Our solar system consists of our star, the Sun, and everything bound to it by gravity — the planets Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune, dwarf planets such as Pluto, dozens of moons, and millions of asteroids, comets and meteoroids. The Solar System was formed by the gravitational collapse of a massive interstellar molecular cloud 4.6 billion years ago. The Sun holds the great bulk of the system's mass, with Jupiter holding the majority of the remaining mass. Mercury, Venus, Earth, and Mars, the four smaller inner system planets, are terrestrial planets made mostly of rock and metal. The four planets in the outer system are gigantic planets, with masses far greater than Earth's. Jupiter and Saturn, the two largest planets, are gas giants, consisting primarily of hydrogen and helium; Uranus and Neptune, the two outermost planets, are ice giants, consisting primarily of substances with a low melting point.