars, art. The Moons gravitational influence produces the ocean tides, body tides, and this matching of apparent visual size will not continue in the far future. The Moons linear distance from Earth is currently increasing at a rate of 3.82 ±0.07 centimetres per year, since the Apollo 17 mission in 1972, the Moon has been visited only by uncrewed spacecraft. The usual English proper name for Earths natural satellite is the Moon, the noun moon is derived from moone, which developed from mone, which is derived from Old English mōna, which ultimately stems from Proto-Germanic *mǣnōn, like all Germanic language cognates. Occasionally, the name Luna is used, in literature, especially science fiction, Luna is used to distinguish it from other moons, while in poetry, the name has been used to denote personification of our moon. The principal modern English adjective pertaining to the Moon is lunar, a less common adjective is selenic, derived from the Ancient Greek Selene, from which is derived the prefix seleno-. Both the Greek Selene and the Roman goddess Diana were alternatively called Cynthia, the names Luna, Cynthia, and Selene are reflected in terminology for lunar orbits in words such as apolune, pericynthion, and selenocentric. The name Diana is connected to dies meaning day, several mechanisms have been proposed for the Moons formation 4.51 billion years ago, and some 60 million years after the origin of the Solar System. These hypotheses also cannot account for the angular momentum of the Earth–Moon system. This hypothesis, although not perfect, perhaps best explains the evidence, eighteen months prior to an October 1984 conference on lunar origins, Bill Hartmann, Roger Phillips, and Jeff Taylor challenged fellow lunar scientists, You have eighteen months. Go back to your Apollo data, go back to computer, do whatever you have to. Dont come to our conference unless you have something to say about the Moons birth, at the 1984 conference at Kona, Hawaii, the giant impact hypothesis emerged as the most popular. Afterward there were only two groups, the giant impact camp and the agnostics. Giant impacts are thought to have been common in the early Solar System, computer simulations of a giant impact have produced results that are consistent with the mass of the lunar core and the present angular momentum of the Earth–Moon system
22.
Io (moon)
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Io /ˈaɪ. oʊ/ is the innermost of the four Galilean moons of the planet Jupiter. It is the fourth-largest moon, has the highest density of all the moons and it was discovered in 1610 and was named after the mythological character Io, a priestess of Hera who became one of Zeuss lovers. With over 400 active volcanoes, Io is the most geologically active object in the Solar System, several volcanoes produce plumes of sulfur and sulfur dioxide that climb as high as 500 km above the surface. Ios surface is dotted with more than 100 mountains that have been uplifted by extensive compression at the base of Ios silicate crust. Some of these peaks are taller than Mount Everest, unlike most satellites in the outer Solar System, which are mostly composed of water ice, Io is primarily composed of silicate rock surrounding a molten iron or iron-sulfide core. Most of Ios surface is composed of extensive plains coated with sulfur, Ios volcanism is responsible for many of its unique features. Numerous extensive lava flows, several more than 500 km in length, the materials produced by this volcanism make up Ios thin, patchy atmosphere and Jupiters extensive magnetosphere. Ios volcanic ejecta also produce a large torus around Jupiter. Io played a significant role in the development of astronomy in the 17th and 18th centuries and it was discovered in January 1610 by Galileo Galilei, along with the other Galilean satellites. This discovery furthered the adoption of the Copernican model of the Solar System, the development of Keplers laws of motion, and the first measurement of the speed of light. In 1979, the two Voyager spacecraft revealed Io to be a geologically active world, with numerous volcanic features, large mountains, the Galileo spacecraft performed several close flybys in the 1990s and early 2000s, obtaining data about Ios interior structure and surface composition. These spacecraft also revealed the relationship between Io and Jupiters magnetosphere and the existence of a belt of high-energy radiation centered on Ios orbit, Io receives about 3,600 rem of ionizing radiation per day. Further observations have made by Cassini–Huygens in 2000 and New Horizons in 2007, as well as from Earth-based telescopes. From the surface of Io, Jupiter would subtend an arc of 19. 5°, although Simon Marius is not credited with the sole discovery of the Galilean satellites, his names for the moons were adopted. Based on a suggestion from Johannes Kepler in October 1613, he devised a naming scheme whereby each moon was named for a lover of the Greek mythological Zeus or his Roman equivalent. He named the innermost large moon of Jupiter after the Greek mythological figure Io, since the surface was first seen up close by Voyager 1, the International Astronomical Union has approved 225 names for Ios volcanoes, mountains, plateaus, and large albedo features. The approved feature categories used for Io for different types of volcanic features include patera, fluctus, vallis, named mountains, plateaus, layered terrain, and shield volcanoes include the terms mons, mensa, planum, and tholus, respectively. Named, bright albedo regions use the term regio, examples of named features are Prometheus, Pan Mensa, Tvashtar Paterae, and Tsũi Goab Fluctus
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Europa (moon)
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Europa /jʊˈroʊpə/, is the smallest of the four Galilean moons orbiting Jupiter, and the sixth-closest to the planet. It is also the sixth-largest moon in the Solar System, Europa was discovered in 1610 by Galileo Galilei and was named after Europa, the legendary mother of King Minos of Crete and lover of Zeus. Slightly smaller than Earths Moon, Europa is primarily made of rock and has a water-ice crust. It has an atmosphere composed primarily of oxygen. Its surface is striated by cracks and streaks, whereas craters are relatively rare, in addition to Earth-bound telescope observations, Europa has been examined by a succession of space probe flybys, the first occurring in the early 1970s. Europa has the smoothest surface of any solid object in the Solar System. The apparent youth and smoothness of the surface have led to the hypothesis that a water ocean exists beneath it, sea salt from a subsurface ocean may be coating some geological features on Europa, suggesting that the ocean is interacting with the seafloor. This may be important in determining if Europa could be habitable, in addition, the Hubble Space Telescope detected water vapor plumes similar to those observed on Saturns moon Enceladus, which are thought to be caused by erupting cryogeysers. The Galileo mission, launched in 1989, provides the bulk of current data on Europa, no spacecraft has yet landed on Europa, although there have been several proposed exploration missions. The European Space Agencys Jupiter Icy Moon Explorer is a mission to Ganymede that is due to launch in 2022, NASAs planned Europa Multiple-Flyby Mission will be launched in the mid-2020s. Europa, along with Jupiters three other large moons, Io, Ganymede, and Callisto, was discovered by Galileo Galilei on 8 January 1610, and possibly independently by Simon Marius. The first reported observation of Io and Europa was made by Galileo Galilei on 7 January 1610 using a 20×-magnification refracting telescope at the University of Padua. However, in observation, Galileo could not separate Io and Europa due to the low magnification of his telescope. The following day,8 January 1610, Io and Europa were seen for the first time as separate bodies during Galileos observations of the Jupiter system, Europa is named after Europa, daughter of the king of Tyre, a Phoenician noblewoman in Greek mythology. Like all the Galilean satellites, Europa is named after a lover of Zeus, Europa was courted by Zeus and became the queen of Crete. The naming scheme was suggested by Simon Marius, who discovered the four satellites independently, Marius attributed the proposal to Johannes Kepler. The names fell out of favor for a time and were not revived in general use until the mid-20th century. In much of the astronomical literature, Europa is simply referred to by its Roman numeral designation as Jupiter II or as the second satellite of Jupiter
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Canyon
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A canyon or gorge is a deep cleft between escarpments or cliffs resulting from the erosive activity of a river over geologic timescales. A canyon may also refer to a rift between two peaks, such as those in ranges including the Rocky Mountains, the Alps. Usually a river or stream and erosion carve out such splits between mountains, examples of mountain-type canyons are Provo Canyon in Utah or Yosemite National Park in Californias Sierra Nevada. Canyons within mountains, or gorges that have an opening on one side are called box canyons. Slot canyons are very narrow canyons, often with smooth walls, steep-sided valleys in the seabed of the continental slope underwater are referred to as submarine canyons. Unlike canyons on land, submarine canyons are thought to be formed by turbidity currents, the word canyon is Spanish in origin, with the same meaning. The word canyon is used in North America while the words gorge and ravine are used in Europe and Oceania, though gorge. In the United States, place names generally use canyon in the southwest and gorge in the northeast, in Canada, a gorge is usually narrow while a ravine is more open and often wooded. The military-derived word defile is occasionally used in the United Kingdom, most canyons were formed by a process of long-time erosion from a plateau or table-land level. The cliffs form because harder rock strata that are resistant to erosion, Canyons are much more common in arid than in wet areas because physical weathering has a more localized effect in arid zones. The wind and water from the combine to erode and cut away less resistant materials such as shales. The freezing and expansion of water also serves to help form canyons, water seeps into cracks between the rocks and freezes, pushing the rocks apart and eventually causing large chunks to break off the canyon walls, in a process known as frost wedging. Canyon walls are formed of resistant sandstones or granite. Sometimes large rivers run through canyons as the result of geological uplift. These are called entrenched rivers, because they are unable to alter their course. In the United States, the Colorado River in the Southwest, Canyons often form in areas of limestone rock. As limestone is soluble to an extent, cave systems form in the rock. When these collapse, a canyon is left, as in the Mendip Hills in Somerset and Yorkshire Dales in Yorkshire, England
25.
