Despite their differences, the members of the solar system probably form a common family. They seem to have originated at the same time; few indications exist of later captures from other stars or interstellar space.

   Early attempts to explain the origin of this system include the nebular hypothesis of the German philosopher Immanuel Kant and the French astronomer and mathematician Pierre Simon de Laplace, according to which a cloud of gas broke into rings that condensed to form planets. Doubts about the stability of such rings led some scientists to consider various catastrophic hypotheses, such as a close encounter of the Sun with another star. Such encounters are extremely rare, and the hot, tidally disrupted gases would dissipate rather than condense to form planets.

   Current theories connect the formation of the solar system with the formation of the Sun itself, about 4.7 billion years ago. The fragmentation and gravitational collapse of an interstellar cloud of gas and dust, triggered perhaps by nearby supernova explosions, may have led to the formation of a primordial solar nebula. The Sun would then form in the densest, central region. It is so hot close to the Sun that even silicates, which are relatively dense, have difficulty forming there. This phenomenon may account for the presence near the Sun of a planet such as Mercury, having a relatively small silicate envelope and a larger than usual dense iron core. (It is easier for iron dust and vapor than for lighter silicates to coalesce near the central region of a solar nebula.) At larger distances from the center of the solar nebula, gases condensed into such solids as are found today from Jupiter outwards.

   Evidence of a possible preformation supernova explosion appears as traces of anomalous isotopes in tiny inclusions in some meteorites. This association of planet formation with star formation suggests that billions of other stars in our galaxy may also have planets. The high frequency of binary and multiple stars, as well as the large satellite systems around Jupiter and Saturn, attest to the tendency of collapsing gas clouds to fragment into multibody systems.

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