The IAU has, in relatively recent years, been fiddling with the definitions of what objects such as planets are. Quite aside from disrupting everyone’s general understanding for no good reason whatsoever, they did this very poorly.

Here’s a set of criteria for defining a body as a planet:

A Planet…

Planet Pluto

Planet Pluto

  • …is a natural object, such as a rocky ball or a ball of gas, or combination thereof, in hydrostatic equilibrium, which is to say it has enough mass to have pulled itself into a long-term stable oblate spheroid (like Earth) or sphere.
  • …isn’t in a long-term stable orbit around a significantly larger object of a similar nature to itself (in other words, it’s not a moon.)
  • …hasn’t lit up its own fusion reaction

That may not be a perfect set of criteria, but I submit that it is at least close. Also much closer than the IAU’s profound lapse of judgement.


Planet Ceres

Planet Ceres


So yes: Pluto is (still) a planet. We could, if we were being really anal, quibble about it being number nine; There’s Ceres, a 950 km diameter planet located in the asteroid belt, between Mars and Jupiter, for instance (at the left) which makes Pluto (at least) planet ten as counted outwards from our star; but Pluto is definitely a planet.

Unlike, for instance, Vesta (below), which is just as clearly an isolated fragment of something larger:

Vesta

Vesta