Solar System

The Solar System is the gravitationally bound system consisting of the Sun and the objects that orbit it, either directly or indirectly. Of the objects that orbit the Sun directly, the largest eight are the planets, with the remainder being significantly smaller objects, such as dwarf planets and small Solar System bodies. Of the objects that orbit indirectly, the moons, two of the largest, Ganymede and Titan, 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 to 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 orbit or cross the ecliptic.

The discovery of extrasolar planets in the 1990s has stimulated interest in the possibility that life might exist beyond the Solar System, and in habitability. The study of habitability also considers Earth's co-orbital astronomical companion, Venus, as it is a second example of a terrestrial planet. As of March 2021, there are 4,437 known extrasolar planets in 3,155 planetary systems, with 665 systems having more than one planet. There are also many more hypothetical extinct or undiscovered planetary systems, including hypothetical halo planets and hot Jupiters.