The Solar System<ref group="lower-alpha">Capitalization of the name varies. The IAU, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects, but uses mixed "Solar System" and "solar system" in their naming guidelines document. The name is commonly rendered in lower case ("solar system"), as, for example, in the Oxford English Dictionary and Merriam-Webster's 11th Collegiate Dictionary.</ref> comprises the Sun and the planetary system that orbits it, either directly or indirectly.<ref group="lower-alpha">The moons orbiting the Solar System's planets are an example of the latter.</ref> Of those objects that orbit the Sun directly, the largest eight are the planets,<ref group="lower-alpha">Historically, several other bodies were once considered planets, including, from its discovery in 1930 until 2006, Pluto. See Former planets.</ref> with the remainder being significantly smaller objects, such as dwarf planets and small Solar System bodies such as comets and asteroids.Unknown extension tag "ref" Of those that orbit the Sun indirectly, two are larger than the smallest planet.
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 most 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 largely of substances with relatively high melting points compared with hydrogen and helium, called ices, such as water, ammonia and methane. All planets have almost circular orbits that lie within a nearly flat disc called the ecliptic.
The Solar System also contains smaller objects.<ref group=lower-alpha name=footnoteB /> The asteroid belt, which lies between 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, 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 to have been rounded by their own gravity.<ref name=Stern2012>"Today we know of more than a dozen dwarf planets in the solar system".The PI's Perspective</ref> Such objects are categorized as dwarf planets. Identified dwarf planets include the asteroid Ceres and the trans-Neptunian objects Pluto and Eris.<ref group=lower-alpha name=footnoteB /> In addition to these two regions, various other small-body populations, including comets, centaurs and interplanetary dust, freely travel between regions. Six of the planets, at least three of the dwarf planets, and many of the smaller bodies are orbited by natural satellites,<ref group=lower-alpha>See List of natural satellites of the Solar System for the full list of natural satellites of the eight planets and first five dwarf planets</ref> 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 interstellar wind; it extends out to the edge of the scattered disc. The Oort cloud, which is believed 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.
Solar System sections
Intro Discovery and exploration Structure and composition Formation and evolution Sun Interplanetary medium Inner Solar System Outer Solar System Comets Trans-Neptunian region Farthest regions Galactic context Visual summary See also Notes References External links
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