Astronomy Down Under

Nine, ten, eleven… how many planets?

26 Sep at 15:25 by Wilson

Funny how things come and go. Last year, when Sedna was discovered, it was announced as a possible 10^th planet; however, there was also talk of not only not considering Sedna as a planet but also demoting Pluto to a more general “Kuiper belt object”, leaving us with 8 planets again.

Then, earlier this year, an as-yet-unnamed object (2003 UB₃₁₃, unofficially referred to as Xena or Lila) was discovered that is larger than Pluto, and considerably farther away (Sedna is smaller than Pluto and, therefore, more easily brushed aside). This is one of possibly thousands of similar objects orbiting the Sun from very, very far away. Are those objects planets?

It turns out that it is very difficult to come up with a scientific definition of planet that will include the current set of nine and nothing else. It would be easy if Pluto were demoted; then you would be able to define based on size (it’s very unlikely, although not impossible, that there’s any undiscovered object larger than Mercury orbiting the sun) or even on distance from the Sun (which is somewhat arbitrary, though), or you could pick the definition currently chosen by many scientists: a large body that dominates a particular orbit. Pluto does not fit (Neptune would be the dominant body in that orbit), and neither do the inner Solar System asteroids or any of the Kuiper Belt objects (no single body dominates those orbits). However, any definition that tries to include Pluto will, certainly, include many other bodies.

The current set of nine planets is a historical accident: when Pluto was discovered, it was thought to be an one-of-a-kind object, which would make it an odd planet but not much of a problem. The subsequent discovery of the Kuiper belt opened a can of worms, of which at least two jumped out so far: Sedna and “Lila”. Nowadays, the working definition of “planet” is “whatever the IAU says is a planet”, and the IAU is working (without much success) to come up with a ruling on the new bodies and, presumably, any new ones that pop up.

Personally, I’d like to see “Lila” accepted as a planet, but I recognize that this would bring a different problem: we could end up with hundreds of planets over the next few decades, and that would trivialize the word “planet”. Nine is ok, ten is fine, eleven is all right… 137 may be a little too much. I think the easiest solution is to keep the name for the current nine planets for historical reasons (or add “Lila” as the 10^th planet to recognize its discoverers), retire the word “planet” as a scientifically significant word (therefore freezing the set of planets in the actual configuration) and adopt an official naming convention based on the characteristics of the objects being named. This would probably make everyone happy without being too disruptive.