Nebular Hypothesis

in astronomy, a theory of the formation of the solar system. It was propounded originally by Swedenborg and was passed on by Kant to La Place, "the French Newton," who elaborated the theory and gave it currency. It is associated usually with the name of La Place. According to this theory the material of which the present solar system--sun, planets, moons, and asteroids--is composed was distributed in a vast gaseous mass extending beyond the orbit of Neptune. Under the influence of gravitation and motion the mass contracted, throwing off planet after planet, the remotest planet first. The sun, at the center, is what was left after the mass became reduced sufficiently to hang together. La Place was of the opinion that each planet disengaged itself from the central mass in the form of a ring, such as now surrounds the planet Saturn. The moons of each planet are supposed to have been thrown off from the planets around which they revolve. The entire theory is undergoing study at the present time. It has been suggested, for instance, that our moon and the other moons are captured planets, or comets, that is to say, that they formerly described orbits about the sun, but that they came so near the planets they now attend, that they were captured, drawn within the influence of their respective planets, and thus became satellites.