OTS 44
OTS 44 is a free-floating planetary-mass object or brown dwarf located at 550 light-years in the constellation Chamaeleon. It is among the lowest-mass free-floating substellar objects, with 11.5 times the mass of Jupiter, or 1.1% that of the Sun. Its radius is not well known and is estimated to be 23–57% that of the Sun. OTS 44 was discovered in 1998 by Oasa and Sugitani as a member of the star-forming region Chamaeleon I. Based upon infrared observations with the Spitzer Space Telescope and the Herschel Space Observatory, OTS 44 emits an excess of infrared radiation for an object of its type, suggesting it has a circumstellar disk of dust and particles of rock and ice; this disk has a mass of at least 10 Earth masses. Observations with the SINFONI spectrograph at the Very Large Telescope show that the disk is accreting matter at the rate of 10−11 of the mass of the Sun per year, it could develop into a planetary system. SCR 1845-6357, a binary system comprising a red dwarf and a brown dwarf Cha 110913-773444, an astronomical object that may be a free-floating planet surrounded by what appears to be a protoplanetary disk Astronomers Discover Beginnings of'Mini' Solar System MPIA Science Release 2013-09 - Blurring the lines between stars and planets: Lonely planets offer clues to star formation
Exoplanet
An exoplanet or extrasolar planet is a planet outside the Solar System. The first evidence of an exoplanet was not recognized as such; the first scientific detection of an exoplanet was in 1988. The first confirmed detection occurred in 1992; as of 1 April 2019, there are 4,023 confirmed planets in 3,005 systems, with 656 systems having more than one planet. There are many methods of detecting exoplanets. Transit photometry and Doppler spectroscopy have found the most, but these methods suffer from a clear observational bias favoring the detection of planets near the star. In several cases, multiple planets have been observed around a star. About 1 in 5 Sun-like stars have an "Earth-sized" planet in the habitable zone. Assuming there are 200 billion stars in the Milky Way, it can be hypothesized that there are 11 billion habitable Earth-sized planets in the Milky Way, rising to 40 billion if planets orbiting the numerous red dwarfs are included; the least massive planet known is Draugr, about twice the mass of the Moon.
The most massive planet listed on the NASA Exoplanet Archive is HR 2562 b, about 30 times the mass of Jupiter, although according to some definitions of a planet, it is too massive to be a planet and may be a brown dwarf instead. There are planets that are so near to their star that they take only a few hours to orbit and there are others so far away that they take thousands of years to orbit; some are so far out. All of the planets detected so far are within the Milky Way. Nonetheless, evidence suggests that extragalactic planets, exoplanets farther away in galaxies beyond the local Milky Way galaxy, may exist; the nearest exoplanet is Proxima Centauri b, located 4.2 light-years from Earth and orbiting Proxima Centauri, the closest star to the Sun. The discovery of exoplanets has intensified interest in the search for extraterrestrial life. There is special interest in planets that orbit in a star's habitable zone, where it is possible for liquid water, a prerequisite for life on Earth, to exist on the surface.
The study of planetary habitability considers a wide range of other factors in determining the suitability of a planet for hosting life. Besides exoplanets, there are rogue planets, which do not orbit any star; these tend to be considered as a separate category if they are gas giants, in which case they are counted as sub-brown dwarfs, like WISE 0855−0714. The rogue planets in the Milky Way number in the billions; the convention for designating exoplanets is an extension of the system used for designating multiple-star systems as adopted by the International Astronomical Union. For exoplanets orbiting a single star, the designation is formed by taking the name or, more designation of its parent star and adding a lower case letter; the first planet discovered in a system is given the designation "b" and planets are given subsequent letters. If several planets in the same system are discovered at the same time, the closest one to the star gets the next letter, followed by the other planets in order of orbital size.
A provisional IAU-sanctioned standard exists to accommodate the designation of circumbinary planets. A limited number of exoplanets have IAU-sanctioned proper names. Other naming systems exist. For centuries scientists and science fiction writers suspected that extrasolar planets existed, but there was no way of detecting them or of knowing their frequency or how similar they might be to the planets of the Solar System. Various detection claims made in the nineteenth century were rejected by astronomers; the first evidence of an exoplanet was not recognized as such. The first suspected scientific detection of an exoplanet occurred in 1988. Shortly afterwards, the first confirmed detection came in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12; the first confirmation of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet was found in a four-day orbit around the nearby star 51 Pegasi. Some exoplanets have been imaged directly by telescopes, but the vast majority have been detected through indirect methods, such as the transit method and the radial-velocity method.
