Planets go missing

Even before the discovery of Eris, an icy body of the outer solar system larger than Pluto, a heated debate was raging over how we should define a planet. Something had to be done. Either Eris was a planet, or Pluto wasn’t.

So the International Astronomical Union set up a committee to consider the issue, to report back to the IAU conference.

The committee came back with the new definition that a planet had to be orbiting a star, big enough to have collapsed into a spherical shape under its own gravity, and not be the satellite of a planet.

I had to think about it for a while, but in the end it was clear that this was a good definition, not because it saved Pluto, but because it was a robust, physical definition. It could be applied to new discoveries without the need for further arguments and conferences. This definition included the nine existing planets and added Eris, Ceres and Charon making a total of 12 planets.

Alas, the IAU didn’t ratify the recommendation. Instead of thinking about what a planet really is, physically, it decided instead to draw an arbitrary line in order to preserve an arbitrary and by no means broadly-held perception of what a planet should be.

The change to the committee’s recommendation was to invent a new category called “Dwarf Planets” to apply to Pluto, Eris and other similar bodies.

Even the name doesn’t make sense. Dwarf planets, the ruling goes, are not determined by size. In fact, they’re the same as normal planets, except that they have not “cleared out the neighbourhood around their orbit” of other bodies.

The orbital neighbourhoods of all planets are cluttered with other bodies. What exactly is an orbital neighbourhood? Here again we have an arbitrary decision and description, not a physical one.

Even Jupiter has a whole retinue of asteroids which share its orbit, the Trojans. We now need to insert a special “secondary” set of definitions - again arbitrary - to exclude the Trojans, just so that Jupiter can still be called a planet.

Space in the outer solar system is huge. If we finally find out, in years to come, that there is nothing else sharing Pluto’s orbit, will we then call it a planet after all? How close to it does another body have to be before it is deemed to have not been “cleared out”?

And what about ellipticity? Just because the orbits of the planets out to Neptune have low ellipticities and obliquities, what logic is there in deciding that all planets have to have this? How small an orbital ellipticity and obliquity does a body need to have before it is a planet? Here is yet another arbitrary, non-physical, line which has been drawn.

What if we find a body the size of Earth out in the Kuiper Belt? Is this still a “dwarf planet”? What if we find a body the size of Neptune out there? Will we suddenly insert some post hoc reason to call it a planet after all? Maybe its ellipticity will be low, so we could just “tweak” the definition to include it that way, without including all that other icy riff-raff out past Neptune.

If a chance gravitational encounter threw a large KBO (Kuiper Belt Object) into an orbit between Uranus and Neptune, would it become a planet? Why is Neptune regarded as a planet anyway, when there are whole families of asteroids and KBO’s - including Pluto - whose orbits cross its own?

And if we exclude Trojans and resonant orbiting bodies from the “cleared out orbit” rule, isn’t Pluto then a planet after all, along with Eris and others?

The IAU has rejected a clean, simple system based on purely physical elements, where you could look at a body and say, yes, that’s a planet. In its stead it has set up an artificial and unworkable one requiring ongoing arbitrary adjustments purely so that what they’ve already decided to call planets, can still be called planets.

What earthly purpose is there in the IAU taking to itself a process of “accreditation” for planets? If it was just a matter of awarding planet status to those bodies they already wanted to be planets, couldn’t they have just published a list, saying, “These are planets and nothing else is, unless we say so”.

It would have been a lot easier, and a lot less dumb. Planets are now no longer a physical class of bodies, but merely a label conferred by committee, subject to ongoing argument.

Worst of all though, is that the word will no longer have any scientific value. It has already been noted by the IAU itself that we can no longer say that the bodies we have found orbiting other stars can be called planets, because they’re too far away, and we can’t test the new criteria for planethood. So now only our own solar system has planets.

In a few years when we start to find significant numbers of terrestrial bodies around other stars, does this mean they too will have to be called something else - not because they’re strange or small or don’t look like planets, but simply because we can’t “accredit” them without first finding out if they’ve “cleared out their orbits of other bodies - except (presumably) Trojans?”

Nobody is going to go for this. The general public already see it as a joke. Certainly the scientific community will cease to use the word “planet” for any constructive purpose, and the “definition” will break down within a few years as we find bodies that don’t fit the description, or whose orbital or physical elements test the limits and the vagueness of the terms used by the IAU in its definition.

They will be obviously “planets” in any sensible sense - but we won’t be able to call them that, because a bunch of scientists didn’t like the idea of accepting the fact that, out past Neptune, planets simply look like that. They have elliptical orbits, and they’re small and icy. So what?

Ceres is a planet. Again, what’s the problem? It’s a little planet because Jupiter stole most of the stuff that would have made a decent-sized body out of it. That’s fine. At least we can look at it and say, “It’s big enough to be spheroidal, so it’s a planet”. We don’t have to investigate its “orbital neighbourhood” (however defined) to decide if it should have planethood “conferred” on it by the IAU.

Losing one planet might be regarded as a mistake, but losing four really does look like carelessness.