No. 10

On the Origin of the Asteroids.

The hypothesis of the origin of the solar systems I had the honour to send you, seems to demand that the production of the asteroids and of Saturn's rings should have been subsequent events to the formation of the solar system. I have consequently investigated these systems, and I will now give a short account of what appears to me to be a possible explanation. It is well known that Olbers, who discovered several of these planets, supposed that they had been produced by the explosion of a planet. He also argued that, if so, all the parts should pass through one common spot in their several orbits, and be searched the heavens on this principle, and persuaded Harding to search also; and he actually discovered Juno in looking at the intersection of the two orbits of Ceres and Pallas. Subsequently, however, many more were found, and several of these departed altogether from this law. The idea was then given up; especially as, on the other band, it was conceived impossible that any force in nature could produce such an explosion. I shall attempt to shew that probably Olbers was right in his original suggestions, but altogether wrong in his idea about the common point in the orbit. Of course, an instantaneous explosion would produce such a result at first, but the altered eccentricity of the orbits of the several pieces, due to the powerful attractions of the larger planets, would soon displace them from the common point. But from the great distance, about 90,000,000 of miles, which exists between the greatest perhelions and least aphelions of these bodies and from other causes, it is generally considered absolutely impossible that these bodies could have been formed from a planet. If we look upon a cosmical explosion as an instantaneous phenomenon, it appears that this conclusion may be sound.

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I shall attempt to show: (1st) that the explosion of a planet is not such a very unlikely event; (2ndly) that as such an explosion will require definite durations for its completion, and from other causes, the usual ideas about these orbits require modifications. That, therefore, the asteroids may very possibly be parts of a planet, notwithstanding the very cogent arguments to the contrary brought forward in a recent article, which appeared in the "Cornhill Magazine," called "A Ring of Worlds."

Actual observation shows that there are bodies travelling in space with a high independent velocity, and reasoning based on the hypothesis of "partial impact," renders it probable that these bodies exist in very considerable numbers. Arguing on principles of energy, it may be shown that a body, having a mass about one-hundredth that of the earth, and moving with the velocity of about one hundred miles a second, would possess sufficient energy to blow the earth into fragments if it struck it fairly. Supposing such a body to strike a planet it would penetrate into its mass until its momentum was destroyed. The heat developed would make it and the material in its path into gas at a pressure of tens of thousands of atmospheres. This pressure would have to overcome the inertia of the mass, which of course it would do slowly, and would follow up the mass as it travelled. The larger the striking mass the greater would be the volume of gas produced; and, as the velocity would not then be required to be so great, the pressure might be smaller and would act for a longer period. It is not improbable that the pressure may act for some days. Again, it is reasonable to suppose that the impact would be eccentric. In such a case it is clear that rotation would be developed, and this rotation would also tend to alter the orbits. It is most important to remember that not until the pressure has completely ceased to act, and the mutual attractions of the various pieces have become too small by distance, to influence the result, will the directions and velocity of the motions of the fragments becomes portion of established orbits. Generally those bodies which were most retarded and most accellerated would have the most eccentric orbits, and clearly any perturbation which tended to render these orbits more circular would increase the distance of these orbits.

With respect to the velocity necessary to be given to a particle in order for it to leave the original body, it is clear that, if the pressure were sufficient to commence motion, both the lessened attraction and acquired motion would enable a lessened pressure to continue the motions, and, consequently, until the pressure ceased, the velocity need never have been enough to take them from effective attraction, and then, as they would be so far from the centre, the final radial velocity might be very small. In fact it is only necessary that the energy of the explosion should be equal to the total energy of gravitation of such a mass to effect the disruption. It is extremely probable, also, that for some time a mass of spray would be travelling approximately in the old orbit of the planet and, although very rare, it would be of considerable extent. This would tell in Jupiter's selective action, in the production of the ring spaces. The existence of these spaces renders it certain that the explosion must have occurred millions of years ago.

Every other fact in connection with these bodies points to the conclusion that they have been so formed. Most strikingly the great inclinations of many of their orbital planes, which exactly agrees with this hypothesis, but appears unaccountable on any other theory. Having thus attempted to clear up the apparent objections offered by the asteroids to this new hypothesis of the origin of the solar system, I shall in my next letter attempt to shew its application to the origin of the visible universe.