We know that the solar system formed from a rotating disk of gas and dust. Matter in the disk with too much angular momentum to fall onto the protosun coalesced at varying orbital distances into planetesimals (see Section 4-2 for more on planetesimals). Many of these chunks of rock and metal eventually collided, forming the planets, dwarf planets, and larger moons of the solar system, as well as myriad pieces of dust, rock, and boulder-sized debris. Others were captured whole by various planets as small, irregularly shaped moons, like Phobos and Deimos in orbit around Mars. Many planetesimals still orbit the Sun today in splendid isolation. These are the asteroids, sometimes called minor planets (not to be confused with dwarf planets).