Impact crater
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An impact crater is an approximately circular depression in the surface of a planet, moon, or other solid body in the Solar System or elsewhere, formed by the hypervelocity impact of a smaller body. In contrast to volcanic craters, which result from explosion or internal collapse, impact craters typically have raised rims, Impact craters are the dominant geographic features on many solid Solar System objects including the Moon, Mercury, Callisto, Ganymede and most small moons and asteroids. Where such processes have destroyed most of the original crater topography and this indicates that there should be far more relatively young craters on the planet than have been discovered so far. Note that the rate of impact cratering in the outer Solar System could be different from the inner Solar System, although Earths active surface processes quickly destroy the impact record, about 190 terrestrial impact craters have been identified. They are also found in the stable interior regions of continents. Impact craters are not to be confused with landforms that may appear similar, including calderas, sinkholes, glacial cirques, ring dikes, salt domes, and others. Daniel Barringer was one of the first to identify an impact crater, Meteor Crater in Arizona, in the 1920s, the American geologist Walter H. Bucher studied a number of sites now recognized as impact craters in the United States. He concluded they had created by some great explosive event. However, in 1936, the geologists John D, albritton Jr. revisited Buchers studies and concluded that the craters that he studied were probably formed by impacts. The concept of impact cratering remained more or less speculative until the 1960s, armed with the knowledge of shock-metamorphic features, Carlyle S. By 1970, they had identified more than 50. Although their work was controversial, the American Apollo Moon landings, because the processes of erosion on the Moon are minimal, craters persist almost indefinitely. Since the Earth could be expected to have roughly the same cratering rate as the Moon, Impact cratering involves high velocity collisions between solid objects, typically much greater than the velocity of sound in those objects. Such hyper-velocity impacts produce physical effects such as melting and vaporization that do not occur in familiar sub-sonic collisions. On Earth, ignoring the effects of travel through the atmosphere. The fastest impacts occur at about 72 km/s in the worst case scenario in which an object in a retrograde near-parabolic orbit hits Earth, the median impact velocity on Earth is about 20 km/s. The larger the meteoroid the more of its initial cosmic velocity it preserves, while an object of 9,000 kg maintains about 6% of its original velocity, one of 900,000 kg already preserves about 70%. Extremely large bodies are not slowed by the atmosphere at all, impacts at these high speeds produce shock waves in solid materials, and both impactor and the material impacted are rapidly compressed to high density
26.
Mountain
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A mountain is a large landform that stretches above the surrounding land in a limited area, usually in the form of a peak. A mountain is steeper than a hill. Mountains are formed through tectonic forces or volcanism and these forces can locally raise the surface of the earth. Mountains erode slowly through the action of rivers, weather conditions, a few mountains are isolated summits, but most occur in huge mountain ranges. High elevations on mountains produce colder climates than at sea level and these colder climates strongly affect the ecosystems of mountains, different elevations have different plants and animals. Because of the less hospitable terrain and climate, mountains tend to be used less for agriculture and more for resource extraction and recreation, the highest mountain on Earth is Mount Everest in the Himalayas of Asia, whose summit is 8,850 m above mean sea level. The highest known mountain on any planet in the Solar System is Olympus Mons on Mars at 21,171 m, there is no universally accepted definition of a mountain. Elevation, volume, relief, steepness, spacing and continuity have been used as criteria for defining a mountain, whether a landform is called a mountain may depend on local usage. The highest point in San Francisco, California, is called Mount Davidson, notwithstanding its height of 300 m, similarly, Mount Scott outside Lawton, Oklahoma is only 251 m from its base to its highest point. Whittows Dictionary of Physical Geography states Some authorities regard eminences above 600 metres as mountains, in addition, some definitions also include a topographical prominence requirement, typically 100 or 500 feet. For a while, the US defined a mountain as being 1,000 feet or taller, any similar landform lower than this height was considered a hill. However, today, the United States Geological Survey concludes that these terms do not have technical definitions in the US, using these definitions, mountains cover 33% of Eurasia, 19% of South America, 24% of North America, and 14% of Africa. As a whole, 24% of the Earths land mass is mountainous, there are three main types of mountains, volcanic, fold, and block. All three types are formed from plate tectonics, when portions of the Earths crust move, crumple, compressional forces, isostatic uplift and intrusion of igneous matter forces surface rock upward, creating a landform higher than the surrounding features. The height of the feature makes it either a hill or, if higher and steeper, major mountains tend to occur in long linear arcs, indicating tectonic plate boundaries and activity. Volcanoes are formed when a plate is pushed below another plate, at a depth of around 100 km, melting occurs in rock above the slab, and forms magma that reaches the surface. When the magma reaches the surface, it builds a volcanic mountain. Examples of volcanoes include Mount Fuji in Japan and Mount Pinatubo in the Philippines, the magma does not have to reach the surface in order to create a mountain, magma that solidifies below ground can still form dome mountains, such as Navajo Mountain in the US
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Volcano
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A volcano is a rupture in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. Earths volcanoes occur because its crust is broken into 17 major, therefore, on Earth, volcanoes are generally found where tectonic plates are diverging or converging. This type of volcanism falls under the umbrella of plate hypothesis volcanism, Volcanism away from plate boundaries has also been explained as mantle plumes. These so-called hotspots, for example Hawaii, are postulated to arise from upwelling diapirs with magma from the boundary,3,000 km deep in the Earth. Volcanoes are usually not created where two plates slide past one another. Erupting volcanoes can pose hazards, not only in the immediate vicinity of the eruption. Historically, so-called volcanic winters have caused catastrophic famines, the word volcano is derived from the name of Vulcano, a volcanic island in the Aeolian Islands of Italy whose name in turn comes from Vulcan, the god of fire in Roman mythology. The study of volcanoes is called volcanology, sometimes spelled vulcanology, at the mid-oceanic ridges, two tectonic plates diverge from one another as new oceanic crust is formed by the cooling and solidifying of hot molten rock. Most divergent plate boundaries are at the bottom of the oceans, therefore, most volcanic activity is submarine, black smokers are evidence of this kind of volcanic activity. Where the mid-oceanic ridge is above sea-level, volcanic islands are formed, for example, subduction zones are places where two plates, usually an oceanic plate and a continental plate, collide. In this case, the plate subducts, or submerges under the continental plate forming a deep ocean trench just offshore. In a process called flux melting, water released from the subducting plate lowers the temperature of the overlying mantle wedge. This magma tends to be very viscous due to its high content, so it often does not reach the surface. When it does reach the surface, a volcano is formed, typical examples of this kind of volcano are Mount Etna and the volcanoes in the Pacific Ring of Fire. Because tectonic plates move across them, each volcano becomes dormant and is eventually re-formed as the plate advances over the postulated plume and this theory is currently under criticism, however. The most common perception of a volcano is of a mountain, spewing lava and poisonous gases from a crater at its summit, however. The features of volcanoes are more complicated and their structure. Some volcanoes have rugged peaks formed by lava domes rather than a summit crater while others have features such as massive plateaus
28.
