Both Jupiter and Earth are considered planets even though Jupiter is clearly larger than Earth. How much smaller than Earth can an object be and still be a planet? (credit: NASA)

Gravity Rules: The nature of planethood

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Three ideas

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 of 4800 km), or maybe even Pluto (diameter of 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 or kind of orbit 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 extrasolar 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 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.

Gravity Rules

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 mass 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. If such a planetary body happens to be orbiting another planet, as Titan, Triton, the Galilean satellites, and other large satellites are in our solar system, then we call it a planetary-scale moon. If it is orbiting a star (or ejected from a solar system), we simply call it a planet.

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 qualities 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 midway 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 schoolkids 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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