Are astronomers searching for the 10th planet?

Astronomers are always on the look out for new members of the solar system. The most active programs involve searches for new comets, asteroids and satellites of the outer planets, but there is an on-again, off-again search for trans-plutonian planets that some ( a hand full) of astronomers have undertaken over the last decade. Part of the difficulty is in deciding what defines a planet. If Pluto is a planet, how about something 2 or 3 times smaller making it just a little bigger than the asteroid Ceres? Astronomers can pretty much rule out an object as big as Jupiter orbiting a little further out than Pluto, since its gravitational perturbation on Pluto's orbit would be pretty obvious. But a planet as large as the Earth at twice Pluto's distance would be almost undetectable.

So, the answer is that we simply don't know if a bonifide 10th planet exists beyond the orbit of Pluto if its mass is too small, but we can rule-out very large new planets if they are just beyond Pluto's orbit and not too far.