Formation and evolution of the Solar System System bodies have been created by both collisions and capture. Mars's two small moons, Deimos and Phobos, are thought to be captured asteroids. The Earth's Moon is thought to have formed as a result of a single, large head-on collision. The impacting object probably had a mass comparable to that of Mars, and the impact probably occurred near the end of the period of giant impacts. The collision kicked into orbit some of the impactor's mantle, which then coalesced into the Moon. The impact was probably the last in the series of mergers that formed the Earth. It has been
Definition of planet object can be irregular in shape, but beyond that point gravity begins to pull an object towards its own centre of mass until the object collapses into an ellipsoid. (None of the large objects of the Solar System are truly spherical. Many are spheroids, and several, such as the larger moons of Jupiter and Saturn and the dwarf planet , have been further distorted into ellipsoids by rapid rotation or tidal forces, but still in hydrostatic equilibrium.) However, there is no precise point at which an object can be said to have reached hydrostatic equilibrium. As Soter noted in his