Vulcanoid asteroids are a special group of asteroids which may orbit close to the Sun, closer than the orbit of the planet Mercury. No vulcanoid has ever been discovered; it is not known if they ever existed. Because they would orbit so close to the Sun they could only be seen during twilight, or a solar eclipse. Their very small size could make them impossible to see. This part of the Solar System is described as being gravitationally stable, and every other similar stable area has asteroids. If they do exist, vulcanoids would be made of material that could help explain the formation of the planets. If they do not exist, then theories about how the planets were formed might have to change. They are called vulcanoids after a planet, Vulcan, which early astronomers thought they had seen close to the Sun.