A gas giant is a giant planet composed of hydrogen and helium. Gas giants are sometimes known as failed stars because they contain the same basic elements as a star. Jupiter and Saturn are the gas giants of the Solar System; the term "gas giant" was synonymous with "giant planet", but in the 1990s it became known that Uranus and Neptune are a distinct class of giant planet, being composed of heavier volatile substances. For this reason and Neptune are now classified in the separate category of ice giants. Jupiter and Saturn consist of hydrogen and helium, with heavier elements making up between 3 and 13 percent of the mass, they are thought to consist of an outer layer of molecular hydrogen surrounding a layer of liquid metallic hydrogen, with a molten rocky core. The outermost portion of their hydrogen atmosphere is characterized by many layers of visible clouds that are composed of water and ammonia; the layer of metallic hydrogen makes up the bulk of each planet, is referred to as "metallic" because the large pressure turns hydrogen into an electrical conductor.
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 low-mass brown dwarf and a gas giant are debated. One school of thought is based on formation. 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 used to refer to all giant planets. 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 upper layers of the atmosphere, all matter is above the critical point, where there is no distinction between liquids and gases; the term has caught on, because planetary scientists use "rock", "gas", "ice" as shorthands for classes of elements and compounds found as planetary constituents, irrespective of what phase the matter may appear in.
In the outer Solar System and helium are referred to as "gases". Because Uranus and Neptune are composed of, in this terminology, not gas, they are referred to as ice giants and separated from the gas giants. Gas giants can, theoretically, be divided into five distinct classes according to their modeled physical atmospheric properties, hence their appearance: ammonia clouds, water clouds, alkali-metal clouds, silicate clouds. 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 combined, hydrogen planets need not be as large as the familiar gas giants from the Solar System. However, smaller gas planets and planets closer to their star will lose atmospheric mass more via hydrodynamic escape than larger planets and planets farther out.
A gas dwarf could be defined as a planet with a rocky core that has accumulated a thick envelope of hydrogen and other volatiles, having as result a total radius between 1.7 and 3.9 Earth-radii. The smallest known extrasolar planet, a "gas planet" is Kepler-138d, which has the same mass as Earth but is 60% larger and therefore has a density that indicates a thick gas envelope. 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 Hot Jupiter
Transhumanism is an international philosophical movement that advocates for the transformation of the human condition by developing and making available sophisticated technologies to enhance human intellect and physiology. Transhumanist thinkers study the potential benefits and dangers of emerging technologies that could overcome fundamental human limitations as well as the ethical limitations of using such technologies; the most common transhumanist thesis is that human beings may be able to transform themselves into different beings with abilities so expanded from the current condition as to merit the label of posthuman beings. The contemporary meaning of the term "transhumanism" was foreshadowed by one of the first professors of futurology, FM-2030, who taught "new concepts of the human" at The New School in the 1960s, when he began to identify people who adopt technologies and worldviews "transitional" to posthumanity as "transhuman"; the assertion would lay the intellectual groundwork for the British philosopher Max More to begin articulating the principles of transhumanism as a futurist philosophy in 1990, organizing in California an intelligentsia that has since grown into the worldwide transhumanist movement.
Influenced by seminal works of science fiction, the transhumanist vision of a transformed future humanity has attracted many supporters and detractors from a wide range of perspectives, including philosophy and religion. According to Nick Bostrom, transcendentalist impulses have been expressed at least as far back as the quest for immortality in the Epic of Gilgamesh, as well as in historical quests for the Fountain of Youth, the Elixir of Life, other efforts to stave off aging and death. There is debate about whether the philosophy of Friedrich Nietzsche can be considered an influence on transhumanism, despite its exaltation of the "Übermensch", due to its emphasis on self-actualization rather than technological transformation; the transhumanist philosophies of Max More and Stefan Lorenz Sorgner have been influenced by Nietzschean thinking. By way of contrast, The Transhumanist Declaration "...advocates the well-being of all sentience". The late 19th to early 20th century movement known as Russian cosmism incorporated some ideas which developed into the core of the transhumanist movement.
