
The
Solar System consists of the
Sun and the
astronomical objects gravitationally bound in
orbit around it, all of which
formed from the collapse of a giant
molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun. Of the many objects that
orbit the Sun, most of the
mass is contained within eight relatively solitary
planets whose orbits are almost circular and lie within a nearly flat disc called the
ecliptic plane. 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, 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
ices, such as water, ammonia and methane, and are often referred to separately as "ice giants".