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Solar System

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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, 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 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 these two regions, various other small-body populations, including comets, centaurs and interplanetary dust clouds, freely travel between regions. Six of the planets, at least four of the 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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Io (moon).
Io (pronounced /ˈaɪoʊ/ eye'-oe, or as Greek Ῑώ) is the innermost of the four Galilean moons of Jupiter and, with a diameter of 3,642 kilometers, the fourth largest moon in the Solar System. It was discovered in 1610 by Galileo Galilei, along with the other Galilean satellites. This discovery furthered the adoption of the Copernican model of the Solar System and the development of Kepler's laws of motion. Unlike most satellites in the outer Solar System (which have a thick coating of ice), Io is primarily composed of silicate rock surrounding a molten iron or iron sulfide core. Io has one of the most geologically active surfaces in the solar system, with over 400 active volcanoes. This extreme geologic activity is the result of tidal heating from friction generated within Io's interior by Jupiter's varying pull. Several volcanoes produce plumes of sulfur and sulfur dioxide that climb as high as 500 km (310 mi). Io's surface is also dotted with more than 100 mountains that have been uplifted by extensive compression at the base of the moon's silicate crust. Some of these peaks are taller than Earth's Mount Everest. Most of Io's surface is characterized by extensive plains coated with sulfur and sulfur dioxide frost.

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Major topics

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 ˑ Planetary habitability ˑ Terrestrial planets ˑ Gas giants ˑ Rings) ˑ Dwarf planets (Plutoid) ˑ Moons ˑ Exploration ˑ Colonization ˑ Discovery timeline

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

Bold articles are featured.
Italicized articles are on dwarf planets or major moons.

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