The nebula theory (alternately, nebular hypothesis) is model of the formation of a solar system. It states that stars are formed from a cloud of gas, primarily hydrogen, that contracts. The pressure at the center of the cloud would be so enormous that nuclear fission is started. The remnants of the cloud then proceed to give rise to planets, asteroids, and various other celestial bodies. Over time, the solar wind pushes away what remains of the nebula, and the star progresses through its lifespan. This theory explains the reason that long period comets, and extremely distant dwarf planets can exist.