Seriously, though, I would think such a metric might be a good way of defining planets, since it represents something which can be measured for distant planets even if we can't see them. Just have to accept that Pluto's an oddball.Thanks, great reply. Out of curiousity, what number does Mercury give you, compared with, say, Neptune? To do the whole broken record thing, the number needed to make a planet would have to be arbitrary; Ceres is known to be spherical (although that was doubted until recently); not sure about Pallas and Vesta, but I think that they qualify as dwarf planets under the IAU "standard".
Mercury and Neptune are apples and oranges. Mercury is a silicate world like the other inner planets (all the way to Mars) and Neptune belongs with the icy gas giants. Totally different worlds, totally different compositions.
I'd say that a planet is a typically spherical world orbiting a star with a gravity that allows one to walk around on it and that is not considered a moon. On Pluto a grown man is about 13 pounds, or half what he would weigh on the moon. And no, you could not launch yourself into space by jumping on Pluto any more than the astronauts on the moon could even reach 10 feet by jumping.
You can land a spaceship on Pluto, get out and walk around and it orbits the sun. Sure, it's in a dance with its moon but it's clearly bigger than it and it clearly has an atmosphere (one that freezes, but so what).
Also, there is Eris, a body which is even bigger than Pluto but much, much farther out. It has a moon (or moons, can't remember). Never made the planet list though.