Astronomers are always on the lookout for new members of the solar system. The most active programs involve searches for new comets, asteroids and satellites of the outer planets, but over the last decade a handful of scientists have undertaken an on-again, off-again search for planets beyond Pluto. Part of the difficulty is in deciding what defines a planet. If Pluto is a planet, how about something two or three times smaller, which would make it just a little bigger than the asteroid Ceres?

Astronomers can rule out an object as big as Jupiter orbiting a little further out than Pluto, since its gravitational effect on Pluto's orbit would be pretty obvious. It would also be easily visible because of its brightness. To go undetected, a planet similar in size and composition to the other outer planets would have to be very far from the sun.

We do know of many other objects orbiting the Sun that are similar to Pluto, which belongs to the Kuiper Belt -- a field of icy, comet-like bodies located beyond Neptune. In 2005, an object likely larger than Pluto, UB313, was located in the Kuiper Belt.

Find out more on HubbleSite:

• What is Planet X?