Formation and evolution of the Solar System
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The formation of the Solar System began 4.6 billion years ago with the gravitational collapse of a small part of a giant molecular cloud. This model, known as the hypothesis, was first developed in the 18th century by Emanuel Swedenborg, Immanuel Kant. Its subsequent development has interwoven a variety of disciplines including astronomy, physics, geology. Since the dawn of the age in the 1950s and the discovery of extrasolar planets in the 1990s. The Solar System has evolved considerably since its initial formation, many moons have formed from circling discs of gas and dust around their parent planets, while other moons are thought to have formed independently and later been captured by their planets. Still others, such as Earths Moon, may be the result of giant collisions, collisions between bodies have occurred continually up to the present day and have been central to the evolution of the Solar System. The positions of the planets often shifted due to gravitational interactions and this planetary migration is now thought to have been responsible for much of the Solar Systems early evolution. In the far distant future, the gravity of passing stars will gradually reduce the Suns retinue of planets, some planets will be destroyed, others ejected into interstellar space. Ultimately, over the course of tens of billions of years, the first step toward a theory of Solar System formation and evolution was the general acceptance of heliocentrism, which placed the Sun at the centre of the system and the Earth in orbit around it. This concept had developed for millennia, but was not widely accepted until the end of the 17th century, the first recorded use of the term Solar System dates from 1704. The most significant criticism of the hypothesis was its apparent inability to explain the Suns relative lack of momentum when compared to the planets. However, since the early 1980s studies of stars have shown them to be surrounded by cool discs of dust and gas, exactly as the nebular hypothesis predicts. Understanding of how the Sun is expected to continue to evolve required an understanding of the source of its power, in 1935, Eddington went further and suggested that other elements also might form within stars. Fred Hoyle elaborated on this premise by arguing that evolved stars called red giants created many elements heavier than hydrogen, when a red giant finally casts off its outer layers, these elements would then be recycled to form other star systems. The nebular hypothesis says that the Solar System formed from the collapse of a fragment of a giant molecular cloud. The cloud was about 20 parsec across, while the fragments were roughly 1 parsec across, the further collapse of the fragments led to the formation of dense cores 0. 01–0.1 pc in size. One of these fragments formed what became the Solar System. The remaining 2% of the mass consisted of elements that were created by nucleosynthesis in earlier generations of stars
29.
Hydrosphere
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The hydrosphere is the combined mass of water found on, under, and above the surface of a planet, minor planet or natural satellite. It has been estimated there are 1386 million cubic kilometers of water on Earth. This includes water in liquid and frozen forms in groundwater, oceans, lakes, saltwater accounts for 97. 5% of this amount. Fresh water accounts for only 2. 5%, of this fresh water,68. 9% is in the form of ice and permanent snow cover in the Arctic, the Antarctic, and mountain glaciers. 30. 8% is in the form of fresh groundwater, only 0. 3% of the fresh water on Earth is in easily accessible lakes, reservoirs and river systems. The total mass of the Earths hydrosphere is about 1.4 ×1018 tonnes, about 20 ×1012 tonnes of this is in Earths atmosphere. Approximately 75% of Earths surface, an area of some 361 million square kilometers, is covered by ocean, the average salinity of Earths oceans is about 35 grams of salt per kilogram of sea water. The hydrological cycle transfers water from one state or reservoir to another, reservoirs include atmospheric moisture, streams, oceans, rivers, lakes, groundwater, subterranean aquifers, polar icecaps and saturated soil. Solar energy, in the form of heat and light, most evaporation comes from the oceans and is returned to the earth as snow or rain. Sublimation refers to evaporation from snow and ice, transpiration refers to the expiration of water through the minute pores or stomata of trees. Evapotranspiration is the used by hydrologists in reference to the three processes together, transpiration, sublimation and evaporation. In his book Water, Marq de Villiers described the hydrosphere as a system in which water exists. The hydrosphere is intricate, complex, interdependent, all-pervading and stable, de Villiers claimed that, On earth, the total amount of water has almost certainly not changed since geological times, what we had then we still have. Water can be polluted, abused, and misused but it is neither created nor destroyed, there is no evidence that water vapor escapes into space. Every year the turnover of water on Earth involves 577,000 km3 of water and this is water that evaporates from the oceanic surface and from land. The same amount of falls as atmospheric precipitation,458,000 km3 on the ocean and 119,000 km3 on land. The difference between precipitation and evaporation from the surface represents the total runoff of the Earths rivers. These are the sources of fresh water to support life necessities
30.
Planetesimal
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Planetesimals /plænᵻˈtɛsᵻməlz/ are solid objects thought to exist in protoplanetary disks and in debris disks. When the bodies reach sizes of one kilometer, then they can attract each other directly through their mutual gravity. This is how planetesimals are often defined, bodies that are smaller than planetesimals must rely on Brownian motion or turbulent motions in the gas to cause the collisions that can lead to sticking. It is generally thought that about 3, a few planetesimals may have been captured as moons, such as Phobos and Deimos and many of the small high-inclination moons of the giant planets. Planetesimals that have survived to the current day are valuable to science because they contain information about the formation of the Solar System, although their exteriors are subjected to intense solar radiation that can alter their chemistry, their interiors contain pristine material essentially untouched since the planetesimal was formed. This makes each planetesimal a time capsule, and their composition might reveal the conditions in the Solar Nebula from which our planetary system was formed, the word planetesimal comes from the mathematical concept infinitesimal and literally means an ultimately small fraction of a planet. This corresponds to objects larger than approximately 1 km in the solar nebula, bodies large enough not only to keep together by gravitation but to change the path of approaching rocks over distances of several radii start to grow faster. These bodies, larger than 100 km to 1000 km, are called embryos or protoplanets, in other words, some planetesimals became other types of body once planetary formation had finished, and may be referred to by either or both names. The above definition is not endorsed by the International Astronomical Union, there is also no exact dividing line between a planetesimal and protoplanet. Discovering the Essential Universe by Neil F. Comins Linda T. Elkins-Tanton, planetesimals - Early Differentiation and Consequences for Planets. Cambridge University Press, Cambridge 2017, ISBN9781107118485
31.
Dwarf planet
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A dwarf planet is a planetary-mass object that is neither a planet nor a natural satellite. The International Astronomical Union currently recognizes five dwarf planets, Ceres, Pluto, Haumea, Makemake, another hundred or so known objects in the Solar System are suspected to be dwarf planets. Individual astronomers recognize several of these, and in August 2011 Mike Brown published a list of 390 candidate objects, Stern states that there are more than a dozen known dwarf planets. Only two of these bodies, Ceres and Pluto, have observed in enough detail to demonstrate that they actually fit the IAUs definition. The IAU accepted Eris as a dwarf planet because it is more massive than Pluto and they subsequently decided that unnamed trans-Neptunian objects with an absolute magnitude brighter than +1 are to be named under the assumption that they are dwarf planets. The classification of bodies in other systems with the characteristics of dwarf planets has not been addressed. Starting in 1801, astronomers discovered Ceres and other bodies between Mars and Jupiter which were for some decades considered to be planets. Between then and around 1851, when the number of planets had reached 23, astronomers started using the asteroid for the smaller bodies. With the discovery of Pluto in 1930, most astronomers considered the Solar System to have nine planets and it was roughly one-twentieth the mass of Mercury, which made Pluto by far the smallest planet. Although it was more than ten times as massive as the largest object in the asteroid belt, Ceres. In the 1990s, astronomers began to find objects in the region of space as Pluto. Many of these shared several of Plutos key orbital characteristics, and Pluto started being seen as the largest member of a new class of objects and this led some astronomers to stop referring to Pluto as a planet. Several terms, including subplanet and planetoid, started to be used for the now known as dwarf planets. By 2005, three trans-Neptunian objects comparable in size to Pluto had been reported and it became clear that either they would also have to be classified as planets, or Pluto would have to be reclassified. Astronomers were also confident that more objects as large as Pluto would be discovered, Eris was discovered in January 2005, it was thought to be slightly larger than Pluto, and some reports informally referred to it as the tenth planet. As a consequence, the became a matter of intense debate during the IAU General Assembly in August 2006. The IAUs initial draft proposal included Charon, Eris, and Ceres in the list of planets, dropping Charon from the list, the new proposal also removed Pluto, Ceres, and Eris, because they have not cleared their orbits. The IAUs final Resolution 5A preserved this three-category system for the bodies orbiting the Sun
32.