Fundamental ideas of transhumanism were first advanced in 1923 by the British geneticist J. B. S. Haldane in his essay Daedalus: Science and the Future, which predicted that great benefits would come from the application of advanced sciences to human biology—and that every such advance would first appear to someone as blasphemy or perversion, "indecent and unnatural". In particular, he was interested in the development of the science of eugenics and the application of genetics to improve human characteristics, such as health and intelligence, his article inspired popular interest. J. D. Bernal, a crystallographer at Cambridge, wrote The World, the Flesh and the Devil in 1929, in which he speculated on the prospects of space colonization and radical changes to human bodies and intelligence through bionic implants and cognitive enhancement; these ideas have been common transhumanist themes since. The biologist Julian Huxley is regarded as the founder of transhumanism after using the term for the title of an influential 1957 article.
The term itself, derives from an earlier 1940 paper by the Canadian philosopher W. D. Lighthall. Huxley describes transhumanism in these terms: Up till now human life has been, as Hobbes described it,'nasty and short'. Huxley's definition differs, albeit not from the one in use since the 1980s; the ideas raised by these thinkers were explored in the science fiction of the 1960s, notably in Arthur C. Clarke's 2001: A Space Odyssey, in which an alien artifact grants transcendent power to its wielder. Japanese Metabolist architects produced a manifesto in 1960 which outlined goals to "encourage active metabolic development of our society" through design and technology. In the Material and Man section of the manifesto, Noboru Kawazoe suggests that:After several decades, with the rapid progress of communication technology, every one will have a “brain wave receiver” in his ear, which conveys directly and what other people think about him and vice versa. What I think will be known by all the people.
There is no more individual consciousness, only the will of mankind as a whole. The concept of the technological singularity, or the ultra-rapid advent of superhuman intelligence, was first proposed by the British cryptologist I. J. Good in 1965: Let an ultraintelligent machine be defined as a machine that can far surpass all the intellectual activities of any man however clever. Since the design of machines is one of these intellectual activities, an ultraintelligent machine could design better machines, thus the first ultraintelligent machine is the last invention that man need make. Computer scientist Marvin Minsky
Titan is the largest moon of Saturn and the second-largest natural satellite in the Solar System. It is the only moon known to have a dense atmosphere, the only object in space, other than Earth, where clear evidence of stable bodies of surface liquid has been found. Titan is the sixth gravitationally rounded moon from Saturn. Described as a planet-like moon, Titan is 50% larger than Earth's moon and 80% more massive, it is the second-largest moon in the Solar System after Jupiter's moon Ganymede, is larger than the planet Mercury, but only 40% as massive. Discovered in 1655 by the Dutch astronomer Christiaan Huygens, Titan was the first known moon of Saturn, the sixth known planetary satellite. Titan orbits Saturn at 20 Saturn radii. From Titan's surface, Saturn subtends an arc of 5.09 degrees and would appear 11.4 times larger in the sky than the Moon from Earth. Titan is composed of ice and rocky material. Much as with Venus before the Space Age, the dense opaque atmosphere prevented understanding of Titan's surface until the Cassini–Huygens mission in 2004 provided new information, including the discovery of liquid hydrocarbon lakes in Titan's polar regions.
The geologically young surface is smooth, with few impact craters, although mountains and several possible cryovolcanoes have been found. The atmosphere of Titan is nitrogen; the climate—including wind and rain—creates surface features similar to those of Earth, such as dunes, lakes and deltas, is dominated by seasonal weather patterns as on Earth. With its liquids and robust nitrogen atmosphere, Titan's methane cycle is analogous to Earth's water cycle, at the much lower temperature of about 94 K. Titan was discovered on March 25, 1655, by the Dutch astronomer Christiaan Huygens. Huygens was inspired by Galileo's discovery of Jupiter's four largest moons in 1610 and his improvements in telescope technology. Christiaan, with the help of his older brother Constantijn Huygens, Jr. began building telescopes around 1650 and discovered the first observed moon orbiting Saturn with one of the telescopes they built. It was the sixth moon discovered, after Earth's Moon and the Galilean moons of Jupiter.
