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Forgetting Pluto and Charon, drag sees.
Those are more of a binary planet than a planet and satellite, to be fair. And that’s even before we get into the whole planet vs dwarf planet debate.
(For the record, I think Ceres should also count as a planet)
Ceres is the issue. People are pissed that they learnt the list of planets and now it’s different, but don’t realise the only viable options were to either drop the last one or add a bunch of them, some right in the middle.
And if Ceres is a planet, shouldn’t Eris be one too?
Exactly. It would get real messy.
What is argument for Ceres and Eris being planets?
The same as for Pluto
Large round bodies orbiting the sun.
Hydrostatic equilibrium. Their gravity is enough to pull their mass into a sphere-ish shape.
Contrast to asteroids that are irregular.