well if i had to create a definition for a planet it would be; a hunk of rock or ball gas of at least ____(enter number here)______km in diameter that orbits primarily the sun (so you don't have any moons being considered as planets)
that is one of the things that we are trying to find out with this mission to Pluto. Scientists are trying to figure out what those numbers are that u mentioned. And once they figure that out we will be able to classify if Pluto is a planet or just a rock and same thing goes for the other 2 rocks past Pluto.
How would they do that...measure the diameter of Pluto? Wouldn't that be hard...haha I don't think that they would have to walk the whole length of Pluto and measure it. But how else would they be able to do it?