Sunday, July 26, 2015

Pluto a planet?

(This post doesn't concern Nethack).

With NASA being a total winner with its New Horizons mission to Pluto, the debate about whether Pluto should be a planet has emerged yet again.

You can look at wikipedia for more details, but the basic story for how Pluto lost its planet status, or rather, how planet gained its definition is as follows: in 2005, astronomers found a dwarf-planet called Eris with a slightly wackier orbit than Pluto's.

Other, similarly-sized objects had been found, but the kicker in this case was that Eris is more massive than Pluto (still, both are less massive than our own pet rock, the Moon).  This prompted the International Astronomical Union (IAU) to actually come up with a formal definition for the word "planet" in 2006.

So, according to the IAU, in order to be a planet, a celestial body must:
  1. is in orbit around the Sun,
  2. has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and
  3. has "cleared the neighbourhood" around its orbit.
(Incidentally, I assume #1 is why we refer to natural satellites of other stars as "exoplanets" instead of "planets").

Pluto apparently has failed the third test because it shares its orbit with other Kuiper belt objects.  This led to Pluto being deemed a "dwarf-planet", which led to everyone losing their shit because they wanted Pluto to be a full-fledged Planet.

I propose to you a new stance on the topic.  Whether Pluto re-gains its status as a full-fledged planet is irrelevant.  My proposal is that the word "planet" should re-gain its status as not being formally defined by the IAU.

The term "planet" is a cultural term that goes back centuries.  Everyone knows what you mean when you say "planet" and almost no one can recite the formal IAU definition of "planet".  At the same time, if people want to use more technical terms, like "Kuiper belt object", they already exist.  Undefining the word "planet", i.e., making its meaning ambiguous, is a win-win solution.