The IAU (International Astronomical Union) says the requirements for being a planet are:
It orbits around the Sun.
It has sufficient mass to assume hydrostatic equilibrium (a nearly round shape).
It has “cleared the neighborhood” around its orbit.
Pluto does all but the 3rd requirement since it's so small it hasn't asserted its "dominance" of the area.
I believe it's currently a dwarf planet but I'm not 100% sure.
Solid mass isn't a requirement, it just needs to be round.
Caption this Meme
