Free Astronomy Magazine November-December 2017

NOVEMBER-DECEMBER 2017 8 SOLAR SYSTEM of asteroids that we see there today would have been added to the belt only during and after the planetary migration phase, when the planets were already in an ad- vanced stage of formation. To support this apparently paradoxical scenario, Ray- mond and Izidoro offer arguments that, for the most part at least, seem quite convincing. The idea of an initially empty asteroid belt arose from a fact that has been established for decades: the innermost part of the belt is dominated by what are called S-type asteroids, particularly rich in silicates on their surfaces (and likely underground as well), while the outermost part of the belt is dominated by C-type asteroids, which feature an abundance of carbon. This rough division is actually