The Solar System consists of the Sun and the astronomical objects (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune) bound to it by gravity, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. 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". Beyond Neptune's orbit lie trans-Neptunian objects composed mostly of ices such as water ammonia and methane. Within these two regions five individual objects Ceres, Pluto, Haumea, Makemake and Eris are recognized to be large enough to have been rounded by their own gravity and are thus termed dwarf planets.