That's one of the reasons Pluto was removed from planet status, and it's size makes it more of a dwarf planet as well. It was determined to be one of several objects of similar size in the Kuiper belt. Step 3 of the IAU resolution for determining a planet was what I was referencing. http://en.wikipedia.org/wiki/Pluto#2006:_IAU_classification One of the many ways we can search for new planets around other stars is looking for cleared paths of dust/debris. So if we see a disk of dust/debris from UV/IR telescopes, and there's a "path" carved out of it, we can assume there was a larger object, probably a planet, that had cleared it's local orbit even if we can't see the planet itself. Here's a picture of one such example... The disk is at an angle almost going up an down, but you can see the elipse that has been cleared by a possible planet/s.In fact I didn't know that this was a defining characteristic of a mature planet.
The debate came to a head in 2006 with an IAU resolution that created an official definition for the term "planet". According to this resolution, there are three main conditions for an object to be considered a 'planet':
1.The object must be in orbit around the Sun.
2.The object must be massive enough to be a sphere by its own gravitational force. More specifically, its own gravity should pull it into a shape of hydrostatic equilibrium.
3.It must have cleared the neighbourhood around its orbit.