Ceres (dwarf planet)
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Ceres is the largest object in the asteroid belt that lies between the orbits of Mars and Jupiter. Its diameter is approximately 945 kilometers, making it the largest of the planets within the orbit of Neptune. The 33rd-largest known body in the Solar System, it is the dwarf planet within the orbit of Neptune. Composed of rock and ice, Ceres is estimated to approximately one third of the mass of the entire asteroid belt. Ceres is the object in the asteroid belt known to be rounded by its own gravity. From Earth, the apparent magnitude of Ceres ranges from 6.7 to 9.3, Ceres was the first asteroid discovered, by Giuseppe Piazzi at Palermo on 1 January 1801. It was originally considered a planet, but was reclassified as an asteroid in the 1850s after many other objects in similar orbits were discovered. Ceres appears to be differentiated into a core and icy mantle. The surface is probably a mixture of ice and various hydrated minerals such as carbonates. In January 2014, emissions of water vapor were detected from several regions of Ceres and this was unexpected, because large bodies in the asteroid belt typically do not emit vapor, a hallmark of comets. The robotic NASA spacecraft Dawn entered orbit around Ceres on 6 March 2015, pictures with a resolution previously unattained were taken during imaging sessions starting in January 2015 as Dawn approached Ceres, showing a cratered surface. Two distinct bright spots inside a crater were seen in a 19 February 2015 image, on 11 May 2015, NASA released a higher-resolution image showing that, instead of one or two spots, there are actually several. In October 2015, NASA released a true portrait of Ceres made by Dawn. In February 2017, organics were reported to have been detected on Ceres in Ernutet crater, Johann Elert Bode, in 1772, first suggested that an undiscovered planet could exist between the orbits of Mars and Jupiter. Kepler had already noticed the gap between Mars and Jupiter in 1596, the pattern predicted that the missing planet ought to have an orbit with a semi-major axis near 2.8 astronomical units. Although they did not discover Ceres, they found several large asteroids. One of the selected for the search was Giuseppe Piazzi. Before receiving his invitation to join the group, Piazzi discovered Ceres on 1 January 1801 and he was searching for the 87th of the Catalogue of the Zodiacal stars of Mr la Caille, but found that it was preceded by another
33.
Pluto
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Pluto is a dwarf planet in the Kuiper belt, a ring of bodies beyond Neptune. It was the first Kuiper belt object to be discovered, Pluto was discovered by Clyde Tombaugh in 1930 and was originally considered to be the ninth planet from the Sun. After 1992, its planethood was questioned following the discovery of objects of similar size in the Kuiper belt. In 2005, Eris, which is 27% more massive than Pluto, was discovered and this led the International Astronomical Union to define the term planet formally in 2006, during their 26th General Assembly. That definition excluded Pluto and reclassified it as a dwarf planet, Pluto is the largest and second-most-massive known dwarf planet in the Solar System and the ninth-largest and tenth-most-massive known object directly orbiting the Sun. It is the largest known trans-Neptunian object by volume but is less massive than Eris, like other Kuiper belt objects, Pluto is primarily made of ice and rock and is relatively small—about one-sixth the mass of the Moon and one-third its volume. It has an eccentric and inclined orbit during which it ranges from 30 to 49 astronomical units or AU from the Sun. This means that Pluto periodically comes closer to the Sun than Neptune, light from the Sun takes about 5.5 hours to reach Pluto at its average distance. Pluto has five moons, Charon, Styx, Nix, Kerberos. Pluto and Charon are sometimes considered a system because the barycenter of their orbits does not lie within either body. The IAU has not formalized a definition for binary dwarf planets, on July 14,2015, the New Horizons spacecraft became the first spacecraft to fly by Pluto. During its brief flyby, New Horizons made detailed measurements and observations of Pluto, on October 25,2016, at 05,48 pm ET, the last bit of data was received from New Horizons from its close encounter with Pluto on July 14,2015. In the 1840s, Urbain Le Verrier used Newtonian mechanics to predict the position of the then-undiscovered planet Neptune after analysing perturbations in the orbit of Uranus. Subsequent observations of Neptune in the late 19th century led astronomers to speculate that Uranuss orbit was being disturbed by another planet besides Neptune, by 1909, Lowell and William H. Pickering had suggested several possible celestial coordinates for such a planet. Lowell and his observatory conducted his search until his death in 1916, unknown to Lowell, his surveys had captured two faint images of Pluto on March 19 and April 7,1915, but they were not recognized for what they were. There are fourteen other known prediscovery observations, with the oldest made by the Yerkes Observatory on August 20,1909. Percivals widow, Constance Lowell, entered into a legal battle with the Lowell Observatory over her late husbands legacy. Tombaughs task was to image the night sky in pairs of photographs, then examine each pair
34.
Eris (dwarf planet)
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Eris is the most massive and second-largest dwarf planet known in the Solar System. It is also the known body directly orbiting the Sun. It is measured to be 2,326 ±12 kilometers in diameter, Eris is 27% more massive than dwarf planet Pluto, though Pluto is slightly larger by volume. Eris mass is about 0. 27% of the Earths mass, Eris was discovered in January 2005 by a Palomar Observatory-based team led by Mike Brown, and its identity was verified later that year. It is an object and a member of a high-eccentricity population known as the scattered disk. It has one moon, Dysnomia. As of February 2016, its distance from the Sun is 96.3 astronomical units, because Eris appeared to be larger than Pluto, NASA initially described it as the Solar Systems tenth planet. This, along with the prospect of other objects of similar size being discovered in the future, motivated the International Astronomical Union to define the term planet for the first time. Observations of an occultation by Eris in 2010 showed that its diameter was 2,326 ±12 kilometers, very slightly less than Pluto. Eris was discovered by the team of Mike Brown, Chad Trujillo, the discovery was announced on July 29,2005, the same day as Makemake and two days after Haumea, due in part to events that would later lead to controversy about Haumea. Routine observations were taken by the team on October 21,2003, in January 2005, the re-analysis revealed Eriss slow motion against the background stars. Follow-up observations were carried out to make a preliminary determination of Eriss orbit. More observations released in October 2005 revealed that Eris has a moon, observations of Dysnomias orbit permitted scientists to determine the mass of Eris, which in June 2007 they calculated to be ×1022 kg, 27%±2% greater than Plutos. Eris is named after the Greek goddess Eris, a personification of strife, the regular adjectival form of Eris is Eridian. As a result, for a time the object known to the wider public as Xena. Xena was a name used internally by the discovery team. It was inspired by the character of the television series Xena. The discovery team had saved the nickname Xena for the first body they discovered that was larger than Pluto
35.