Huygens named his discovery Saturni Luna, publishing in the 1655 tract De Saturni Luna Observatio Nova. After Giovanni Domenico Cassini published his discoveries of four more moons of Saturn between 1673 and 1686, astronomers fell into the habit of referring to these and Titan as Saturn I through V. Other early epithets for Titan include "Saturn's ordinary satellite". Titan is numbered Saturn VI because after the 1789 discoveries the numbering scheme was frozen to avoid causing any more confusion. Numerous small moons have been discovered closer to Saturn since then; the name Titan, the names of all seven satellites of Saturn known, came from John Herschel, in his 1847 publication Results of Astronomical Observations Made during the Years 1834, 5, 6, 7, 8, at the Cape of Good Hope. He suggested the names of the mythological Titans and sisters of Cronus, the Greek Saturn. In Greek mythology, the Titans were a race of powerful deities, descendants of Gaia and Uranus, that ruled during the legendary Golden Age.
Titan orbits Saturn once 22 hours. Like the Moon and many of the satellites of the giant planets, its rotational period is identical to its orbital period; because of this, there is a sub-Saturnian point on its surface, from which the planet would always appear to hang directly overhead. Longitudes on Titan are measured westward, starting from the meridian passing through this point, its orbital eccentricity is 0.0288, the orbital plane is inclined 0.348 degrees relative to the Saturnian equator. Viewed from Earth, Titan reaches an angular distance of about 20 Saturn radii from Saturn and subtends a disk 0.8 arcseconds in diameter. The small, irregularly shaped satellite Hyperion is locked in a 3:4 orbital resonance with Titan. A "slow and smooth" evolution of the resonance—in which Hyperion migrated from a chaotic orbit—is considered unlikely, based on models. Hyperion formed in a stable orbital island, whereas the massive Titan absorbed or ejected bodies that made close approaches. Titan is 5,149.46 kilometers in diameter, 1.06 times that of the planet Mercury, 1.48 that of the Moon, 0.40 that of Earth.
Before the arrival of Voyager 1 in 1980, Titan was thought to be larger than Ganymede and thus the largest moon in the Solar System. Titan's diameter and mass are similar to those of the Jovian moons Callisto. Based on its bulk density of 1.88 g/cm3, Titan's composition is half water ice and half rocky material. Though similar in composition to Dione and Enceladus, it is denser due to gravitational compression, it has a mass 1/4226 that of Saturn, making it the largest moon of the gas giants relative to the ma
Jupiter is the fifth planet from the Sun and the largest in the Solar System. It is a giant planet with a mass one-thousandth that of the Sun, but two-and-a-half times that of all the other planets in the Solar System combined. Jupiter and Saturn are gas giants. Jupiter has been known to astronomers since antiquity, it is named after the Roman god Jupiter. When viewed from Earth, Jupiter can reach an apparent magnitude of −2.94, bright enough for its reflected light to cast shadows, making it on average the third-brightest natural object in the night sky after the Moon and Venus. Jupiter is composed of hydrogen with a quarter of its mass being helium, though helium comprises only about a tenth of the number of molecules, it may have a rocky core of heavier elements, but like the other giant planets, Jupiter lacks a well-defined solid surface. Because of its rapid rotation, the planet's shape is that of an oblate spheroid; the outer atmosphere is visibly segregated into several bands at different latitudes, resulting in turbulence and storms along their interacting boundaries.
A prominent result is the Great Red Spot, a giant storm, known to have existed since at least the 17th century when it was first seen by telescope. Surrounding Jupiter is a powerful magnetosphere. Jupiter has 79 known moons, including the four large Galilean moons discovered by Galileo Galilei in 1610. Ganymede, the largest of these, has a diameter greater than that of the planet Mercury. Jupiter has been explored on several occasions by robotic spacecraft, most notably during the early Pioneer and Voyager flyby missions and by the Galileo orbiter. In late February 2007, Jupiter was visited by the New Horizons probe, which used Jupiter's gravity to increase its speed and bend its trajectory en route to Pluto; the latest probe to visit the planet is Juno, which entered into orbit around Jupiter on July 4, 2016. Future targets for exploration in the Jupiter system include the probable ice-covered liquid ocean of its moon Europa. Astronomers have discovered nearly 500 planetary systems with multiple planets.
