The Solar System[a] is the Sun and the objects that orbit the Sun. These are a planetary system of eight planets[b]and various secondary bodies, dwarf planets and small Solar System objects that orbit the Sun directly,[c] as well as satellites (moons) that orbit many planets and smaller objects. The Solar System formed 4.6 billion years agofrom 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 andMars, 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 theecliptic plane.