Free Astronomy Magazine November-December 2017

7 NOVEMBER-DECEMBER 2017 SOLAR SYSTEM vented and interrupt- ed by Jupiter’s grow- ing mass, which was more than enough to throw that part of the solar system into disar- ray. This scenario grad- ually gained credibility during the twentieth century, but when the complexity of that pop- ulation of asteroids became clearer in re- cent decades and it was determined that the planetary systems were characterised by an initial migration of the planets within them, the theory of the planet that failed to form began to show its weaknesses. The best mathematical models, in fact, that explain the origin and evolution of the main asteroid belt are propped up by a rather forced assumption: that during the first 100 million years of its existence, the belt must have somehow lost at least 99% of its initial mass. The migration of the planets (Jupiter, essentially) and the re- sulting dynamic chaos, along with the col- lisional evolution of the asteroids, surely O n the right, an overview of the asteroid belt. Below, an animation that shows the orbital motions of over 100,000 of the as- teroids observed by the Sloan Digi- tal Sky Survey, with colors illus- trating the com- positional diversity mea- sured by the SDSS five-color camera. The rela- tive sizes of each asteroid are also illustrated. All main-belt aster- oids and Trojan asteroids with or- bits known to high precision are shown. The ani- mation is ren- dered with a timestep of 3 days. The compo- sitional gradient of the asteroid belt is clearly visi- ble, with green Vesta-family members in the inner belt fading through the blue C-class asteroids in the outer belt, and the deep red Trojan swarms beyond that.The average orbital distances of Mer- cury, Venus, Earth, Mars, and Jupiter are illus- trated with rings. [Alex H. Parker: alexharrison- parker.com - Music: Tamxr by LJ Kruzer (ljkruzer.co.uk ) contributed to removing a significant mass from the belt, but explaining why that small fraction remained is a problem for which no elegant and wholly convincing solution exists. Among the least convinced are Sean N. Raymond (Université de Bor- deaux, CNRS) and Andre Izidoro (Univer- sité de Bordeaux, CNRS, and Universidade Estadual Paulista), who proposed an al- ternative hypothesis last September in the magazine Sci- ence Advances on the origin of the main asteroid belt— one that is diametri- cally opposed to the classic one. The two researchers claim that at the dawn of the solar system that area was not more populated than to- day but, on the con- trary, was complete- ly empty. The hun- dreds of thousands