Small Solar System body
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A Small Solar System Body is an object in the Solar System that is neither a planet, nor a dwarf planet, nor a natural satellite. The term was first defined in 2006 by the International Astronomical Union, all other objects, except satellites, orbiting the Sun shall be referred to collectively as Small Solar System Bodies. These currently include most of the Solar System asteroids, most Trans-Neptunian Objects, comets and this encompasses all comets and all minor planets other than those that are dwarf planets. Except for the largest, which are in equilibrium, natural satellites differ from small Solar System bodies not in size. The orbits of satellites are not centered on the Sun, but around other Solar System objects such as planets, dwarf planets. Some of the larger small Solar System bodies may be reclassified in future as dwarf planets, the orbits of the vast majority of small Solar System bodies are located in two distinct areas, namely the asteroid belt and the Kuiper belt. These two belts possess some internal structure related to perturbations by the planets, and have fairly loosely defined boundaries. Other areas of the Solar System also encompass small bodies in smaller concentrations and these include the near-Earth asteroids, centaurs, comets, and scattered disc objects
36.
Haumea
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Haumea, minor-planet designation 136108 Haumea, is a dwarf planet located beyond Neptunes orbit. On September 17,2008, it was recognized as a planet by the International Astronomical Union and named after Haumea. Haumeas mass is about one-third that of Pluto, and 1/1400 that of Earth, although its shape has not been directly observed, calculations from its light curve indicate that it is a triaxial ellipsoid, with its major axis twice as long as its minor. Its gravity is thought to be sufficient for it to have relaxed into hydrostatic equilibrium, two teams claim credit for the discovery of Haumea. Mike Brown and his team at Caltech discovered Haumea in December 2004 on images they had taken on May 6,2004, on July 20,2005, they published an online abstract of a report intended to announce the discovery at a conference in September 2005. At around this time, José Luis Ortiz Moreno and his team at the Instituto de Astrofísica de Andalucía at Sierra Nevada Observatory in Spain found Haumea on images taken on March 7–10,2003. Ortiz emailed the Minor Planet Center with their discovery on the night of July 27,2005, Ortiz later admitted he had accessed the Caltech observation logs but denied any wrongdoing, stating he was merely verifying whether they had discovered a new object. However, the IAU announcement on September 17,2008, that Haumea had been accepted as a dwarf planet, did not mention a discoverer. Until it was given a permanent name, the Caltech discovery team used the nickname Santa among themselves, because they had discovered Haumea on December 28,2004, the Spanish team were the first to file a claim for discovery to the Minor Planet Center, in July 2005. On July 29,2005, Haumea was given the provisional designation 2003 EL61, on September 7,2006, it was numbered and admitted into the official minor planet catalogue as 2003 EL61. The names were proposed by David Rabinowitz of the Caltech team, Haumea is the matron goddess of the island of Hawaiʻi, where the Mauna Kea Observatory is located. The two known moons, also believed to have formed in this manner, are named after two of Haumeas daughters, Hiʻiaka and Nāmaka. The proposal by the Ortiz team, Ataecina, did not meet IAU naming requirements, Haumea is a plutoid, a dwarf planet beyond Neptunes orbit. Haumea appears to have an ellipsoid shape resulting its rapid rotation complicated by tidal interactions with its moons. This contrasts with the simpler oblate shape typically assumed by less rapidly rotating astronomical bodies such as the Earth, in other words, Haumea is spinning so fast that if it spun much faster these bulges would distort into a dumbbell shape and split the planet in two. Haumea was initially listed as a classical Kuiper belt object in 2006 by the Minor Planet Center, the nominal trajectory suggests that it is in the weak 7,12 resonance with Neptune, because its perihelion distance of 35 AU is near the limit of stability with Neptune. There are precovery images of Haumea dating back to March 22,1955 from the Palomar Mountain Digitized Sky Survey, further observations of the orbit will be required to verify its dynamic status. Haumea has a period of 284 Earth years, a perihelion of 35 AU
37.
2 Pallas
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Pallas, minor-planet designation 2 Pallas, is the second asteroid to have been discovered, and is one of the largest asteroids in the Solar System. With an estimated 7% of the mass of the belt, it is the third-most-massive asteroid. It is 512 kilometers in diameter, somewhat smaller than Vesta and it is likely a remnant protoplanet. When Pallas was discovered by the German astronomer Heinrich Wilhelm Matthäus Olbers on 28 March 1802, it was counted as a planet, the discovery of many more asteroids after 1845 eventually led to their reclassification. Pallass surface is most likely composed of a material, its spectrum. It was formerly considered a dwarf planet for its size. In 1801, the astronomer Giuseppe Piazzi discovered an object which he believed to be a comet. Shortly thereafter he announced his observations of this object, noting that the slow, uniform motion was uncharacteristic of a comet, suggesting it was a different type of object. This was lost from sight for months, but was recovered later that year by the Baron von Zach and Heinrich W. M. Olbers after a preliminary orbit was computed by Carl Friedrich Gauss. This object came to be named Ceres, and was the first asteroid to be discovered, a few months later, Olbers was again attempting to locate Ceres when he noticed another moving object in the vicinity. This was the asteroid Pallas, coincidentally passing near Ceres at the time, the discovery of this object created interest in the astronomy community. Before this point it had been speculated by astronomers that there should be a planet in the gap between Mars and Jupiter, now, unexpectedly, a second such body had been found. When Pallas was discovered, some estimates of its size were as high as 3,380 km in diameter, even as recently as 1979, Pallas was estimated to be 673 km in diameter, 26% greater than the currently accepted value. The orbit of Pallas was determined by Gauss, who found the period of 4.6 years was similar to the period for Ceres, Pallas has a relatively high orbital inclination to the plane of the ecliptic. In 1917, the Japanese astronomer Kiyotsugu Hirayama began to study asteroid motions, by plotting the mean orbital motion, inclination and eccentricity of a set of asteroids, he discovered several distinct groupings. In a later paper he reported a group of three associated with Pallas, which became named the Pallas family, after the largest member of the group. Since 1994 more than 10 members of family have been identified. The validity of this grouping was confirmed in 2002 by a comparison of their spectra and these resulted in the first accurate measurements of its diameter
38.
Pressure
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Pressure is the force applied perpendicular to the surface of an object per unit area over which that force is distributed. Gauge pressure is the relative to the ambient pressure. Various units are used to express pressure, Pressure may also be expressed in terms of standard atmospheric pressure, the atmosphere is equal to this pressure and the torr is defined as 1⁄760 of this. Manometric units such as the centimetre of water, millimetre of mercury, Pressure is the amount of force acting per unit area. The symbol for it is p or P, the IUPAC recommendation for pressure is a lower-case p. However, upper-case P is widely used. The usage of P vs p depends upon the field in one is working, on the nearby presence of other symbols for quantities such as power and momentum. Mathematically, p = F A where, p is the pressure, F is the normal force and it relates the vector surface element with the normal force acting on it. It is incorrect to say the pressure is directed in such or such direction, the pressure, as a scalar, has no direction. The force given by the relationship to the quantity has a direction. If we change the orientation of the element, the direction of the normal force changes accordingly. Pressure is distributed to solid boundaries or across arbitrary sections of normal to these boundaries or sections at every point. It is a parameter in thermodynamics, and it is conjugate to volume. The SI unit for pressure is the pascal, equal to one newton per square metre and this name for the unit was added in 1971, before that, pressure in SI was expressed simply in newtons per square metre. Other units of pressure, such as pounds per square inch, the CGS unit of pressure is the barye, equal to 1 dyn·cm−2 or 0.1 Pa. Pressure is sometimes expressed in grams-force or kilograms-force per square centimetre, but using the names kilogram, gram, kilogram-force, or gram-force as units of force is expressly forbidden in SI. The technical atmosphere is 1 kgf/cm2, since a system under pressure has potential to perform work on its surroundings, pressure is a measure of potential energy stored per unit volume. It is therefore related to density and may be expressed in units such as joules per cubic metre. Similar pressures are given in kilopascals in most other fields, where the prefix is rarely used
39.