These systems include a few planets with masses several times greater than Earth's, orbiting closer to their star than Mercury is to the Sun, sometimes Jupiter-mass gas giants close to their star. Earth and its neighbor planets may have formed from fragments of planets after collisions with Jupiter destroyed those super-Earths near the Sun; as Jupiter came toward the inner Solar System, in what theorists call the grand tack hypothesis, gravitational tugs and pulls occurred causing a series of collisions between the super-Earths as their orbits began to overlap. Researchers from Lund University found that Jupiter's migration went on for around 700,000 years, in a period 2-3 million years after the celestial body started its life as an ice asteroid far from the sun; the journey inwards in the solar system followed a spiraling course in which Jupiter continued to circle around the sun, albeit in an tight path. The reason behind the actual migration relates to gravitational forces from the surrounding gases in the solar system.
Jupiter moving out of the inner Solar System would have allowed the formation of inner planets, including Earth. Jupiter is composed of gaseous and liquid matter, it is the largest of hence its largest planet. It has a diameter of 142,984 km at its equator; the average density of Jupiter, 1.326 g/cm3, is the second highest of the giant planets, but lower than those of the four terrestrial planets. Jupiter's upper atmosphere is about 88–92% hydrogen and 8–12% helium by percent volume of gas molecules. A helium atom has about four times as much mass as a hydrogen atom, so the composition changes when described as the proportion of mass contributed by different atoms. Thus, Jupiter's atmosphere is 75% hydrogen and 24% helium by mass, with the remaining one percent of the mass consisting of other elements; the atmosphere contains trace amounts of methane, water vapor and silicon-based compounds. There are traces of carbon, hydrogen sulfide, oxygen and sulfur; the outermost layer of the atmosphere contains crystals of frozen ammonia.
The interior contains denser materials—by mass it is 71% hydrogen, 24% helium, 5% other elements. Through infrared and ultraviolet measurements, trace amounts of benzene and other hydrocarbons have been found; the atmospheric proportions of hydrogen and helium are close to the theoretical composition of the primordial solar nebula. Neon in the upper atmosphere only consists of 20 parts per million by mass, about a tenth as abundant as in the Sun. Helium is depleted to about 80% of the Sun's helium composition; this depletion is a result of precipitation of these elements into the interior of the planet. Based on spectroscopy, Saturn is thought to be similar in composition to Jupiter, but the other giant planets Uranus and Neptune have less hydrogen and helium and more ices and are thus now termed ice giants. Jupiter's mass is 2.5 times that of all the other planets in the Solar System combined—this is so massive that its barycenter with the Sun lies above the Sun's surface at 1.068 solar radii from the Sun's center.
Jupiter is much larger than Earth and less dense: its volume is that of about 1,321 Earths, but it is only 318 times as massive. Jupiter's radius is about 1/10 the radius of the Sun, its mass is 0.001 times the mass of the Sun, so the densities of the two bodies are similar. A "Jupiter mass" is used as a u
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System after Mercury. In English, Mars carries a name of the Roman god of war, is referred to as the "Red Planet" because the reddish iron oxide prevalent on its surface gives it a reddish appearance, distinctive among the astronomical bodies visible to the naked eye. Mars is a terrestrial planet with a thin atmosphere, having surface features reminiscent both of the impact craters of the Moon and the valleys and polar ice caps of Earth; the days and seasons are comparable to those of Earth, because the rotational period as well as the tilt of the rotational axis relative to the ecliptic plane are similar. Mars is the site of Olympus Mons, the largest volcano and second-highest known mountain in the Solar System, 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 and may be a giant impact feature. Mars has two moons and Deimos, which are small and irregularly shaped.
