Most planets form when a molecular cloud collapses into a young star. The leftover gas and dust form a disk around the star, and the particulates inside the disk begin to collide and coalesce over millions of years, forming larger and larger objects until a planet eventually takes shape. - See more at: http://www.astrobio.net/topic/deep-space/cosmic-evolution/the-birth-of-a-planet/#sthash.RjGXT4xY.dpuf