4 Vesta
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Vesta, minor-planet designation 4 Vesta, is one of the largest objects in the asteroid belt, with a mean diameter of 525 kilometres. It was discovered by the German astronomer Heinrich Wilhelm Olbers on 29 March 1807 and is named after Vesta, Vesta is the second-most-massive and second-largest body in the asteroid belt after the dwarf planet Ceres, and it contributes an estimated 9% of the mass of the asteroid belt. It is slightly larger than Pallas, though more massive. Vesta is the last remaining rocky protoplanet of the kind that formed the terrestrial planets, numerous fragments of Vesta were ejected by collisions one and two billion years ago that left two enormous craters occupying much of Vestas southern hemisphere. Debris from these events has fallen to Earth as howardite–eucrite–diogenite meteorites, Vesta is the brightest asteroid visible from Earth. Its maximum distance from the Sun is slightly greater than the distance of Ceres from the Sun. NASAs Dawn spacecraft entered orbit around Vesta on 16 July 2011 for an exploration and left orbit on 5 September 2012 en route to its final destination. Researchers continue to examine data collected by Dawn for additional insights into the formation, Heinrich Olbers discovered Pallas in 1802, the year after the discovery of Ceres. He proposed that the two objects were the remnants of a destroyed planet and these orbital intersections were located in the constellations of Cetus and Virgo. Olbers commenced his search in 1802, and on 29 March 1807 he discovered Vesta in the constellation Virgo—a coincidence, because Ceres, Pallas, and Vesta are not fragments of a larger body. Because the asteroid Juno had been discovered in 1804, this made Vesta the fourth object to be identified in the region that is now known as the asteroid belt, the discovery was announced in a letter addressed to German astronomer Johann H. Schröter dated 31 March. Gauss decided on the Roman virgin goddess of home and hearth, Vesta was the fourth asteroid to be discovered, hence the number 4 in its formal designation. The name Vesta, or national variants thereof, is in use with two exceptions, Greece and China. In Greek, the name adopted was the Hellenic equivalent of Vesta, Hestia, in English, in Chinese, Vesta is called the hearth-god star, 灶神星 zàoshénxīng, in contrast to the goddess Vesta, who goes by her Latin name. Upon its discovery, Vesta was, like Ceres, Pallas, the symbol representing the altar of Vesta with its sacred fire and was designed by Gauss. In Gausss conception, this was drawn, in its modern form, after the discovery of Vesta, no further objects were discovered for 38 years, and the Solar System was thought to have eleven planets. However, in 1845, new asteroids started being discovered at a rapid pace and it soon became clear that it would be impractical to continue inventing new planetary symbols indefinitely, and some of the existing ones proved difficult to draw quickly. That year, the problem was addressed by Benjamin Apthorp Gould, who suggested numbering asteroids in their order of discovery, thus, the fourth asteroid, Vesta, acquired the generic symbol ④
40.
Super-Earth
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The term super-Earth refers only to the mass of the planet, and so does not imply anything about the surface conditions or habitability. The alternative term gas dwarfs may be accurate for those at the higher end of the mass scale, as suggested by MIT professor Sara Seager. In general, super-Earths are defined exclusively by their masses, while sources generally agree on an upper bound of 10 Earth masses, the lower bound varies from 1 or 1.9 to 5, with various other definitions appearing in the popular media. The term super-Earth is also used by astronomers to refer to planets bigger than Earth-like planets and this definition was made by the Kepler Mission. Planets above 10 Earth masses are termed massive solid planets/mega-Earths or gas giant planets depending on whether they are mostly rock, the first super-Earths were discovered by Aleksander Wolszczan and Dale Frail around the pulsar PSR B1257+12 in 1992. The two outer planets of the system have masses approximately four times Earth—too small to be gas giants, the first super-Earth around a main-sequence star was discovered by a team under Eugenio Rivera in 2005. It orbits Gliese 876 and received the designation Gliese 876 d and it has an estimated mass of 7.5 Earth masses and a very short orbital period of just about 2 days. Due to the proximity of Gliese 876 d to its host star, it may have a temperature of 430–650 kelvin. With Gliese 581 c having a mass of at least 5 Earth masses, subsequent research suggested Gliese 581 c had likely suffered a runaway greenhouse effect like Venus. Two further super-Earths were discovered in 2006, OGLE-2005-BLG-390Lb with a mass of 5.5 Earth masses, which was found by gravitational microlensing, the smallest super-Earth found as of 2008 was MOA-2007-BLG-192Lb. The planet was announced by astrophysicist David P. Bennett for the international MOA collaboration on June 2,2008 and this planet has approximately 3.3 Earth masses and orbits a brown dwarf. It was detected by gravitational microlensing, in June 2008, European researchers announced the discovery of three super-Earths around the star HD40307, a star that is only slightly less massive than our Sun. The planets have at least the minimum masses,4.2,6.7. The planets were detected by the radial velocity method by the HARPS in Chile, in addition, the same European research team announced a planet 7.5 times the mass of Earth orbiting the star HD181433. This star also has a Jupiter-like planet that orbits every three years, planet COROT-7b, with a mass estimated at 4.8 Earth masses and an orbital period of only 0.853 days, was announced on 3 February 2009. The density estimate obtained for COROT-7b points to a composition including rocky silicate minerals, similar to the four planets of the Solar System. COROT-7b, discovered right after HD7924 b, is the first super-Earth discovered that orbits a main star that is G class or larger. The discovery of Gliese 581 e with a mass of 1.9 Earth masses was announced on 21 April 2009
41.
Gas giant
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A gas giant is a giant planet composed mainly of hydrogen and helium. Jupiter and Saturn are the gas giants of the Solar System, for this reason, Uranus and Neptune are now often classified in the separate category of ice giants. Jupiter and Saturn consist mostly of hydrogen and helium, with heavier elements making up between 3 and 13 percent of the mass and they are thought to consist of an outer layer of molecular hydrogen surrounding a layer of liquid metallic hydrogen, with probably a molten rocky core. The outermost portion of their hydrogen atmosphere is characterized by layers of visible clouds that are mostly composed of water. The layer of metallic hydrogen makes up the bulk of each planet, the gas giants cores are thought to consist of heavier elements at such high temperatures and pressures that their properties are poorly understood. The defining differences between a very low-mass brown dwarf and a gas giant are debated, one school of thought is based on formation, the other, on the physics of the interior. Part of the debate concerns whether brown dwarfs must, by definition, have experienced nuclear fusion at some point in their history, the term gas giant was coined in 1952 by the science fiction writer James Blish and was originally used to refer to all giant planets. Arguably it is something of a misnomer, because throughout most of the volume of all giant planets the pressure is so high that matter is not in gaseous form. Other than solids in the core and the layers of the atmosphere, all matter is above the critical point. In the outer Solar System, hydrogen and helium are referred to as gases, water, methane, and ammonia as ices, and silicates and metals as rock. Because Uranus and Neptune are primarily composed of, in this terminology, ices, not gas, they are referred to as ice giants. Jupiter and Saturn are both class I, hot Jupiters are class IV or V. A cold hydrogen-rich gas giant more massive than Jupiter but less than about 500 M⊕ will only be larger in volume than Jupiter. For masses above 500 M⊕, gravity will cause the planet to shrink, kelvin–Helmholtz heating can cause a gas giant to radiate more energy than it receives from its host star. Although the words gas and giant are often combined, hydrogen planets need not be as large as the gas giants from the Solar System. However, smaller gas planets and planets closer to their star will lose atmospheric mass more quickly via hydrodynamic escape than larger planets and planets farther out. The smallest known extrasolar planet that is likely a gas planet is Kepler-138d, a low-mass gas planet can still have a radius resembling that of a gas giant if it has the right temperature. List of gravitationally rounded objects of the Solar System List of planet types
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Pulsar
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A pulsar is a highly magnetized, rotating neutron star or white dwarf, that emits a beam of electromagnetic radiation. This radiation can be observed only when the beam of emission is pointing toward Earth, neutron stars are very dense, and have short, regular rotational periods. This produces a very precise interval between pulses that range from milliseconds to seconds for an individual pulsar, Pulsars are believed to be one of the candidates of high and ultra-high energy astroparticles. The precise periods of pulsars make them very useful tools, observations of a pulsar in a binary neutron star system were used to indirectly confirm the existence of gravitational radiation. The first extrasolar planets were discovered around a pulsar, PSR B1257+12, certain types of pulsars rival atomic clocks in their accuracy in keeping time. The first pulsar was observed on November 28,1967, by Jocelyn Bell Burnell and they observed pulses separated by 1.33 seconds that originated from the same location on the sky, and kept to sidereal time. When observations with another telescope confirmed the emission, it eliminated any sort of instrumental effects and it is an interesting problem—if one thinks one may have detected life elsewhere in the universe, how does one announce the results responsibly. Even so, they nicknamed the signal LGM-1, for little green men and it was not until a second pulsating source was discovered in a different part of the sky that the LGM hypothesis was entirely abandoned. Their pulsar was later dubbed CP1919, and is now known by a number of designators including PSR 1919+21, PSR B1919+21, although CP1919 emits in radio wavelengths, pulsars have, subsequently, been found to emit in visible light, X-ray, and/or gamma ray wavelengths. The word pulsar is a portmanteau of pulsating star, and first appeared in print in 1968, An entirely novel kind of came to light on Aug.6 last year and was referred to, by astronomers. Now it is thought to be a novel type between a white dwarf and a neutron, the name Pulsar is likely to be given to it. Dr. A. Hewish told me yesterday, … I am sure that today every radio telescope is looking at the Pulsars. The existence of stars was first proposed by Walter Baade and Fritz Zwicky in 1934. The discovery of the Crab pulsar later in 1968 seemed to provide confirmation of the neutron star model of pulsars. The Crab pulsar has a 33-millisecond pulse period, which was too short to be consistent with other proposed models for pulsar emission. Moreover, the Crab pulsar is so named because it is located at the center of the Crab Nebula, consistent with the 1933 prediction of Baade and Zwicky. Considerable controversy is associated with the fact that Professor Hewish was awarded the prize while Bell, Bell claims no bitterness upon this point, supporting the decision of the Nobel prize committee. In 1974, Joseph Hooton Taylor, Jr. and Russell Hulse discovered for the first time a pulsar in a binary system and this pulsar orbits another neutron star with an orbital period of just eight hours
43.
