Portal:Solar System
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Portal:Solar System

The Solar System Portal

The sun and planets of the solar system (distances not to scale)

The Solar System is the gravitationally bound 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, the 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, Mercury, Venus, Earth and Mars, are terrestrial planets, being primarily composed of rock and metal. The four outer planets are giant planets, being substantially more massive than the terrestrials. The two largest, Jupiter and Saturn, are gas giants, being composed mainly of hydrogen and helium; the two outermost planets, Uranus and Neptune, are ice giants, being composed mostly of substances with relatively high melting points compared with hydrogen and helium, called volatiles, such as water, ammonia and methane. All eight planets have almost circular orbits that lie within a nearly 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 Neptune's orbit lie the Kuiper belt and scattered disc, which are populations of trans-Neptunian objects composed mostly of ices, and beyond them a newly discovered population of sednoids. Within these populations, some objects are large enough to have rounded under their own gravity, though there is considerable debate as to how many there will prove to be. Such objects are categorized as dwarf planets. Identified or accepted dwarf planets include the asteroid Ceres and the trans-Neptunian objects Pluto and Eris. In addition to these two regions, various other small-body populations, including comets, centaurs and interplanetary dust clouds, freely travel between regions. Six of the planets, the six largest possible dwarf planets, and many of the smaller bodies are orbited by natural satellites, usually 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; it extends out to the edge of the scattered disc. The Oort cloud, which is thought to be the source for long-period comets, may also exist at a distance roughly 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.

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Neptune, as photographed by Voyager 2.
Neptune is the eighth and farthest planet from the Sun in the Solar System. It is the fourth largest planet by diameter, and the third largest by mass. The planet is named after the Roman god of the sea. Discovered on September 23, 1846, Neptune was the first planet found by mathematical prediction rather than regular observation. Unexpected changes in the orbit of Uranus led astronomers to deduce the gravitational perturbation of an unknown planet. Neptune was found within a degree of the predicted position. The moon Triton was found shortly thereafter, but none of the planet's other 12 moons were discovered before the 20th century. Neptune has been visited by only one spacecraft, Voyager 2, which flew by the planet on August 25, 1989. Neptune is similar in composition to Uranus, and both have different compositions from those of the larger gas giants Jupiter and Saturn. Traces of methane in the atmosphere, in part, account for the planet's blue appearance. At the time of the 1989 Voyager 2 flyby, its southern hemisphere possessed a Great Dark Spot comparable to the Great Red Spot on Jupiter. Neptune has a faint and fragmented ring system, which may have been detected during the 1960s but was only indisputably confirmed by Voyager 2.

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Earthrise, as seen by Apollo 8

"Earthrise," the first occasion in which humans saw the Earth seemingly rising above the surface of the Moon, taken during the Apollo 8 mission on December 24, 1968. This view was seen by the crew at the beginning of its fourth orbit around the Moon, although the very first photograph taken was in black-and-white. Note that the Earth is in shadow here. A photo of a fully lit Earth would not be taken until the Apollo 17 mission.

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The SunMercuryVenusThe MoonEarthMarsPhobos and DeimosCeresThe main asteroid beltJupiterMoons of JupiterSaturnMoons of SaturnUranusMoons of UranusNeptuneMoons of NeptunePlutoMoons of PlutoHaumeaMoons of HaumeaMakemakeThe Kuiper BeltErisDysnomiaThe Scattered DiscThe Hills CloudThe Oort CloudSolar System Template Final.png

Solar System: Planets (Definition ? Planetary habitability ? Terrestrial planets ? Gas giants ? Rings) ? Dwarf planets (Plutoid) ? Moons ? Exploration ? Colonization ? Discovery timeline

Sun: Sunspot ? Solar wind ? Solar flare ? Solar eclipse
Mercury: Geology ? Exploration (Mariner 10 ? MESSENGER ? BepiColombo) ? Transit
Venus: Geology ? Atmosphere ? Exploration (Venera ? Mariner program 2/5/10 ? Pioneer ? Vega 1/2? Magellan ? Venus Express) ? Transit
Earth: History ? Geology ? Geography ? Atmosphere ? Rotation
Moon: Geology ? Selenography ? Atmosphere ? Exploration (Luna ? Apollo 8/11) ? Orbit ? Lunar eclipse
Mars: Moons (Phobos ? Deimos) ? Geology ? Geography ? Atmosphere ? Exploration (Mariner ? Mars ? Viking 1/2 ? Pathfinder ? MER)
Ceres: Exploration (Dawn)
Jupiter: Moons (Amalthea, Io ? Europa ? Ganymede ? Callisto) ? Rings ? Atmosphere ? Magnetosphere ? Exploration (Pioneer 10/11 ? Voyager 1/2 ? Ulysses ? Cassini ? Galileo ? New Horizons)
Saturn: Moons (Mimas ? Enceladus ? Tethys ? Dione ? Rhea ? Titan ? Iapetus) ? Rings ? Exploration (Pioneer 11 ? Voyager 1/2 ? Cassini-Huygens)
Uranus: Moons (Miranda ? Ariel ? Umbriel ? Titania ? Oberon) ? Rings ? Exploration (Voyager 2)
Neptune: Moons (Triton) ? Rings ? Exploration (Voyager 2)
Planets beyond Neptune
Pluto: Moons (Charon, Nix, Hydra, Kerberos, Styx) ? Exploration (New Horizons)
Haumea: Moons (Hi'iaka, Namaka)
Makemake
Eris: Dysnomia
Small bodies: Meteoroids ? Asteroids (Asteroid belt) ? Centaurs ? TNOs (Kuiper belt ? Scattered disc ? Oort cloud) ? Comets (Hale-Bopp ? Halley's ? Hyakutake ? Shoemaker-Levy 9)
Formation and evolution of the Solar System: History of Solar System formation and evolution hypotheses ? Nebular hypothesis
See also: Featured content ? Featured topic ? Good articles ? List of objects

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Italicized articles are on dwarf planets or major moons.

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