Formation of Planets

     It is not exactly known how planets are formed. The leading theory is that they are formed during the collapse of a nebula. A protostar, or pre-star, forms at the core, surrounded by a rotating disk of dust and other material. Through collision dust particles in the disk steadily accumulate to form ever-larger bodies. Local concentrations of mass form, and these accelerate the collision process by drawing in additional material with their gravitational field. These concentrations become denser until they collapse inward under gravity to form protoplanets or pre-planets. After a planet reaches a diameter larger than the Earth's moon, it begins to accumulate an atmosphere, greatly increasing the capture rate of material by means of atmospheric drag.  When the protostar has grown large enough to ignite and form a star, the surviving disk is removed from the inside outward by the heat of the star and many other factors. Afterward there still may be many protoplanets orbiting the star or each other, but over time many will collide, either to form a single larger planet or release material for other larger protoplanets or planets to absorb.  Those objects that have become massive enough will capture most matter in their orbital neighborhoods to become planets. Meanwhile, protoplanets that have avoided collision become natural satellites of planets through a process of gravitational capture.