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.
Mercury and Neptune are apples and oranges.Not in the context of supercat's idea.
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.Yes, I agree, with the slight change (as seen in David Levy's suggestion) that the gravity was sufficient to turn it into a spheroid. :') That brings in Ceres, Pluto, Eris, and some other outer Solar System objects. I've been trying to avoid using the term Kuiper Belt Object (KBO) because I don't think it's relevant any longer.