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 four outer planets are giant planets, being substantially more massive than the terrestrials. 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. Such objects are categorized as dwarf planets. Identified dwarf planets include the asteroid Ceres and the trans-Neptunian objects Pluto and Eris. This was the first evidence that anything other than the planets orbited the Sun. The remaining objects of the Solar System (including the four terrestrial planets, the dwarf planets, moons, asteroids, and comets) 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 overall structure of the charted regions of the Solar System consists of the Sun, four relatively small inner planets surrounded by a belt of mostly rocky asteroids, and four giant planets surrounded by the Kuiper belt of mostly icy objects. The inner Solar System includes the four terrestrial planets and the asteroid belt. The outer Solar System is beyond the asteroids, including the four giant planets. Kepler's laws of planetary motion describe the orbits of objects about the Sun. The orbits of the planets are nearly circular, but many comets, asteroids, and Kuiper belt objects follow highly elliptical orbits. Objects farther from the Sun are composed largely of materials with lower melting points. With a few exceptions, the farther a planet or belt is from the Sun, the larger the distance between its orbit and the orbit of the next nearer object to the Sun. Some Solar System models attempt to convey the relative scales involved in the Solar System on human terms. The Sun is the Solar System's star and by far its most massive component. The Sun is a G2-type main-sequence star. The inner Solar System is the region comprising the terrestrial planets and the asteroid belt. The term inner planet should not be confused with inferior planet, which designates those planets that are closer to the Sun than Earth is (i.e. Mercury and Venus).
Mercury (0.4 AU from the Sun) is the closest planet to the Sun and on average, all seven other planets. The smallest planet in the Solar System (0.055 M), Mercury has no natural satellites. It has one natural satellite, the Moon, the only large satellite of a terrestrial planet in the Solar System.
The asteroid belt occupies the orbit between Mars and Jupiter, between 2.3 and 3.3 AU from the Sun. Asteroid moons are asteroids that orbit larger asteroids. The inner Solar System also contains near-Earth asteroids, many of which cross the orbits of the inner planets. The outer region of the Solar System is home to the giant planets and their large moons. The four outer planets, or giant planets (sometimes called Jovian planets), collectively make up 99% of the mass known to orbit the Sun. The term superior planet designates planets outside Earth's orbit and thus includes both the outer planets and Mars.
Ganymede, the largest satellite in the Solar System, is larger than Mercury.
Titan, the second-largest moon in the Solar System, is larger than Mercury and the only satellite in the Solar System with a substantial atmosphere.
Some comets with hyperbolic orbits may originate outside the Solar System, but determining their precise orbits is difficult. Though it is estimated to contain anything from dozens to thousands of dwarf planets, it is composed mainly of small Solar System bodies. The dwarf planet Pluto (39 AU average) is the largest known object in the Kuiper belt. Pluto has a 3:2 resonance with Neptune, meaning that Pluto orbits twice round the Sun for every three Neptunian orbits. Kuiper belt objects whose orbits share this resonance are called plutinos. It is thought to be composed of comets that were ejected from the inner Solar System by gravitational interactions with the outer planets. Much of the Solar System is still unknown. Objects may yet be discovered in the Solar System's uncharted regions. This revolution is known as the Solar System's galactic year. The closest solitary Sun-like star to the Solar System is Tau Ceti at 11.9 light-years. Compared to many other planetary systems, the Solar System stands out in lacking planets interior to the orbit of Mercury. The known Solar System also lacks super-Earths (Planet Nine could be a super-Earth beyond the known Solar System). The orbits of Solar System planets are nearly circular. Solar System
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