With few exceptions the asteroids are only to be seen in large telescopes and then only as star-like points of light. Most of them are simply huge rocks and all are necessarily devoid of life since such small bodies have not sufficient gravitational force to hold an atmosphere.
The revolution of the planets around the sun and of the satellites of the planets around the primary planets are performed according to known laws of motion that make it possible to foretell the positions of these bodies years in advance. Asteroids and comets also obey these same laws, and after three observations of the positions of one of these bodies have been obtained its future movements can be predicted. All the planets and their satellites are nearly perfect spheres. They all, with few exceptions, rotate on their axes and revolve around the sun, or, in the case of moons, around their primaries, in the same direction, from west to east. Only the two outermost satellites of Jupiter, the outermost satellites of Saturn and the satellites of Uranus and Neptune retrograde or travel in their orbits from east to west, which is opposite to the direction of motion of all the other planets and satellites.
The paths of all the planets around the sun are ellipses that are nearly circular, and they all lie nearly in the same plane. The asteroids have orbits that are more flattened or elliptical and these orbits are in some instances highly inclined to the planetary orbits. The comets have orbits that are usually very elongated ellipses or parabolas. Some of the comets may be only chance visitors to our solar system, though astronomers generally believe that they are all permanent members. Paths of comets pass around the sun at all angles and some comets move in their orbits from west to east, while others move in the opposite direction or retrograde. The behavior of the asteroids and comets is not at all in accord with the theory that was, until recently, universally advanced to explain the origin of the solar system.
Some astronomers have made attempts to modify the nebular hypothesis that has held sway for so many years, in order to make it fit in with more recent discoveries, but others feel that a new theory is now required to explain the origin of the solar system. Several theories have been advanced but no new theory has yet definitely replaced the famous nebular hypothesis of the noted French astronomer La Place.