Our solar system began to form about 4.6 billion years ago. Astronomers think small rocky and icy grains within the solar nebula began sticking together, growing into even larger objects. Although the process remains poorly understood, solid objects miles or more across eventually populated the disk. Astronomers call these bodies planetesimals. Within a million years after the proto-Sun formed, collisions among planetesimals created larger bodies called planetary embryos, which were roughly as massive as Mars. After another few million years, radiation from the young Sun and nearby stars dispersed the disk’s remaining gas, leaving behind only solid objects, which continued to collide, shatter, and merge to form the planets, asteroids, and other bodies we see today.