Meet the primordial asteroid family

One of the major goals of planetary science is to understand the formation of all the bodies within our solar system, including the nearly one million known asteroids. There are two main competing theories (see the figure). The first and classical theory suggests that these bodies formed incrementally, starting as dust grains and accumulating bit by bit until they reached their final size. The second and more recent theory (1, 2) suggests that these bodies formed almost instantly through the gravitational collapse of clusters of pebble-sized material in the protoplanetary disk into single bodies hundreds or thousands of kilometers in diameter. This method skips the meter-to-kilometer intermediate size range that has been problematic to quantify with the classical method. On page 1026 of this issue, Delbo' et al. (3) find compelling observational evidence that when the asteroids formed, they were initially of large size, thus favoring the second model.