These may be captured asteroids, similar to 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, less than 1% of the Earth's, except at the lowest elevations for short periods; the two polar ice caps appear to be made of water. The volume of water ice in the south polar ice cap, if melted, would be sufficient to cover the entire planetary surface to a depth of 11 meters. In November 2016, NASA reported finding a large amount of underground 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 be seen from Earth with the naked eye, as can its reddish coloring, its apparent magnitude reaches −2.94, surpassed only by Jupiter, the Moon, the Sun.
Optical ground-based telescopes are limited to resolving features about 300 kilometers across when Earth and Mars are closest because of Earth's atmosphere. Mars is half the diameter of Earth with a surface area only less than the total area of Earth's dry land. Mars is less dense than Earth, having about 15% of Earth's volume and 11% of Earth's mass, resulting in about 38% of Earth's surface gravity; the red-orange appearance of the Martian surface is caused by rust. It can look like butterscotch. Like Earth, Mars has differentiated into a dense 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 of iron and nickel with about 16–17% sulfur; this iron sulfide core is thought to be twice as rich in lighter elements as Earth's. The core is surrounded by a silicate mantle that formed many of the tectonic and volcanic features on the planet, but it appears to be dormant. Besides silicon and oxygen, the most abundant elements in the Martian crust are iron, aluminum and potassium.
The average thickness of the planet's crust is about 50 km, with a maximum thickness of 125 km. Earth's crust averages 40 km. Mars is a terrestrial planet that consists of minerals containing silicon and oxygen and other elements that make up rock; the surface of Mars is composed of tholeiitic basalt, although parts are more silica-rich than typical basalt and may be similar to andesitic rocks on Earth or silica glass. Regions of low albedo suggest concentrations of plagioclase feldspar, with northern low albedo regions displaying higher than normal concentrations of sheet silicates and high-silicon glass. Parts of the southern highlands include detectable amounts of high-calcium pyroxenes. Localized concentrations of hematite and olivine have been found. Much of the surface is covered by finely grained iron oxide dust. Although Mars has no evidence of a structured global magnetic field, observations show that parts of the planet's crust have been magnetized, suggesting that alternating polarity reversals of its dipole field have occurred in the past.
This paleomagnetism of magnetically susceptible minerals is similar to the alternating bands found on Earth's ocean floors. One theory, published in 1999 and re-examined in October 2005, is that these bands suggest plate tectonic activity on Mars four billion years ago, before the planetary dynamo ceased to function and the planet's magnetic field faded, it is thought that, during the Solar System's formation, Mars was created as the result of a stochastic process of run-away accretion of material from the protoplanetary disk that orbited the Sun. Mars has many distinctive chemical features caused by its position in the Solar System. Elements with comparatively low boiling points, such as chlorine and sulphur, are much more common on Mars than Earth. After the formation of the planets, all were subjected to the so-called "Late Heavy Bombardment". About 60% of the surface of Mars shows a record of impacts from that era, whereas much of the remaining surface is underlain by immense impact basins caused by those events.
There is evidence of an enormous impact basin in the northern hemisphere of Mars, spanning 10,600 by 8,500 km, or four times the size of the Moon's South Pole – Aitk
The John Varley Reader
The John Varley Reader is a representative collection of 18 of the science fiction short stories by John Varley, first published in paperback in September 2004. It features 5 new stories; each story is preceded by an autobiographical introduction. Introduction "Picnic on Nearside" "Overdrawn at the Memory Bank" "In the Hall of the Martian Kings" "Gotta Sing, Gotta Dance" "The Barbie Murders" "The Phantom of Kansas" "Beatnik Bayou" "Air Raid" "The Persistence of Vision" "Press Enter" "The Pusher" "Tango Charlie and Foxtrot Romeo" "Options" "Just Another Perfect Day" "In Fading Suns and Dying Moons" "The Flying Dutchman" "Good Intentions" "The Bellman" The John Varley Reader title listing at the Internet Speculative Fiction Database
The Solar System is the gravitationally bound planetary system of the Sun and the objects that orbit it, either directly or indirectly. Of the objects that orbit the Sun directly, the largest are the eight planets, with the remainder being smaller objects, such as the five dwarf planets and small Solar System bodies. 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 gravitational collapse of a giant interstellar molecular cloud. The vast majority of the system's mass is in the Sun, with the majority of the remaining mass contained in Jupiter; the four smaller inner planets, Venus and Mars, are terrestrial planets, being composed of rock and metal. The four outer planets are giant planets, being more massive than the terrestrials; the two largest and Saturn, are gas giants, being composed of hydrogen and helium. All eight planets have circular orbits that lie within a nearly flat disc called the ecliptic.
