Planetesimals are the fundamental building blocks of the planets as well as the ancestors of asteroids and comets. To understand them and their importance, one must first understand how planets form. The solar system formed 4.6 billion years ago from...

Planetesimals are solid objects thought to exist in protoplanetary disks and in debris disks. A widely accepted theory of planet formation, the so-called planetesimal hypothesis of Viktor Safronov, states that planets form out of dust grains that...