The Solar System[d] is the gravitationally bound system of the Sun and the masses that orbit it, most prominently its eight planets, of which Earth is one.[11] The system formed about 4.6 billion years ago when a dense region of a molecular cloud collapsed, creating the Sun and a protoplanetary disc from which the orbiting bodies assembled. Inside the Sun's core hydrogen is fused into helium for billions of years, releasing energy which is over even longer periods of time emitted through the Sun's outer layer, the photosphere. This creates the heliosphere and a decreasing temperature gradient across the Solar System.
The mass of the Solar System is by 99.86% almost completely made up of the Sun's mass. The next most massive objects of the system are the eight planets, which by definition dominate the orbits they occupy. Closest to the Sun in order of increasing distance are the four terrestrial planets – Mercury, Venus, Earth and Mars. These are the planets of the inner Solar System. Earth and Mars are the only planets in the Solar System which orbit within the Sun's habitable zone, in which the sunlight can make surface water under atmospheric pressure liquid. Beyond the frost line at about five astronomical units (AU),[e] are two gas giants – Jupiter and Saturn – and two ice giants – Uranus and Neptune. These are the planets of the outer Solar System. Jupiter and Saturn possess nearly 90% of the non-stellar mass of the Solar System.
Additionally to the planets there are in the Solar System other planetary-mass objects, but which do not dominate their orbits, such as dwarf planets and planetary-mass moons. Dwarf planets there are generally nine Solar System objects identified as such: Ceres, Orcus, Pluto, Haumea, Quaoar, Makemake, Gonggong, Eris, and Sedna.[f] Natural satellites, which are commonly called 'moons', can be found throughout the Solar System and in sizes from planetary-mass moons to much less massive moonlets at their smallest. The largest two moons (Ganymede of Jupiter and Titan of Saturn) are larger than the smallest planet (Mercury), while the seven most massive, which includes Earth's Moon, are more massive and larger than any of the dwarf planets. Less massive than these planetary-mass objects are the vast number of small Solar System bodies, such as asteroids, comets, centaurs, meteoroids, and interplanetary dust clouds. All dwarf planets and many of the smaller bodies are within the asteroid belt (between Mars's and Jupiter's orbit) and the Kuiper belt (just outside Neptune's orbit).[g]