In February 2018, researchers using the Chandra X-ray Observatory, combined with a planet detection technique called microlensing, found evidence of planets in a distant galaxy, stating "Some of these exoplanets are as small as the moon, while others are as massive as Jupiter. Unlike Earth, most of the exoplanets are not bound to stars, so they're wandering through space or loosely orbiting between stars. We can estimate. In the sixteenth century the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that Earth and other planets orbit the Sun, put forward the view that the fixed stars are similar to the Sun and are accompanied by planets. In the eighteenth century the same possibility was mentioned by Isaac Newton in the "General Scholium" that concludes his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centres of similar systems, they will all be constructed according to a similar design and subject to the dominion of One."In 1952, more than 40 years before the first hot Jupiter was discovere
University of Warsaw
The University of Warsaw, established in 1816, is the largest university in Poland. It employs over 6,000 staff including over 3,100 academic educators, it provides graduate courses for 53,000 students. The University offers some 37 different fields of study, 18 faculties and over 100 specializations in Humanities, technical as well as Natural Sciences, it was founded as a Royal University on 19 November 1816, when the Partitions of Poland separated Warsaw from the oldest and most influential University of Kraków. Alexander I granted permission for the establishment of five faculties – law and political science, philosophy and the humanities; the university expanded but was closed during November Uprising in 1830. It was reopened in 1857 as the Warsaw Academy of Medicine, now based in the nearby Staszic Palace with only medical and pharmaceutical faculties. All Polish-language campuses were closed in 1869 after the failed January Uprising, but the university managed to train 3,000 students, many of whom were important part of the Polish intelligentsia.
The university was resurrected during the First World War and the number of students reached 4,500 in 1918. After Poland's independence the new government focused on improving the university, in the early 1930s it became the country's largest. New faculties were established and the curriculum was extended. Following the Second World War and the devastation of Warsaw, the University reopened in 1945. Today, the University of Warsaw consists of 126 buildings and educational complexes with over 18 faculties: biology, chemistry and political science and sociology, physics and regional studies, history, applied linguistics and Slavic philology, philology, Polish language and public administration, applied social sciences and mathematics, computer science and mechanics; the University of Warsaw is one of the top Polish universities. It was ranked by Perspektywy magazine as best Polish university in 2010, 2011, 2014 and 2016. International rankings such as ARWU and University Web Ranking rank the university as the best Polish higher level institution.
On the list of 100 best European universities compiled by University Web Ranking, the University of Warsaw was placed as 61st. QS World University Rankings positioned the University of Warsaw as the best higher level institution among the world's top 400. In 1795 the partitions of Poland left Warsaw with access only to the Academy of Vilnius. In 1815, the newly established autonomous Congress Poland de facto belonging to the Russian Empire found itself without a university at all, as Vilnius was incorporated into Russia; the first to be established in Congress Poland were the Medical School. In 1816 Tsar Alexander I permitted the Polish authorities to create a university, comprising five departments: Law and Administration, Philosophy and Art and Humanities; the university soon grew to 50 professors. After most of the students and professors took part in the November 1830 Uprising the university was closed down. After the Crimean War, Russia entered a brief period of liberalization, the permission was given to create a Polish medical and surgical academy in Warsaw.
In 1862 departments of Law and Administration and History, Mathematics and Physics were opened. The newly established academy gained importance and was soon renamed the "Main School". However, after the January 1863 Uprising the liberal period ended and all Polish-language schools were closed down again. During its short existence, the Main School educated over 3,000 students, many of whom became part of the backbone of the Polish intelligentsia; the Main School was replaced with a Russian-language "Imperial University of Warsaw". Its purpose was to provide education for the Russian military garrison of Warsaw, the majority of students were Poles; the tsarist authorities believed that the Russian university would become a perfect way to Russify Polish society and spent a significant sum on building a new university campus. However, various underground organizations soon started to grow and the students became their leaders in Warsaw. Most notable of these groups joined the ranks of the 1905 Revolution.
