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Gravity Rules: The Nature and Meaning of Planethood

by S. Alan Stern
Boulder - Mar 22, 2004
I am a planetary scientist, so you won't find it surprising that this past Monday evening, March 15th, 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 for similar reasons to the reason we don't exclude dwarf stars from the list of stars - 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.

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.

This 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). With her range of view, and therefore her range of experience, limited this make believe way, 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 story, and there isn't much real variation in the range of compositions. If Kate were 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 that 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 far away 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.

Where the controversy comes in is at the small end - i.e., in deciding what the lower size boundary should be for planet classification. In that regard, I have heard a lot of suggestions as to how we might go about deciding whether any given object is too small to be a planet, or not. The ones I don't like fall into three categories.

Idea 1: Formation Mechanism Rules. "If an object forms like a planet then it is a planet; if it forms like a star, then it is a star."

A nice try, I say, but this is fatally flawed in at least two different ways. First, we do not know how to determine how any given object formed without ambiguity. Just how did those pulsar planets form? No one knows. How about those super Jupiters? There are least five separate proposed formation mechanisms for these bodies in the technical literature. How about our own Jupiter for that matter? We can't even agree yet on this - because we don't have sufficient data to distinguish between two well-developed, plausible models. Another problem with the Formation Mechanism idea is that both stars and planets can each occasionally form by mergers and also by the fission of a rapidly rotating parent body - so in those cases the formation criterion can't distinguish whether such objects are stars, or planets, or some kind of astrophysical hermaphrodite. We simply have to find a better criterion than this.

Idea 2: Just legislate it. "Adopt some minimum size or mass-say the size or mass of Mercury (diameter=4800 km), or maybe even Pluto (diameter=2400 km), as the minimum size for a planet."

Using this criterion, anything above the legislated line (that isn't so massive as to turn itself into a star) would qualify. This idea is nicer than the first one because you can actually hope to measure an object's size or mass. It also allows one to keep from jarring the public who were taught for so long that Pluto is a planet. However, legislation like this certainly isn't a very scientific way to proceed. In fact, I'd say it's at best a lazy person's way out because it's completely arbitrary, and has no connection to the physical attributes of planetary bodies. If biologists had adopted this kind of size rule for species classification, babies would be excluded from their own species, despite the fact that we know they are genetically related to adults by their DNA! Ridiculous; search on.

Idea 3: Location Rules. "Let's use an object's location as the criterion to establish or reject it from planethood."

I like this one least of all because it is nothing but quicksand. The most common form of this idea is to classify an object as a planet if it is the largest thing in its region. By this criterion, objects like Ceres and Sedna are planets, for they are the largest known things in their regions of the solar system. But what happens when we later discover something out there past Sedna in the Oort Cloud that is larger still. Will we declassify Sedna and replace it with Mr. New Planet? And what if we then find still larger and bigger bodies there? And what do we do about the extra-solar planetary systems where we have no idea what else lurks out there beyond the one or two or three bodies we have spotted so far in each system? Just like the Formation Mechanism criterion, the Location Rules criterion either leaves us paralyzed, unable to render classifications, or living with the threat of endless reclassification. Moreover, we know that planets can migrate around their planetary systems, changing orbits and therefore location for various reasons. By the Location Rules criterion, which objects in a given system are planets becomes a function of when you look, which is nuts. The root of the problem with the Location Rules criterion is that it, like the Formation Mechanism and Legislative criteria, fails because it doesn't recognize any physical attribute of about the nature of a given object, simply its size relative to its cohort population. ("Pluto can't be a planet because it is in the Kuiper Belt.") If biologists adopted this kind of criterion for species classification, a cowboy would become a cow when he herds his cattle! Location is an important factor for realtors, but I don't think it serves anybody satisfactorily for planet classification.

Well, if none of these three ideas work, what are we to do?

The idea that I do like is very simple. It identifies a physical characteristic for setting a lower boundary to planet classification, akin to the "fusion energy generation" criterion for stars. Any kid knows that when you draw a picture of a planet, you have to draw something round. So the idea I like is this: If an object is large enough for gravity to round its shape, then it is no longer just a structure ruled by mechanical strength, like a rock, a building, or a mountain - instead, it is a wholly different kind of structure that we call a planet. I like to call this criterion, "Gravity Rules."

One can calculate the minimum size body that will become rounded by its own gravity starting from very basic principles of physics. Doing so, you find the boundary is a diameter of a few hundred kilometers.

A great number of scientists like this idea. I like it for a number of reasons. For one thing, it's based on physics. In fact, it is ultimately the same kind of physics (the effects of gravitational forces) that stars are classified by - for the thing that turns a large enough body to fusion is its self gravity - which heats an object's interior sufficiently to ignite nuclei in a chain reaction. As a result, this criterion provides a satisfying connection across major classification schemes in astronomy. For another thing, Gravity Rules is comparatively easy to apply - by simply measuring an object's mass or radius (some of the easiest things to determine from afar), we can perform the test to decide if an object should be classified as a planet, or not. Furthermore, the Gravity Rules criterion provides welcome stability - objects don't change classification as they evolve or change location.

Adopting Gravity Rules, all of the planets cited in textbooks, all of the pulsar planets, the super Jupiters, and Pluto, Sedna, Ceres, along with a handful of other asteroids and numerous large Kuiper Belt Objects, fall into the broad category of planets because gravity has rounded them. Some are giants, some are dwarfs, but all are planets, in the same way that some people are giants and some are dwarfs, but all are homo sapiens that share a deeper connection than just a size criterion.

Interestingly, the Gravity Rules criterion just happens to put the Earth about mid-way in size, in a logarithmic sense, between the tiniest dwarf planets and the largest giant planets.

The Gravity Rules criterion of course means that planetary systems (including our own) have very many planets - and most of them dwarfs. I tell school kids that the old view of the solar system that I was taught had nine planets; but things are changing and their kids are likely to hear a number closer to nine hundred than nine. This seems to be a problem for some of my colleagues, but frankly, I don't see why. It simply involves a situation for planetary systems that is analogous to the established fact that galaxies have very many stars, and most stars are dwarf stars (by the way, it is also known that most galaxies are dwarf galaxies). Frankly, this is the first time I can ever remember large numbers scaring any astronomers.

Fewer and fewer astronomers find they can compellingly argue against the Gravity Rules criterion. Alas, not so my teen, who is sticking to her guns. "Dad," Kate told me yesterday over coffee, "I can deal with too many stars to name. I can deal with Pluto, which is obviously a planet because, duh, it is round and it is in my textbook - but there are only nine planets and there should never be any more. Otherwise it's like I told you, we will just have a mess on our hands when it comes time to name them all on tests."

Planetary scientist Alan Stern is an Executive Director of the Space Science and Engineering Division of the Southwest Research Institute, and the Principal Investigator of NASA's Pluto-Kuiper Belt mission.

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