It is not possible to consider the question of the origin of the earth apart from the question of the origin of the solar system. That all the planets, as well as the asteroids, originated from a common parent-mass has never been seriously questioned. All of these bodies revolve about the sun, and rotate upon their axes in the same direction--from west to east. Moreover, all of the planetary orbits lie very nearly in the same plane and are nearly circular in form.
The orbits of the asteroids are more elliptical and more highly inclined to one another than are the orbits of the planets, but on the average they are neither very elliptical nor very highly inclined to the planetary orbits.
The sun rotates upon its axis in the same direction in which the planets rotate and perform their revolutions, and the orbits of the planets are inclined at small angles to the plane of the sun's equator.
These facts are all significant and cannot be overlooked in formulating a theory to explain the origin of the planetary system in general and of the earth in particular. Presumably the planets and asteroids formed at one time a part of a central body which rotated on its axis in the direction in which they now revolve about the sun.
When and by the operation of what force, external or internal, they were separated from this central body is the question.
In 1796 La Place advanced his celebrated _nebular hypothesis_ to explain the origin of the solar system. It was received with favor both by scientists and laymen, and in a short time was almost universally accepted as closely approximating to the truth.
According to the nebular hypothesis the solar nebula from which the planetary system was formed, originally extended at least as far as the orbit of Neptune and rotated slowly in the direction in which the planets now revolve. As it lost heat by radiation and contracted under the gravitation of its parts its rate of rotation increased. When the centrifugal (center-fleeing) force at the equator equalled the gravitational force directed toward the center, a ring would be left behind by the contracting nebula. Such a ring would not be absolutely uniform and would break at some point and gather into a planetary mass under the gravitation of its parts. This planetary mass would abandon rings in turn and these would break up to form satellites. Successive rings were supposed to have been abandoned at intervals by the solar nebula at the present distances of the planets from the sun in the manner described above until the original solar nebula had contracted to its present size.
