(But the IAU is Wrong, Too.)
In 2006, the International Astronomical Union (IAU) voted that Pluto was not a planet, just months after the New Horizons spacecraft was launched with a mission to…visit the last planet that had never been visited by a spacecraft.
Well…that was awkward.
The IAU vote was a weird and highly politicized affair in which planet-hunting scientist Mike Brown, the person with the most to gain from keeping Pluto a planet, was one of the loudest voices for demoting it. Check out this excerpt from Dr. Brown’s book for an idea of how it went down. In fact, Alan Stern, the scientist in charge of New Horizons (rightly) says that the IAU doesn’t speak for all planetary scientists and rejects their definition.
The IAU definition says that a planet must orbit the Sun, be large enough to become round under its own gravity (larger than a few hundred kilometers across) and must have “cleared the neighborhood” around its orbit. Pluto orbits the Sun and is round, but it sits in the Kuiper belt with a bunch of other similar objects, some of which are nearly as large, so you can’t say that it has “cleared its neighborhood”. Therefore Pluto is not a planet, and the IAU created the new category of “dwarf planets” to describe these objects.
I think that the IAU did the right thing for the wrong reasons. It really doesn’t make sense for Pluto to be a planet, but the IAU definition is clunky, unclear, and incomplete.
The reason Pluto shouldn’t be a planet is that it just doesn’t fit in with the other planets, and, crucially, it fits in a lot better with its fellow dwarf planets. If you were a space alien seeing our Solar System for the first time, you would conclude that there are eight large, mostly isolated objects orbiting the Sun, and two belts of small objects in different orbits. You would most easily conclude that the eight large, isolated objects are “planets”, and the many objects in the two belts are not. They are asteroids, comets, and dwarf planets. (We don’t really have a unified term for them.) Pluto, although it is the largest, looks like just another Kuiper belt object much more than it does a planet.
Now, this categorizes Pluto quite well, but notice that it’s not an actual definition, and there’s a reason for that: you don’t have enough enough information to make a definition, and the IAU’s mistake was trying to make a definition based on this limited information. Let’s look at the three parts of the definition to see why.
1. A planet must orbit the Sun.
Yes, that means our Sun. Yes, the definition specifically says our Sun. And yes, that excludes more than 99% of all known planets because the definition doesn’t apply at all to extrasolar planets. From start to finish in the debate, the IAU never even tried to address the problem of extrasolar planets (and this was 2006, when we already knew about 100 or so exoplanets). With out the full picture of the universe of planets that we get from exoplanets (and we’re still trying to understand it today), you can’t construct a definition intelligently.
2. A planet must be round.
This part of the definition is not bad in itself. I agree that planets should be large enough to be round under their own gravity. The problem is that it’s hard to tell whether an object is large enough to be round in the Outer Solar System. You can make a good guess based on how bright it is, but you can’t be sure. Worse, the IAU has appointed itself as the gatekeeper of which objects are declared to be “dwarf planets” (large enough to be round). Not only is this not at all normal scientific practice, but they have also been very, very cautious about it, not certifying objects that are so large they are dead sure to be round because the telescope technology doesn’t exist to see it directly. Objects that are dwarf planets are not listed as such. Mike Brown made the same complaint here.
3. A planet must “clear the neighborhood” around its orbit.
This statement is both unclear and misleading–misleading because no part of the Solar System is truly “cleared” and unclear because there is no accepted definition for what it means. Basically, what this rule really says is that a planet must be gravitationally dominant over all the other objects around it. This actually isn’t so bad. Asteroids and/or comets can be found all throughout the Solar System, but in most parts of it, they are either penned in a particular area by the gravity of a planet, or on an unstable orbit that will see them kicked out of the area after a few million years or so. However, the IAU definition fails to capture that nuance.
(Granted, it is thought that planetary systems evolve in such a way that they are always just barely stable over the age of the system, but that’s another story.)
Honorable Mention: Small objects that are neither planets nor dwarf planets are not called something simple like “asteroids” and “comets” by the IAU. Instead, they are given the clunky moniker, “small Solar System bodies”.
So the IAU definition needs a lot of work, if we should even have it at all. It was adopted in a rush and for unscientific reasons. Unfortunately, it probably won’t be revisited unless we discover something that directly challenges our concept of a planet again, like a Mars-sized object in the Outer Solar System (which is possible), which would not be called a planet under the current definition. More on that next time.