Observations of the universe make us think matter is much more common than antimatter, and I believe no "probably-antimatter planets" have been found.
But, if Pluto, for example, somehow were mainly antimatter, would that contradict any specific observation?
Is it simply a judgment based on whether we see high-energy photon emissions (signifying matter-antimatter annihilation) from the planet? If so, do we get enough data from Pluto to be confident?
(I'm just curious if Antimatter Pluto could be a believable science fiction movie and can't decide despite answers here and here.)