Most asteroids in our Solar System are in the asteroid belt between Mars and Jupiter . There are millions of asteroids , and like most other small Solar System bodies the asteroids are thought to be remnants of planetesimals , material within the young Sunâ s solar nebula that have not grown large enough to form planets . The large majority of known asteroids orbit in the main asteroid belt between the orbits of Mars and Jupiter , however many different orbital families exist with significant populations including Jupiter Trojans and near-Earth asteroids .