A dwarf planet, according to the IAU.

So the IAU has declared that Pluto is not a planet. I agree with this assessment, but I also think that the IAU’s definition is poorly written. So how should that word “planet” actually be defined? It’s not simple.

The basic problem here is that “planet” isn’t a scientific term, and it never has been. Scientists often compare it to defining the word “continent“. How many continents are there? Seven…except that Europe and Asia are only continents by convention, not because they’re separate land masses. What’s more, geologists don’t care what people mean when they say “continent”, and if they did, they could reasonably declare that Madagascar is a continent.

It’s hard to define a continent in scientific terms. If you try to, you would probably say there are six of them, but that’s not totally crazy because people do talk about Eurasia as a continent.

Similarly, most astronomers don’t particularly care scientifically about the definition of a planet, and there are plenty of scientific papers that call objects planets that aren’t really planets. For example, the object DENIS-P J082303.1-491201b is described by the NASA Exoplanet Archive as a planet, but at 28.5 times the mass of Jupiter, it must be a brown dwarf, which most people agree is not a planet. The reason it’s called a planet there is probably that, as seen in a telescope, it looks very similar, and you use the same techniques to study it.

So in practice, astronomers don’t have a stake in this, but the public very much does, and astronomers do care quite a bit about social outreach, so it is fairly important to establish a clear and defensible concept of what a planet is.

Astronomer Mike Brown says that scientists work by “concepts” rather than “definitions”. His meaning is a little vague, but essentially, a concept is a general set of properties that a group of objects has in common, and it doesn’t have to be as precise as a formal definition.

So what is our concept of a planet? Here, we can pin down a few things.

A planet should be big. It should be smaller than a star, of course, but it should be bigger than all of the asteroids and comets and such in the solar system. This includes being big enough to be round, which is part of the IAU definition.

A planet should not be a moon. This even supersedes the “big” qualifier because there are some moons that are bigger than some planets. But it is pretty well agreed that a planet shouldn’t orbit a bigger planet. (This is ignoring the concept of a double planet, where the two planets are close to the same size because that’s a whole other can of worms.) Note that I did not say a planet must orbit a star, even though most astronomers say it should. This is because the cultural concept of a planet  includes rouge planets, which do not orbit a star.

Planets should be few enough in our Solar System that any five-year-old can memorize them with ease. This is not a scientific concept. Not by a long shot. But it has been the reality from time immemorial. In all of recorded history, there have never been fewer than five planets and never more than twenty-three (and that for only a couple of years). This was the problem with the proposed definition of a planet that would have included the dwarf planets–everything large enough to be round. This definition would eventually lead to hundreds of planets, once they’re better characterized, completely wrecking our cultural concept of the word.

With this in mind, I think that we cannot rigorously define the word “planet”, even for our own Solar System, because we don’t know what’s still out there for us to find, but I think that we can divide our known Solar System into planets and non-planets, although there are still some pitfalls to watch out for. I’ll do that in my next post.