The Solar System contains smaller objects. The asteroid belt, which lies between the orbits of Mars and Jupiter contains objects composed, like the terrestrial planets, of rock and metal. Beyond Neptune's orbit lie the Kuiper belt and scattered disc, which are populations of trans-Neptunian objects composed of ices, beyond them a newly discovered population of sednoids. Within these populations are several dozen to 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 trans-Neptunian objects Pluto and Eris. In addition to these two regions, various other small-body populations, including comets and interplanetary dust clouds travel between regions. Six of the planets, at least four of the dwarf planets, many of the smaller bodies are orbited by natural satellites termed "moons" after the Moon; 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, creates a bubble-like region in the interstellar medium known as the heliosphere. The heliopause is the point at which pressure from the solar wind is equal to the opposing pressure of the interstellar medium; the Oort cloud, thought to be the source for long-period comets, may exist at a distance a thousand times further than the heliosphere. The Solar System is located in the Orion Arm, 26,000 light-years from the center of the Milky Way galaxy. For most of history, humanity did not understand the concept of the Solar System. Most people up to the Late Middle Ages–Renaissance believed Earth to be stationary at the centre of the universe and categorically different from the divine or ethereal objects that moved through the sky. Although the Greek philosopher Aristarchus of Samos had speculated on a heliocentric reordering of the cosmos, Nicolaus Copernicus was the first to develop a mathematically predictive heliocentric system.
In the 17th century, Galileo discovered that the Sun was marked with sunspots, that Jupiter had four satellites in orbit around it. Christiaan Huygens followed on from Galileo's discoveries by discovering Saturn's moon Titan and the shape of the rings of Saturn. Edmond Halley realised in 1705 that repeated sightings of a comet were recording the same object, returning once every 75–76 years; this was the first evidence that anything other than the planets orbited the Sun. Around this time, the term "Solar System" first appeared in English. In 1838, Friedrich Bessel measured a stellar parallax, an apparent shift in the position of a star created by Earth's motion around the Sun, providing the first direct, experimental proof of heliocentrism. Improvements in observational astronomy and the use of unmanned spacecraft have since enabled the detailed investigation of other bodies orbiting the Sun; the principal component of the Solar System is the Sun, a G2 main-sequence star that contains 99.86% of the system's known mass and dominates it gravitationally.
The Sun's four largest orbiting bodies, the giant planets, account for 99% of the remaining mass, with Jupiter and Saturn together comprising more than 90%. The remaining objects of the Solar System together comprise less than 0.002% of the Solar System's total mass. Most large objects in orbit around the Sun lie near the plane of Earth's orbit, known as the ecliptic; the planets are close to the ecliptic, whereas comets and Kuiper belt objects are at greater angles to it. All the planets, most other objects, orbit the Sun in the same direction that the Sun is rotating. There are exceptions, such as Halley's Comet; the overall structure of the charted regions of the Solar System consists of the Sun, four small inner planets surrounded by a belt of rocky asteroids, four giant planets surrounded by the Kuiper belt of icy objects. Astronomers sometimes informally divide this structure into separate regions; the inner Solar System includes the asteroid belt. The outer Solar System is including the four giant planets.
Since the discovery of the Kuiper belt, the outermost parts of the Solar Sys