PSR B1257+12
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PSR B1257+12, previously designated PSR 1257+12, alternatively designated PSR J1300+1240, also named Lich, is a pulsar located 2300 light years from the Sun in the constellation of Virgo. The pulsar has a system with three known extrasolar planets, named Draugr, Poltergeist and Phobetor, respectively. They were both the first extrasolar planets and the first pulsar planets to be discovered, A and B in 1992, a is the lowest mass planet yet discovered by any observational technique, with somewhat less than twice the mass of Earths moon. The convention that arose for designating pulsars was that of using the letters PSR followed by the right ascension. The modern convention prefixes the older numbers with a B meaning the coordinates are for the 1950.0 epoch, all new pulsars have a J indicating 2000.0 coordinates and also have declination including minutes. Pulsars that were discovered before 1993 tend to retain their B names rather than use their J names, on their discovery, the planets were designated PSR 1257+12 A, B, and C, ordered by increasing distance. However, they are listed under the convention on astronomical databases such as SIMBAD and the Extrasolar Planets Encyclopedia, with A becoming b, B becoming c. In July 2014, the International Astronomical Union launched a process for giving proper names to certain exoplanets, the process involved public nomination and voting for the new names. In December 2015, the IAU announced the names were Lich for this pulsar and Draugr, Poltergeist. The winning names were submitted by the Planetarium Südtirol Alto Adige in Karneid. In 2016, the IAU organized a Working Group on Star Names to catalog and this stellar remnant is now so entered in the IAU Catalog of Star Names. PSR B1257+12 was discovered by the Polish astronomer Aleksander Wolszczan on February 9,1990 using the Arecibo radio telescope, in 1992 Wolszczan and Dale Frail published a famous paper on the first confirmed discovery of planets outside our solar system. Using refined methods one more planet was found orbiting this pulsar in 1994, the pulsar is estimated to have a mass of 1.4 M☉, which is typical for most neutron stars and pulsars. The radius is estimated to be around 10 kilometres, also common for pulsars. The pulsar is extremely hot, with a temperature of either less than or equal to around 28856 K. The pulsar formed about one to three years ago as the result of a white dwarf transforming into a rapidly spinning neutron star during the process of two white dwarfs merging with each other. The discovery stimulated a search for planets orbiting pulsars, but it turned out such planets are rare, only one other pulsar planet. In 1992, Wolszczan and Frail discovered that the pulsar had two planets and these were the first discovery of extrasolar planets to be confirmed, as pulsar planets, they surprised many astronomers who expected to find planets only around main-sequence stars
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Methods of detecting exoplanets
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Any planet is an extremely faint light source compared to its parent star. For example, a star like the Sun is about a billion times as bright as the light from any of the planets orbiting it. In addition to the difficulty of detecting such a faint light source. For those reasons, very few of the extrasolar planets reported as of April 2014 have been observed directly, instead, astronomers have generally had to resort to indirect methods to detect extrasolar planets. As of 2016, several different indirect methods have yielded success and this leads to variations in the speed with which the star moves toward or away from Earth, i. e. the variations are in the radial velocity of the star with respect to Earth. The radial velocity can be deduced from the displacement in the parent stars spectral lines due to the Doppler effect, the radial-velocity method measures these variations in order to confirm the presence of the planet using the binary mass function. The speed of the star around the center of mass is much smaller than that of the planet. An especially simple and inexpensive method for measuring radial velocity is externally dispersed interferometry, until 2014, the radial-velocity method was by far the most productive technique used by planet hunters. It is also known as Doppler spectroscopy and it is also not possible to simultaneously observe many target stars at a time with a single telescope. Planets of Jovian mass can be detectable around stars up to a few light years away. This method easily finds massive planets that are close to stars, modern spectrographs can also easily detect Jupiter-mass planets orbiting 10 astronomical units away from the parent star, but detection of those planets requires many years of observation. It is easier to detect planets around stars, for two reasons, First, these stars are more affected by gravitational tug from planets. The second reason is that low-mass main-sequence stars generally rotate relatively slowly, fast rotation makes spectral-line data less clear because half of the star quickly rotates away from observers viewpoint while the other half approaches. Detecting planets around more massive stars is easier if the star has left the main sequence, sometimes Doppler spectrography produces false signals, especially in multi-planet and multi-star systems. Magnetic fields and certain types of activity can also give false signals. When the host star has multiple planets, false signals can also arise from having insufficient data, so that solutions can fit the data. Planets with orbits highly inclined to the line of sight from Earth produce smaller visible wobbles, one of the advantages of the radial velocity method is that eccentricity of the planets orbit can be measured directly. One of the disadvantages of the radial-velocity method is that it can only estimate a planets minimum mass
45.
