Astronomers are always on the look out for new members of the solar system.
Part of the difficulty is in deciding what defines a planet. 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. Copyright 1997 Dr. Sten Odenwald Return to Ask the Astronomer .