DAVID A. HARDY / SCIENCE PHOTO LIBRARY DAVID A. HARDY / SCIENCE PHOTO LIBRARY
Solar system formation. Artwork of the primordial nebula from which all the solar system components originated. The planets and other smaller bodies such as the asteroids formed about 4.6 billion years ago following a process of condensation within the nebula. It is estimated that the mass of the nebula was a few hundred times greater than the mass of the present system. The planets in the early days underwent a heavy bombardment from smaller bodies which were populating the interplanetary space; the craters seen on the surface of some planets and their satellites are the remains of this epoch. The rate of collisions has decreased by about 2000 times since then.
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