Vulcanoid Zone 
In the late 18th century, astronomers around the world frantically searched for a planet orbiting inside the orbit of Mercury, the closest planet to the Sun. Some oddities in Mercury's orbit led them to believe that it was being tugged by the gravity of an unseen planet just a few million miles from the Sun.
Despite some excited claims to the contrary, no one ever found such a planet, which was known as Vulcan. But calculations showed that other objects could orbit at that distance from the Sun: asteroids. Astronomers have been searching for them ever since. But like Vulcan itself, possible vulcanoids have remained elusive.
The most recent search was conducted with a pair of Sun-watching satellites known as STEREO. They observe the Sun from different angles, providing a 3-D view. They also see the space around the Sun, including what's called the "vulcanoid zone" -- a band around the Sun where vulcanoids could maintain stable orbits.
If there are any objects in this zone as small as three miles across, STEREO should be able to see them. But a recent search through 40 days of its observations found nothing.
The astronomers who sifted through STEREO's data say that if any asteroids larger than that ever did inhabit the vulcanoid zone, they most likely were ground down by repeated collisions, or were thrown out of the zone by close encounters.
So the search will continue -- for small inhabitants of the vulcanoid zone.
Script by Damond Benningfield, Copyright 2010