The New Student's Reference Work/Nebular Hypothesis
Neb′ular Hypoth′esis is a theory advanced by three different men, Swedenborg (1688–1772), Kant (1724–1804) and Laplace (1749–1827), to account for the observed facts of planetary motion. The more important phenomena to be accounted for are these: (1) The orbits of all the planets are nearly circular and lie all nearly in one plane. (2) The direction of revolution about the sun is the same for all planets. (3) Except in the case of Uranus and Neptune, the direction of rotation of the planet on its axis is the same as its direction of revolution. (4) The larger planets rotate (not revolve) more rapidly. (5) The plane of rotation is not very different from the plane of the orbit. (6) The satellites generally have a direction of revolution which coincides with that of the planet’s rotation. To explain this rather orderly state of affairs it was supposed that the matter now constituting this solar system was at some earlier date in its existence distributed in the form of an immense nebula; and that, as this nebula condensed and therefore increased its rate of rotation (while preserving a constant moment of momentum), the centrifugal force became so great that some of the outer portions of the nebula were set free. That is, the centrifugal force reached a point where it balanced the attraction of the rest of the nebula. The portions thus set free, whether as a ring or as a “hump,” condensed still farther and formed the earlier planets. As the concentration of the original nebula proceeded, the rate of rotation kept on increasing and again “threw off” or set free other planets. So also with the planets themselves; as they became more and more compact, their rate of rotation increased sufficiently for them to set free their satellites.
In a general way this hypothesis satisfactorily explains the six facts enumerated above. With later modifications it explains even many of the anomalies of the solar system. See Laplace. For recent criticism of this hypothesis see article by Moulton in Astrophystcal Journal, Vol. II. (1900).