The Solar System consists of
the Sun and its planetary system of eight planets, their moons, and other
non-stellar objects. It formed 4.6 billion years ago from the gravitational
collapse of a giant molecular cloud. The vast majority of the system's mass is
in the Sun, with most of the remaining mass contained in Jupiter. The four
smaller inner planets, Mercury, Venus, Earth and Mars, also called the
terrestrial planets, are primarily composed of rock and metal. The four outer
planets, called the gas giants, are substantially more massive than the
terrestrials. The two largest, Jupiter and Saturn, are composed mainly of
hydrogen and helium; the two outermost planets, Uranus and Neptune, are composed
largely of substances with relatively high melting points (compared with
hydrogen and helium), called ices, such as water, ammonia and methane, and are
often referred to separately as "ice giants". All planets have almost circular
orbits that lie within a nearly flat disc called the ecliptic plane.