Afterwards a boycott of Russian educational facilities was proclaimed and the number of Polish students dropped to below 10%. Most of the students who wanted to continue their education left for Western Europe. After the fall of the January Uprising, the Tsarist authorities' decided to convert the Main School into a Russian-language university, which functioned under the name of Imperial University for 46 years. There were two times. During the 1905–1907 revolution, such a proposal was made by some of the professors, in the face of a boycott of the university by Polish students. Talks on that subject were conducted with a number of Russian cities, including Voronezh and Saratov; the Russian government decided to keep a university in Warsaw, but as a result of the boycott, the university was Russian not only in the sense of the language used, but of the nationality of its professors and students. For the second time the question emerged during th
Geothermal energy
Geothermal energy is thermal energy generated and stored in the Earth. Thermal energy is the energy; the geothermal energy of the Earth's crust originates from the original formation of the planet and from radioactive decay of materials. The geothermal gradient, the difference in temperature between the core of the planet and its surface, drives a continuous conduction of thermal energy in the form of heat from the core to the surface; the adjective geothermal originates from the Greek roots γη, meaning earth, θερμος, meaning hot. Earth's internal heat is thermal energy generated from radioactive decay and continual heat loss from Earth's formation. Temperatures at the core–mantle boundary may reach over 4000 °C; the high temperature and pressure in Earth's interior cause some rock to melt and solid mantle to behave plastically, resulting in portions of the mantle convecting upward since it is lighter than the surrounding rock. Rock and water is heated in the crust, sometimes up to 370 °C. With water from hot springs, geothermal energy has been used for bathing since Paleolithic times and for space heating since ancient Roman times, but it is now better known for electricity generation.
Worldwide, 11,700 megawatts of geothermal power was available in 2013. An additional 28 gigawatts of direct geothermal heating capacity is installed for district heating, space heating, industrial processes and agricultural applications as of 2010. Geothermal power is cost-effective, reliable and environmentally friendly, but has been limited to areas near tectonic plate boundaries. Recent technological advances have expanded the range and size of viable resources for applications such as home heating, opening a potential for widespread exploitation. Geothermal wells release greenhouse gases trapped deep within the earth, but these emissions are much lower per energy unit than those of fossil fuels; the Earth's geothermal resources are theoretically more than adequate to supply humanity's energy needs, but only a small fraction may be profitably exploited. Drilling and exploration for deep resources is expensive. Forecasts for the future of geothermal power depend on assumptions about technology, energy prices, plate boundary movement and interest rates.
Pilot programs like EWEB's customer opt in Green Power Program show that customers would be willing to pay a little more for a renewable energy source like geothermal. But as a result of government assisted research and industry experience, the cost of generating geothermal power has decreased by 25% over the past two decades. In 2001, geothermal energy costs between ten US cents per kWh. Hot springs have been used for bathing at least since Paleolithic times; the oldest known spa is a stone pool on China's Lisan mountain built in the Qin Dynasty in the 3rd century BC, at the same site where the Huaqing Chi palace was built. In the first century AD, Romans conquered Aquae Sulis, now Bath, Somerset and used the hot springs there to feed public baths and underfloor heating; the admission fees for these baths represent the first commercial use of geothermal power. The world's oldest geothermal district heating system in Chaudes-Aigues, has been operating since the 14th century; the earliest industrial exploitation began in 1827 with the use of geyser steam to extract boric acid from volcanic mud in Larderello, Italy.
In 1892, America's first district heating system in Boise, Idaho was powered directly by geothermal energy, was copied in Klamath Falls, Oregon in 1900. The first known building in the world to utilize geothermal energy as its primary heat source was the Hot Lake Hotel in Union County, whose construction was completed in 1907. A deep geothermal well was used to heat greenhouses in Boise in 1926, geysers were used to heat greenhouses in Iceland and Tuscany at about the same time. Charlie Lieb developed the first downhole heat exchanger in 1930 to heat his house. Steam and hot water from geysers began heating homes in Iceland starting in 1943. In the 20th century, demand for electricity led to the consideration of geothermal power as a generating source. Prince Piero Ginori Conti tested the first geothermal power generator on 4 July 1904, at the same Larderello dry steam field where geothermal acid extraction began, it lit four light bulbs. In 1911, the world's first commercial geothermal power plant was built there.
It was the world's only industrial producer of geothermal electricity until New Zealand built a plant in 1958. In 2012, it produced some 594 megawatts. Lord Kelvin invented the heat pump in 1852, Heinrich Zoelly had patented the idea of using it to draw heat from the ground in 1912, but it was not until the late 1940s that the geothermal heat pump was implemented. The earliest one was Robert C. Webber's home-made 2.2 kW direct-exchange system, but sources disagree as to the exact timeline of his invention. J. Donald Kroeker designed the first commercial geothermal heat pump to heat the Commonwealth Building and demonstrated it in 1946. Professor Carl Nielsen of Ohio State University built the first residential open loop version in his home in 1948; the technology became popular in Sweden as a result of the 1973 oil crisis, has been growing in worldwide acceptance since then. The 1979 development of polybutylene pipe augmented the heat pump's economic viability. In 1960, Pacific Gas and Electric began operation of the first successful geothermal electric power plant in the United States at The Geysers in California.
