Free Astronomy Magazine November-December 2017

NOVEMBER-DECEMBER 2017 T his diagram compares our solar system with that of Epsilon Eri- dani, whose struc- tures are similar, as both have belts of asteroids and comets. raised the perihelion of the orbits of the planetesimals in transit in its vicinity. Jupiter could then have done the rest, reducing the eccentricity of those orbits and trapping the planetesimals within dangerous planetary resonances, whose transience (the planetary migration was at its end) could nevertheless have allowed the planetesimals to become stabilised in relatively peaceful final orbits. The planetesimals that survived the forma- tion of the terrestrial planets in the ring and were rerouted into the belt would comprise the S-type population, whose mineralogy is similar to that of the inner planets (from Mercury to Mars). According to what the two authors of the study stated in Science Advances , their simulations placed within the inner part of the belt a mass of plan- etesimal from 3 to 4 times larger than the total actual mass of the S-type asteroids pre- sent today, a proportion compatible with the mechanisms for depleting the belt used in the classic theory. The simulations by Ray- mond and Izidoro also provide a rather con- vincing scenario for the origin of the outer- most part of the asteroid belt (more than 2.7 AU from the Sun), the one dominated by the C-type population, whose total mass is roughly triple of that of the S-types. In this case, the growth of Jupiter and Saturn would have destabilised a large number of planetesimals, distributed mainly between 4 and 9 AU from the Sun. The gas attracted by the planets in those areas of the solar system would have put the brakes on the orbital motion of the planetesimals, encour- aging their relocation in orbits closer to the Sun and especially in the outer part of the belt. Altogether, the new scenario proposed by Raymond and Izidoro is definitely in- triguing and shows that our knowledge of the origin of the main asteroid belt is not as solid as we previously believed. Rather than being the traces of a planet that never formed between Mars and Jupiter, those asteroids would be a by-product of the formation of the terrestrial planets and the gaseous planets. !