nebular hypothesis

Hypothesis that the Solar System evolved from a nebula. 18th-century Swedish mystic and scientist Emanuel Swedenborg and 18th-century German philosopher Immanuel Kant both put forward explanations of this kind, but 18th–19th-century French astronomer Pierre Laplace was the first to develop a nebular hypothesis on strictly scientific lines.

Laplace's hypothesis was that rotating nebulae were first formed by the condensation of gaseous matter, and that the Sun was originally a nebula of this type. As the nebula continued to shrink, it rotated faster and faster, until fragments broke away from the main body of the Sun. These fragments condensed under their own gravitational attraction and formed the planets. Laplace's hypothesis is untenable, as it fails to explain the fact that nearly all the angular momentum of the Solar System resides in the planets.

The tidal hypothesis of English mathematician and astrophysicist James Jeans, that the planets were formed from material torn out of the Sun by the close approach of a passing star, is also untenable, as the ejected material would disperse into space. Modern theories offer no unique solution, but agree that the Sun and planets condensed simultaneously from a rotating cloud of gas and dust. Magnetic interactions between the early Sun and the surrounding disc of matter are believed to account for the high proportion of the system's angular momentum possessed by the planets.