51 Pegasi b
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51 Pegasi b, unofficially dubbed Bellerophon, later named Dimidium, is an extrasolar planet approximately 50 light-years away in the constellation of Pegasus. It was the first exoplanet to be discovered orbiting a star, the Sun-like 51 Pegasi. It is the prototype for a class of planets called hot Jupiters, in 2017, traces of water were discovered in the planets atmosphere. 51 Pegasi is the Flamsteed designation of the host star, the planet was originally designated 51 Pegasi b by Michel Mayor and Didier Queloz, who discovered the planet on December 1995. The following year it was unofficially dubbed Bellerophon by astronomer Geoffrey Marcy, in July 2014, the International Astronomical Union launched a process for giving proper names to certain exoplanets and their host stars. The process involved public nomination and voting for the new names, in December 2015, the IAU announced the winning name for this planet was Dimidium. The name was submitted by the Astronomische Gesellschaft Luzern, Switzerland, Dimidium is Latin for half, referring to the planets mass of at least half the mass of Jupiter. The exoplanets discovery was announced on October 6,1995, by Michel Mayor and they used the radial velocity method with the ELODIE spectrograph on the Observatoire de Haute-Provence telescope in France and made world headlines with their announcement. The planet was discovered using a sensitive spectroscope that could detect the slight and these changes are caused by the planets gravitational effects from just 7 million kilometres distance from the star. Within a week of the announcement, the planet was confirmed by another team using the Lick Observatory in California and this was the first discovery of an exoplanet orbiting a Sun-like star. It marked a point and forced astronomers to accept that giant planets could exist in short-period orbits. After its discovery, many teams confirmed the existence and obtained more observations of its properties. It was discovered that the planet orbits the star in around 4 days and it is much closer to it than Mercury is to the Sun, moves at an orbital speed of 136 km/s, yet has a minimum mass about half that of Jupiter. At the time, the presence of a world so close to its star was not compatible with theories of planet formation and was considered an anomaly. However, since then, numerous other hot Jupiters have been discovered, assuming the planet is perfectly grey with no greenhouse or tidal effects, and a Bond albedo of 0.1, the temperature would be 1265 K. This is between the temperatures of HD189733 b and HD209458 b, before they were measured. It is sufficiently massive that its atmosphere is not blown away by the stars solar wind. 51 Pegasi b probably has a greater radius than that of Jupiter despite its lower mass and this is because its superheated atmosphere must be puffed up into a thick but tenuous layer surrounding it
46.
Stellar nucleosynthesis
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Stellar nucleosynthesis is the process by which the natural abundances of the chemical elements within stars change due to nuclear fusion reactions in the cores and their overlying mantles. Stars are said to evolve with changes in the abundances of the elements within, a star loses most of its mass when it is ejected late in the stars stellar lifetimes, thereby increasing the abundance of elements heavier than helium in the interstellar medium. The term supernova nucleosynthesis is used to describe the creation of elements during the evolution and explosion of a presupernova star, a stimulus to the development of the theory of nucleosynthesis was the discovery of variations in the abundances of elements found in the universe. Those abundances, when plotted on a graph as a function of number of the element, have a jagged sawtooth shape that varies by factors of tens of millions. This suggested a natural process other than random, such a graph of the abundances can be seen at History of nucleosynthesis theory article. Of the several processes of nucleosynthesis, stellar nucleosynthesis is the contributor to elemental abundances in the universe. The fusion of nuclei in a star, starting from its initial hydrogen and helium abundance, provides it energy and this became clear during the decade prior to World War II. The fusion-produced nuclei are restricted to only slightly heavier than the fusing nuclei. Nonetheless, this raised the plausibility of explaining all of the natural abundances of elements in this way. The prime energy producer in our Sun is the fusion of hydrogen to form helium, in 1920, Arthur Eddington, on the basis of the precise measurements of atomic masses by F. W. This was a step toward the idea of nucleosynthesis. In 1939, in a paper entitled Energy Production in Stars and he defined two processes that he believed to be the sources of energy in stars. The first one, the chain reaction, is the dominant energy source in stars with masses up to about the mass of the Sun. The second process, the cycle, which was also considered by Carl Friedrich von Weizsäcker in 1938, is most important in more massive stars. These works concerned the energy capable of keeping stars hot. A clear physical description of the chain and of the CNO cycle appears in a 1968 textbook. Bethes two papers did not address the creation of heavier nuclei, however and that theory was begun by Fred Hoyle in 1946 with his argument that a collection of very hot nuclei would assemble into iron. Hoyle followed that in 1954 with a paper describing how advanced fusion stages within stars would synthesize elements between carbon and iron in mass
47.
Gliese 876 d
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Gliese 876 d is an exoplanet approximately 15 light-years away in the constellation of Aquarius. The planet was the planet discovered orbiting the red dwarf Gliese 876. At the time of its discovery, the planet had the lowest mass of any extrasolar planet apart from the pulsar planets orbiting PSR B1257+12. Due to this low mass, it can be categorized as a super-Earth, the mass of Gliese 876 d from radial velocity has one problem, it is that only a lower limit on the mass can be obtained. This is because the mass value also depends on the orbital inclination. However, models incorporating the gravitational interactions between the resonant outer planets enables the inclination of the orbits to be determined and this reveals that the outer planets are nearly coplanar with an inclination of around 59° with respect to the plane of the sky. Assuming that Gliese 876 d orbits in the plane as the other planets. The low mass of the planet has led to suggestions that it may be a terrestrial planet and this type of massive terrestrial planet could be formed in the inner part of the Gliese 876 system from material pushed towards the star by the inward migration of the gas giants. Alternatively the planet could have formed further from Gliese 876, as a gas giant and this would result in a composition richer in volatile substances, such as water. As it arrived in range, the star would have blown off the hydrogen layer via coronal mass ejection. In this model, the planet would have an ocean of water separated from the silicate core by a layer of ice kept frozen by the high pressures in the planetary interior. Such a planet would have an atmosphere containing water vapor and free oxygen produced by the breakdown of water by ultraviolet radiation, distinguishing between these two models would require more information about the planets radius or composition. The planet does not appear to transit its star, which makes obtaining this information impossible with current observational capabilities, based on models, the radius of the exoplanet, based on the mass, can probably be estimated around 1.65 R⊕. The equilibrium temperature of Gliese 876 d, is estimated to be around 614 K, the planet orbits a star named Gliese 876. The star has a mass of 0.33 M☉ and a radius of around 0.36 R☉ and it has a surface temperature of 3350 K and is 2.55 billion years old. In comparison, the Sun is about 4.6 billion years old and has a temperature of 5778 K. Gliese 876 d is located in an orbit with a semimajor axis of only 0.0208 AU. Predicted total heat flux is approximately 104–5 W/m2 at the planets surface and this is similar to the radiative energy it receives from its parent star of about 40,000 W/m2. Gliese 876 d was discovered by analysing changes in its stars radial velocity as a result of the planets gravity, the radial velocity measurements were made by observing the Doppler shift in the stars spectral lines
48.
Gliese 876
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Gliese 876 is a red dwarf approximately 15 light-years away from Earth in the constellation of Aquarius. It is the star to the Sun confirmed to possess a planetary system. As of 2011, four extrasolar planets have been found to orbit the star, the planetary system is also notable for the orbital properties of its planets. It is the known system of orbital companions to exhibit a triple conjunction in the rare phenomenon of Laplace resonance. It is also the first extrasolar system around a star with measured coplanarity. Two of the planets are located in the systems habitable zone, however. Gliese 876 is located close to the Solar System. Despite being located so close to Earth, the star is so faint that it is invisible to the naked eye, as a red dwarf, Gliese 876 is much less massive than the Sun, estimates suggest it has only 32% of the mass of the Sun. The surface temperature of Gliese 876 is cooler than the Sun and these factors combine to make the star only 1. 24% as luminous as the Sun, and most of this is at infrared wavelengths. Estimating the age and metallicity of cool stars is due to the formation of diatomic molecules in their atmospheres. By fitting the observed spectrum to model spectra, it is estimated that Gliese 876 has a lower abundance of heavy elements compared to the Sun. Based on chromospheric activity the star is likely to be around 6.5 to 9.9 billion years old, depending on the theoretical model used. However, the low period of the star as well as its membership among the young disk population suggest that the star is between 0. 1–5 billion years old. Like many low-mass stars, Gliese 876 is a variable star and its variable star designation is IL Aquarii and it is classified as a BY Draconis variable. Its brightness fluctuates by around 0.04 magnitudes and this type of variability is thought to be caused by large starspots moving in and out of view as the star rotates. On June 23,1998, a planet was announced in orbit around Gliese 876 by two independent teams led by Geoffrey Marcy and Xavier Delfosse. The planet was designated Gliese 876 b and was detected by Doppler spectroscopy, based on luminosity measurement, the circumstellar habitable zone is believed to be located between 0.116 and 0.227 AU. On April 4,2001, a planet designated Gliese 876 c was detected
. Bibcode:2009A&A...507..487M. doi:10.1051/0004-6361/200912172.
