What are asteroids? Asteroids are rocky-metallic objects which range in size from about the size of pebbles to around 600 miles (~1,000 km) across. Although they orbit the sun, they are too small to be considered planets. Asteroids are thought to be leftover material from the formation of our solar system. Most are found in the Asteroid Belt, a doughnut-shaped ring which lies between the orbits of Mars and Jupiter. Astronomers have also identified a group of asteroids whose orbits cross Earth's orbit. Several hundred thousand asteroids are known to exist in our solar system, and many are yet to be discovered. Most of the undiscovered asteroids are the smaller ones (less than 100 km across) which are more difficult to detect. It is estimated that there are over a million of these smaller asteroids.