Tweaking definition of planet

Well, the Solar System reforms of 2006, which I strongly supported, have not been as successful as I’d hoped. As many of you probably know, the International Astronomical Union then came up with the first formal definition of planet. The definition stated that a celestial object is a planet if and only if it:

  1. is in orbit around the Sun,
  2. has sufficient mass so that it assumes a hydrostatic equilibrium (nearly round) shape, and
  3. has “cleared the neighbourhood” around its orbit.

Furthermore, objects that met the first two criteria were termed “Dwarf Planets.” Pluto, Ceres (the largest inhabitant of the asteroid belt), and Eris were immediately classified as dwarf planets, and many other objects were candidates for the designation, once more was known about their sizes.

This resulted in there being eight planets in our Solar System: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune; Pluto was “demoted.” It was this last item, Pluto no longer being a planet, that has caused most of the popular discontentment and news stories over the reforms. Personally, this doesn’t bother me any and I find the use of school children, who are told to write letters to astronomers and museums complaining about “Pluto being taken away” to be totally shameless.

However, I do now recognize legitimate problems with the 2006 definition, as much progress as it represented. It is somewhat confusing that “dwarf planets” are not planets, though it sounds like they should be a subset thereof. Beside the other technical ambiguities, the whole debate is also something of a distraction, and draws attention away from the important things.

For these reasons, I am glad that the Great Planet Debate is resuming, according to, my previous support for the present definition notwithstanding. I now support redefining as a planet any object that:

  1. is in orbit around a star and
  2. has sufficient mass so that it assumes a hydrostatic equilibrium (nearly round) shape.

That would give us twelve planets. But, I would furthermore subdivide planets into major planets and minor planets, the former being those that have “cleared the neighbourhood” around its orbit and the later being those that haven’t. This would give us eight major planets–the current ones, or the old nine less Pluto–and several minor planets, including Ceres, Pluto, Makemake, and Eris–with more minor planets to come as we learn more about the thousands of objects in the Kuiper Belt.

I think this definition would be clear, for both astronomers and laymen, and would be very useful, which, after all, is the sine qua non of a good definition. The only problem is that the term “minor planet” is already in use; it is virtually synonymous with asteroid. Those objects, which will not be planets or minor planets under my suggested definition, will need to be called planetoids, asteroids, or any of many other suitable terms.

Finally, the definition will need to be tweaked to exclude brown dwarfs and to address the issue of so-called rogue planets–round objects that have been ejected from any stellar system–and binary planets, which would include systems like Pluto-Charon where the barycenter (gravitational center) of the system lies above the surface of both objects. This would make Charon and Pluto binary (minor) planets.

I think these reforms could greatly aid discussions about the fascinating Solar System in which we find ourselves situated. Eight (major planets) is great and more (minor planets) are good too!


No comments yet

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: