How big does a planet have to be to be considered a planet?

Answer 1

In order to be classified as a dwarf planet, an object has to be large enough to have achieved hydrostatic equilibrium - that is, forced by gravity into a roughly spherical shape, unlike the odd shapes of asteroids. The lower limit to the size or mass of a dwarf planet is thus much more a matter of observation than formal definition. The size would thus depend upon its mass, the lower limit of which would be dictated by properties of matter. To be considered a proper dwarf planet it would also need to be in solar orbit and not, for example, a moon of another planet; it would further not have cleared the neighborhood around its orbit, as, for example, Pluto has not.