The Solar System is the gravitationally bound system of the Sun and the objects that orbit it. It formed 4.6 billion years ago from the gravitational collapse of a giant interstellar molecular cloud. The vast majority (99.86%) of the system's mass is in the Sun, with most of the remaining mass contained in the planet Jupiter. The four inner system planets—Mercury, Venus, Earth and Mars—are terrestrial planets, being composed primarily of rock and metal. The four giant planets of the outer system are substantially larger and more massive than the terrestrials. The two largest, Jupiter and Saturn, are gas giants, being composed mainly of hydrogen and helium; the next two, Uranus and Neptune, are ice giants, being composed mostly of volatile substances with relatively high melting points compared with hydrogen and helium, such as water, ammonia, and methane. All eight planets have nearly circular orbits that lie near the plane of Earth's orbit, called the ecliptic.
There are an unknown number of smaller dwarf planets
and innumerable small Solar System bodies
orbiting the Sun. Six of the major planets, the six largest possible dwarf planets, and many of the smaller bodies are orbited by natural satellites
, commonly called "moons" after the Moon
. Two natural satellites, Jupiter's moon Ganymede
and Saturn's moon Titan
, are larger but not more massive than Mercury, the smallest terrestrial planet, and Jupiter's moon Callisto
is nearly as large. Each of the giant planets and some smaller bodies are encircled by planetary rings
of ice, dust and moonlets. The asteroid belt
, which lies between the orbits of Mars and Jupiter, contains objects composed of rock, metal and ice. Beyond Neptune's orbit lie the Kuiper belt
and scattered disc
, which are populations of objects composed mostly of ice and rock. (Full article...