The original turbine lasted for more t
Planetary system
A planetary system is a set of gravitationally bound non-stellar objects in or out of orbit around a star or star system. Speaking, systems with one or more planets constitute a planetary system, although such systems may consist of bodies such as dwarf planets, natural satellites, comets and circumstellar disks; the Sun together with its planetary system, which includes Earth, is known as the Solar System. The term exoplanetary system is sometimes used in reference to other planetary systems; as of 1 April 2019, there are 4,023 confirmed planets in 3,005 systems, with 656 systems having more than one planet. Debris disks are known to be common, though other objects are more difficult to observe. Of particular interest to astrobiology is the habitable zone of planetary systems where planets could have surface liquid water, thus the capacity to harbor Earth-like life. Heliocentrism was opposed to geocentrism; the notion of a heliocentric Solar System, with the Sun at the center, is first suggested in the Vedic literature of ancient India, which refer to the Sun as the "centre of spheres".
Some interpret Aryabhatta's writings in Āryabhaṭīya as implicitly heliocentric. The idea was first proposed in Western philosophy and Greek astronomy as early as the 3rd century BC by Aristarchus of Samos, but received no support from most other ancient astronomers. De revolutionibus orbium coelestium by Nicolaus Copernicus, published in 1543, was the first mathematically predictive heliocentric model of a planetary system. 17th-century successors Galileo Galilei, Johannes Kepler, Isaac Newton developed an understanding of physics which led to the gradual acceptance of the idea that the Earth moves round the Sun and that the planets are governed by the same physical laws that governed the Earth. In the 16th century the Italian philosopher Giordano Bruno, an early supporter of the Copernican theory that the Earth and other planets orbit the Sun, put forward the view that the fixed stars are similar to the Sun and are accompanied by planets, he was burned at the stake for his ideas by the Roman Inquisition.
In the 18th century the same possibility was mentioned by Isaac Newton in the "General Scholium" that concludes his Principia. Making a comparison to the Sun's planets, he wrote "And if the fixed stars are the centers of similar systems, they will all be constructed according to a similar design and subject to the dominion of One."His theories gained traction through the 19th and 20th centuries despite a lack of supporting evidence. Long before their confirmation by astronomers, conjecture on the nature of planetary systems had been a focus of the search for extraterrestrial intelligence and has been a prevalent theme in fiction science fiction; the first confirmed detection of an exoplanet was in 1992, with the discovery of several terrestrial-mass planets orbiting the pulsar PSR B1257+12. The first confirmed detection of exoplanets of a main-sequence star was made in 1995, when a giant planet, 51 Pegasi b, was found in a four-day orbit around the nearby G-type star 51 Pegasi; the frequency of detections has increased since particularly through advancements in methods of detecting extrasolar planets and dedicated planet finding programs such as the Kepler mission.
Planetary systems come from protoplanetary disks that form around stars as part of the process of star formation. During formation of a system much material is gravitationally scattered into far-flung orbits and some planets are ejected from the system becoming rogue planets. Planets orbiting pulsars have been discovered. Pulsars are the remnants of the supernova explosions of high-mass stars, but a planetary system that existed before the supernova would be destroyed. Planets would either evaporate, be pushed off of their orbits by the masses of gas from the exploding star, or the sudden loss of most of the mass of the central star would see them escape the gravitational hold of the star, or in some cases the supernova would kick the pulsar itself out of the system at high velocity so any planets that had survived the explosion would be left behind as free-floating objects. Planets found around pulsars may have formed as a result of pre-existing stellar companions that were entirely evaporated by the supernova blast, leaving behind planet-sized bodies.
