The growing number of known solar system bodies—and their wide array of sizes—has raised the question of what should be called a planet. (credit: NASA)

Gravity Rules: The nature of planethood

by S. Alan Stern
Monday, March 29, 2004

I am a planetary scientist, so you won’t find it surprising that one night recently, the dinner table conversation at our home eventually turned to the discovery of the largest ever Kuiper Belt Object, Sedna (2003 VB12). When I remarked that I was amused by the fact that some astronomers don’t consider Sedna a planet, our teenage daughter Kate joined in—agreeing that Sedna shouldn’t be classified a planet. Surprised, I asked why. “Dad, if Sedna is a planet, then Ceres is too, and there are probably lots and lots more things this big that we haven’t discovered. You all should leave it to just the normal nine we learned about in school. We can’t have so many planets that you can’t name them all!”

Flawed as it was, Kate’s logic about exactly what should and should not qualify as a planet is about as good what I have heard lately from some professional astronomers. I explained to Kate that no one knows the names of all the stars, or all the galaxies, but that doesn’t mean we limit the number of stars and galaxies to just the first few handfuls that were named. For that matter, I remarked, if your brain was so completely full of names of people that it just couldn’t take any more, would anyone new who you met after that, therefore not be a person? Of course not! We decide whether a person is a person based on their genetics, just as we do when classifying any given living thing into its species. Likewise, astronomers decide whether a star is a star or not, and whether a galaxy is a galaxy or not, based on its physical properties. It might be a dwarf star or a giant star, a dwarf galaxy or a giant galaxy, but the basic qualification is based on some physical characteristic of the object.

Stars, for example, are objects that generate the bulk of their energy as a result of sustained nuclear fusion in their interiors. If an object is too small to generate the bulk of its energy as a result of sustained nuclear fusion in its interior, then it isn’t termed a star—period. Astronomers do not exclude tiny stars—called dwarf stars—as stars because they are too small; if they have the salient characteristic of a star, i.e., energy generation by fusion, they are termed a star. Despite that, however, some of my brethren think that dwarf planetary bodies like Sedna shouldn’t be termed planets.

I’m amused by this. One doesn’t deny a Chihuahua a place among dogs because it is too small. And we don’t deny a gnat a place among insects, or a Japanese bonsai a place among trees. Why? Because something deeply characteristic—“genetic” if you will—binds the classification across a wide range of sizes.

Owing to the recent discoveries of objects as mind-bending as Sedna, pulsar planets, and super-Jupiters, planetary astronomers are now facing the question of determining formal planet classification criteria. What is needed is a clear, unambiguous criterion (or a set of criteria) that can be applied to test any given astronomical object to determine whether it is a planet.

New discoveries, new questions

Why hadn’t we astronomers faced this issue long ago? It’s because until recently, technological limits kept us from seeing very many examples, and therefore much real variety, among planets.

The situation astronomers are facing now is rather as if Kate had grown up entirely in our house, having never left it or seen any of the outside world, except through our windows (there are days, mind you, that I think this might have been a good thing). Given such a limited range of view, and therefore experience, Kate would only know of a handful or so of other homes that one can see from ours. Several are one-story homes, several are two stories, and there isn’t much real variation in the range of compositions. If Kate was then one day able to ascend to our roof, or, better, to roam the streets of our town, seeing neighborhood after neighborhood, she’d suddenly be confronted with a much greater population of houses. Moreover, in this larger population, she’d see much greater variations in the sizes, styles, compositions, and settings which houses can take on.

This is exactly analogous to what has happened in astronomy over the past dozen years or so with regard to our knowledge of the range of bodies that one might classify as a planet. Simply put, the growing capabilities of telescopes and detector systems available since the early 1990s have enabled the discovery of bodies with masses about that of the Earth that orbit pulsars (“pulsar planets”), objects many times the mass of Jupiter that orbit distant stars (“super Jupiters”), and a growing bevy of tiny worlds in the icy Kuiper Belt beyond Neptune (“ice dwarfs”). These findings dramatically broadened our knowledge horizon and forced us to confront what is and isn’t a planet.

Oddly, there isn’t much controversy to the upper boundary line above which an object is no longer called a planet. If there is enough mass that the object ignites in fusion, such an object is simply termed a star. I have yet to hear anyone call for a separate category for those objects that generate most of their energy by gravitational contraction, as objects like Saturn and Jupiter, and the giant “super Jupiters” do.

page 2: three ideas >>