Planet

From RationalWiki

A planet is a celestial body not sufficiently massive enough to generate photons in the range of visible light, but larger than a comet or asteroid. On both ends of the mass spectrum, it is a subjective call whether a body is a planet or not.

Planets can have atmospheres ranging from thick to none, and they can have lesser bodies orbiting them as satellites. The largest satellite (Ganymede) of the largest planet (Jupiter) is bigger than the smallest planet (Mercury). Humans have photographed all the planets in the Solar System with probes.

Pluto isn't a planet, however. Some people decided it, plainly and simply, sucked too much to be a legitimate planet. Sorry Hades.