Which is the Smallest Planet?
Mercury is the smallest planet in our Solar System. Specifically it's the smallest of the major planets, excluding dwarf planets.
As you can imagine, dwarf planets are smaller than the main planets so there are plenty of smaller planet-like objects. Ceres is the smallest of the known dwarf planets with a diameter of 950 km. In time it's very likely that we'll discover even smaller dwarf planets.
Of course this is only considering planets in our Solar System. There are billions more planets in the Universe that we can't see, so we don't know which is the smallest planet in the whole Universe.