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| system_mass = 1.0014 Solar masses | neareststar = }} | nearestplanetary = | semimajoraxis = | Kuiper_cliff = 50 AU | stars = 1 | noknown_stars = yes | planets = }} | outerplanetname = Neptune | noknown_planets = yes | dwarfplanets = }} | satellites = |}}}} | minorplanets = 698,523 | comets = 3,372 | roundsat = 19 | roundsatlink = List of gravitationally rounded objects of the Solar System#Satellite planemos | inclination = 60.19° | galacticcenter = 27,000 ± 1,000 ly | orbitalspeed = 220 km/s | orbitalperiod = 225–250 Myr | spectral = G2V | frostline = ≈5 AU | heliopause = ≈120 AU | hillsphere = ≈1–2 ly }} The Solar System〔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'' ).〕 comprises the Sun and the planetary system that orbits it, either directly or indirectly.〔The moons orbiting the Solar System's planets are an example of the latter.〕 Of those objects that orbit the Sun directly, the largest eight are the planets,〔Historically, several other bodies were once considered planets, including, from its discovery in 1930 until 2006, Pluto. See Former planets.〕 with the remainder being significantly smaller objects, such as dwarf planets and small Solar System bodies such as comets and asteroids. Of those 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 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 mostly 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. 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 to have been rounded by their own gravity.〔"Today we know of more than a dozen dwarf planets in the solar system".(The PI's Perspective )〕 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, 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,〔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〕 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. == Discovery and exploration == (詳細は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 Galilei, Johannes Kepler, and Isaac Newton developed an understanding of physics that led to the gradual acceptance of the idea that Earth moves around the Sun and that the planets are governed by the same physical laws that governed Earth. The invention of the telescope led to the discovery of further planets and moons. Improvements in the telescope and the use of unmanned spacecraft have enabled the investigation of geological phenomena, such as mountains, craters, seasonal meteorological phenomena, such as clouds, dust storms and ice caps on the other planets. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Solar System」の詳細全文を読む スポンサード リンク
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