Modern Ideas

In the early decades of the 20th century, several scientists decided that the deficiencies of the nebular hypothesis made it no longer tenable. The Americans Thomas Chrowder Chamberlin and Forest Ray Moulton, as well as James Jeans and Harold Jeffreys of Great Britain, independently developed variations on the idea that the planets were formed catastrophically—i.e., by a close encounter of the Sun with another star. The basis of this model was that material was drawn out from one or both stars when the two bodies passed at close range, and this material later coalesced to form planets. A discouraging aspect of the theory was the implication that the formation of solar systems in the Milky Way Galaxy must be extremely rare, because sufficiently close encounters between stars would occur very seldom.

The next significant development took place in the mid-20th century as scientists acquired a more mature understanding of the processes by which stars themselves must form and of the behaviour of gases within and around stars. They realized that hot gaseous material stripped from a stellar atmosphere would simply dissipate in space; it would not condense to form planets. Hence, the basic idea that a solar system could form through stellar encounters was untenable. Furthermore, the growth in knowledge about the interstellar medium indicated that large clouds of such matter exist and that stars form in these clouds. Planets must somehow be created in the process that forms the stars themselves. This awareness encouraged scientists to reconsider certain basic processes that resembled some of the earlier notions of Kant and Laplace.

The current approach to the origin of the solar system treats it as part of the general process of star formation. As observational information has steadily increased, the field of plausible models for this process has narrowed. This information ranges from observations of star-forming regions in giant interstellar clouds to subtle clues revealed in the existing chemical composition of the objects present in the solar system. Many scientists have contributed to the modern perspective, most notably the Canadian-born American astrophysicist Alistair G. W. Cameron.

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