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 with 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, some objects are large enough to have rounded under their own gravity, though there is considerable debate as to how many there will prove to be.
[9][10] Such objects are categorized as
dwarf planets. Identified or accepted 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 travel between regions. Six of the planets, the six largest possible dwarf planets, and many of the smaller bodies are orbited by
natural satellites,
[f] usually termed "moons" after the
Moon. Each of the outer planets is encircled by
planetary rings of dust and other small objects.
