Wikipedia:Today's featured article/September 5, 2014

The nebular hypothesis is the most widely accepted model in cosmogony explaining the formation and evolution of the Solar System, which suggests that it formed from nebulous material in space. The hypothesis offers explanations for some of the Solar System's properties, including the nearly circular and coplanar orbits of the planets, and their motion in the same direction as the Sun's rotation. According to the hypothesis, Sun-like stars form over about 100 million years, in massive, gravitationally unstable clouds of molecular hydrogen (giant molecular clouds). Matter coalesces to smaller, denser clumps within, which then rotate, collapse, and form stars. Star formation produces a gaseous protoplanetary disk around the young star, which may give birth to planets (protoplanetary disk pictured in the Orion Nebula). The formation of planetary systems is thought to be a natural result of star formation, with dense terrestrial planets forming closer to the star and colder giant planets forming further away, beyond the so-called frost line. Originally applied only to our own Solar System, the nebular hypothesis is now thought to be at work throughout the universe. (Full article...)

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