Alternatively, planets may form in an accretion disk of fallback matter surrounding a pulsar. Fallback disks of matter that failed to escape orbit during a supernova may form planets around black holes; as stars evolve and turn into red giants, asymptotic giant branch stars, planetary nebulae they engulf the inner planets, evaporating or evaporating them depending on how massive they are. As the star loses mass, planets that are not engulfed move further out from the star. If an evolved star is in a binary or multiple system the mass it loses can transfer to another star, creating new protoplanetary disks and second- and third-generation planets which may differ in composition from the original planets which may be affected by the mass transfer. Planets in evolved binary systems, Hagai B. Perets, 13 Jan 2011 Can Planets survive Stellar Evolution?, Eva Villaver, Mario Livio, Feb 2007 The Orbital Evolution of Gas Giant Planets around Giant Stars, Eva Villaver, Mario Livio, 13 Oct 2009 On the survival of brown dwarfs and planets engulfed by their giant host star, Jean-Claude Passy, Mordecai-Mark Mac Low, Orsola De Marco, 2 Oct 2012 Foretellings of Ragnarök: World-engulfing Asymptotic Giants and the Inheritance of White Dwarfs, Alexander James Mustill, Eva Villaver, 5 Dec 2012 The Solar System consists of an
Exomoon
An exomoon or extrasolar moon is a natural satellite that orbits an exoplanet or other non-stellar extrasolar body. It is inferred from the empirical study of natural satellites in the Solar System that they are to be common elements of planetary systems; the majority of detected exoplanets are giant planets. In the Solar System, the giant planets have large collections of natural satellites. Therefore, it is reasonable to assume that exomoons are common. Though exomoons are difficult to detect and confirm using current techniques, observations from missions such as Kepler have observed a number of candidates, including some that may be habitats for extraterrestrial life and one that may be a rogue planet. To date there are no confirmed exomoon detections. Although traditional usage implies moons orbit a planet, the discovery of planet-sized satellites around brown dwarfs blurs the distinction between planets and moons, due to the low mass of such failed stars. To resolve this confusion, the International Astronomical Union declared, "Objects with true masses below the limiting mass for thermonuclear fusion of deuterium, that orbit stars or stellar remnants, are planets."
Characteristics of any extrasolar satellite are to vary, as do the Solar System's moons. For extrasolar giant planets orbiting within their stellar habitable zone, there is a prospect a terrestrial planet-sized satellite may be capable of supporting life. For impact-generated moons of terrestrial planets not too far from their star, with a large planet–moon distance, it is expected that the orbital planes of moons will tend to be aligned with the planet's orbit around the star due to tides from the star, but if the planet–moon distance is small it may be inclined. For gas giants, the orbits of moons will tend to be aligned with the giant planet's equator because these formed in circumplanetary disks. Planets close to their stars on circular orbits will become tidally locked; as the planet's rotation slows down the radius of a synchronous orbit of the planet moves outwards from the planet. For planets tidally locked to their stars, the distance from the planet at which the moon will be in a synchronous orbit around the planet is outside the Hill sphere of the planet.
The Hill sphere of the planet is the region where its gravity dominates that of the star so it can hold on to its moons. Moons inside the synchronous orbit radius of a planet will spiral into the planet. Therefore, if the synchronous orbit is outside the Hill sphere all moons will spiral into the planet. If the synchronous orbit is not three-body stable moons outside this radius will escape orbit before they reach the synchronous orbit. A study on tidal-induced migration offered a feasible explanation for this lack of exomoons, it showed the physical evolution of host planets plays a major role in their final fate: synchronous orbits can become transient states and moons are prone to be stalled in semi-asymptotic semimajor axes, or ejected from the system, where other effects can appear. In turn, this would have a great impact on the detection of extrasolar satellites; the existence of exomoons around many exoplanets is theorized. Despite the great successes of planet hunters with Doppler spectroscopy of the host star, exomoons cannot be found with this technique.
This is because the resultant shifted stellar spectra due to the presence of a planet plus additional satellites would behave identically to a single point-mass moving in orbit of the host star. In recognition of this, there have been several other methods proposed for detecting exomoons, including: Direct imaging Microlensing Pulsar timing Transit timing effects Transit method Direct imaging of an exoplanet is challenging due to the large difference in brightness between the star and exoplanet as well as the small size and irradiance of the planet; these problems are greater for exomoons in most cases. However, it has been theorized that tidally heated exomoons could shine as brightly as some exoplanets. Tidal forces can heat up an exomoon. Io, a tidally heated moon orbiting Jupiter, has volcanoes powered by tidal forces. If a tidally heated exomoon is sufficiently tidally heated and is distant enough from its star for the moon's light not to be drowned out, it would be possible for future telescopes to image it.
