Steve Watson

What is a Planet?

The dispute over the criteria used for the categorization of celestial bodies as planets reveals either a confusion concerning the reason for a definition, or an unhappy compromise between independent reasons. The criteria adopted by the International Astronomical Union (IAU) are that the body

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

These criteria are clearly intended to include the classical planets while excluding most of the new and unfamiliar bodies from the Kuiper belt (and beyond,) without simply naming them as such. Controversy over the exclusion of Pluto means the second option would probably have been preferable, but that would not have looked like a scientifically defensible definition.

A really scientific definition, of course, would attempt to determine some sort of Natural Kind amongst such bodies. In this respect, a pretty standard view is that Natural kinds are the classes of real objects that fill the positions of variables or class names in the best scientific theories relating to the relevant domain. (For a very relevant example, when Copernicus determined/theorized that the Moon differed from all the other planets in the traditional system according to its orbital character, he removed it from the category of planets.) In that case, the class of planets should be a class of astronomical bodies that relatively narrowly includes the classical planets and that features in theories of stellar formation, solar system dynamics and development, and so on. Such a class would not obviously be required to regard either condition 1 or 3 of the IAU definition. A better definition would simply require that a planet be any body that:

1. is not massive enough to produce fusion reactions, and
2. has sufficient mass to assume hydrostatic equilibrium (a nearly round shape)

The IAU itself accepts that their condition 1 should be generalised to say that the body is in orbit about a host star. This would still, however, exclude interstellar objects from the class and invalidate the term ‘rogue planet.’ It isn’t clear why this should be necessary: if we desired to speak of only such objects in orbit about a star we could simply talk of the star’s planetary system. (For convenience the planetary system of Tau Ceti, for example, would be the ‘Tau Ceti system.’) The fact that planets that we know of are generally in orbit about a star is a fact about that class but not a definitive one. (That most Zebras live in Africa is a fact about them, a consequence of their origin and history, but it is not definitive.)

Given that the requirement of IAU condition 1 can be discounted, we might further be relieved of the necessity of distinguishing planets from satellites in those cases where two bodies that both satisfy the other criteria for a planet orbit each other. Previously, we might have achieved this by declaring that where the barycentre of the system lay within one of a pair of bodies, that one would be the planet and the other would be the satellite; or we could have insisted that the secondary body has to be significantly smaller than the primary. A reasonable limit for the second would perhaps be a mass ratio greater than 10:1 given that the Pluto-Charon mass ratio of about 1:8 is enough for some – but not for everyone – to describe it as a double planet. It might be more convenient now to speak rather of a planetary sub-system, and, in the case that one of the bodies is clearly the primary, to speak of that planet’s subsystem. We could unambiguously speak, for example, of the ‘Jovian sub-system,’ or even of the ‘Terran sub-system.’

We might, however, need to modify the conditions in order to exclude neutron stars and white dwarfs from the planet class, because they are not massive enough to produce fusion reactions in their matter, as required by condition 1, and yet they are clearly not of the same natural kind as what we intend to refer to as ‘planets.’ We might achieve this by requiring that the body be composed of non-degenerate matter, but that doesn’t really get to the heart of the problem. In fact, this indicates that purely observational criteria are not adequate for distinguishing the class of planets, because we would insist that any body which had been a star in the past, but through natural processes had become non-fusing should not be in that class – regardless of mass or composition or shape. Natural kinds, so many theorists insist, have historical depth or causal boundaries. (A painted horse does not become a zebra.)