How big does Pluto need to be to be a planet again?

Answer 1

It's not, strictly speaking, a matter of size. The bigger problem is that Pluto's orbit crosses Neptune's, which means that Pluto has not "cleared its orbit" (part of the definition of "planet").

Strictly speaking, planets that have not "cleared their orbits" include Neptune (in addition to Pluto, there are a lot of Neptunian Trojan asteroids), Jupiter (the Trojans), and Earth (there's at least one body in one of Earth's Trojan points). However, in all these cases, the bodies in question are much, much smaller than the planet. Pluto's orbit crossing Neptune's means that it would have to be larger than Neptune to claim with a straight face that it had "cleared its orbit" (in practice, if Pluto were, say, half or even a quarter the size of Neptune, we'd probably create a new category called "co-planets" or something for them).

However, it's not. It's not even close. It's smaller than the Moon by a noticeable fraction.

It's hypothetically possible that the definition of "planet" may be changed in the future (it may even be likely, given that there are quite a few people that aren't entirely happy with the current definition). However, it's not likely that any such redefinition will elevate Pluto to "planet" status once again. It's only due to a mistake (or, better, "incomplete information") that it was ever considered a planet in the first place, and rigging the definition so that Pluto gets included would be perpetuating a historical error.

One way the definition might change is to incorporate an actual mathematical parameter (there have been a couple proposed, including the "planetary discriminant" mu and the Stern-Levison parameter lambda).

The "planetary discrimant" is determined by dividing the mass of the candidate body by the mass of all other bodies sharing its orbit. For the "real" planets in the Solar System, this is orders of magnitude greater than 100; for the dwarf planets and other objects, it's orders of magnitude less than 100 (to give an example: mu for Earth is about 1,700,000; mu for Pluto is around 0.08).

The Stern-Levison parameter is more complicated and depends on the mass of the object (squared) and, essentially, the length of its "year", but again there's a clear difference between planets (orders of magnitude more than 1) and dwarf planets and other "junk" (orders of magnitude less than 1). Using the same two bodies as before, Earth's lambda is about 150,000 while Pluto's is 0.0003.

Since the Stern-Levison parameter depends on the distance from the Sun (in a slightly complicated way), it's possible to determine the distance at which a body of that mass would have a lambda value of 1 (and therefore be a "planet"). For Pluto, that distance is about 80% of the Earth-Sun distance., while its actual distance from the Sun is about 50 times greater than that.

Stern himself is the head of the New Horizons project, and personally considers Pluto a planet, but even by his own parameters it doesn't qualify, and he's also proposed the term "uberplanet" to distinguish between bodies with lambda >= 1 and lambda < 1. "Uberplanet/planet" vs. "planet/dwarf planet" seems like a minor quibble, really.