Doppler spectroscopy is an indirect detection method that measures the velocity shift and result stellar spectrum shift associated with an orbiting planet. This method is known as the Radial Velocity method, it is most successful for main sequence stars The spectra of exoplanets have been partially retrieved for several cases, including HD 189733 b and HD 209458 b. The quality of the retrieved spectra is more affected by noise than the stellar spectrum; as a result, the spectral resolution, number of retrieved spectral features, is much lower than the level required to perform doppler spectroscopy of the exoplanet. During its orbit, Io’s ionosphere interacts with Jupiter’s magnetosphere, to create a frictional current that causes radio wave emissions; these are called “Io-controlled decametric emissions” and the researchers believe finding similar emissions near known exoplanets could be key to predicting where other moons exist. In 2002, Cheongho Han & Wonyong Han proposed microlensing be used to detect exomoons.
The authors found detecting satellite signals in lensing light curves will be difficult because the signals are smeared out by the severe finite-source effect for events involved with source stars with small angular radii. In 2008, Sackett
Planet
A planet is an astronomical body orbiting a star or stellar remnant, massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, has cleared its neighbouring region of planetesimals. The term planet is ancient, with ties to history, science and religion. Five planets in the Solar System are visible to the naked eye; these were regarded by many early cultures as emissaries of deities. As scientific knowledge advanced, human perception of the planets changed, incorporating a number of disparate objects. In 2006, the International Astronomical Union adopted a resolution defining planets within the Solar System; this definition is controversial because it excludes many objects of planetary mass based on where or what they orbit. Although eight of the planetary bodies discovered before 1950 remain "planets" under the modern definition, some celestial bodies, such as Ceres, Pallas and Vesta, Pluto, that were once considered planets by the scientific community, are no longer viewed as such.
The planets were thought by Ptolemy to orbit Earth in epicycle motions. Although the idea that the planets orbited the Sun had been suggested many times, it was not until the 17th century that this view was supported by evidence from the first telescopic astronomical observations, performed by Galileo Galilei. About the same time, by careful analysis of pre-telescopic observational data collected by Tycho Brahe, Johannes Kepler found the planets' orbits were elliptical rather than circular; as observational tools improved, astronomers saw that, like Earth, each of the planets rotated around an axis tilted with respect to its orbital pole, 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 other planets share characteristics such as volcanism, hurricanes and hydrology. Planets are divided into two main types: large low-density giant planets, smaller rocky terrestrials. There are eight planets in the Solar System.
In order of increasing distance from the Sun, they are the four terrestrials, Venus and Mars the four giant planets, Saturn 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; as of 1 April 2019, 4,023 known extrasolar planets in 3,005 planetary systems, ranging in size from just above the size of the Moon to gas giants about twice as large as Jupiter have been discovered, out of which more than 100 planets are the same size as Earth, nine of which are at the same relative distance from their star as Earth from the Sun, i.e. in the circumstellar 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. A 2012 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 divine lights of antiquity to the earthly objects of the scientific age; the concept has expanded to include worlds not only in the Solar System, but in hundreds of other extrasolar systems. 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, ancient astronomy. In ancient times, astronomers noted how certain lights moved across the sky, as opposed to the "fixed stars", which maintained a constant relative position in the sky. Ancient Greeks called these lights πλάνητες ἀστέρες or πλανῆται, from which today's word "planet" was derived. In ancient Greece, China and indeed all pre-modern civilizations, it was universally believed that Earth was the center of the Universe and that all the "planets" circled Earth.
The reasons for this perception were that stars and planets appeared to revolve around Earth each day and the common-sense perceptions that Earth was solid and stable and that it was not moving but at rest. The first civilization known to have a functional theory of the planets were the Babylonians, who lived in Mesopotamia in the first and second millennia BC; the oldest surviving planetary astronomical text is the Babylonian Venus tablet of Ammisaduqa, a 7th-century BC copy of a list of observations of the motions of the planet Venus, that dates as early as the second millennium BC. The MUL. APIN is a pair of cuneiform tablets dating from the 7th century BC that lays out the motions of the Sun and planets over the course of the year; the Babylonian astrologers laid the foundations of what would become Western astrology. The Enuma anu enlil, written during the Neo-Assyrian period in the 7th century BC, comprises a list of omens and their relationships with various celestial phenomena including the motions of the planets.
Venus and the outer planets Mars and Saturn were all identified by Babylonian astronomers. These would remain the only known planets until the invention of the telescope in early modern times; the ancient Greeks did not attach as much significance to the planets as the Babylonians. The Pythagoreans, in the 6th and 5t
