The Eight Planets

Mike Brown

On August 24th 2006 the word "planet" was given its first-ever scientific definition by a vote of the International Astronomical Union. With the raising of a few yellow cards in Prague Pluto was demoted from full-fledged planet to "dwarf planet." The object 2003 UB313, sometimes called Xena, sometimes called the "10th planet," which in many ways precipitated this final debate, becomes the largest known dwarf planet. Unless astronomers revisit this issue at some point in the future, it is unlikely that there will ever be more than eight planets.

What was the problem with nine (or ten) planets?

Pluto and 2003 UB313 are significantly smaller than the other planets. If you were to start to classify things in the solar system from scratch, with no preconceived notions about which things belong in which categories, you would likely come to only one conclusion. The four giant planets -- Jupiter, Saturn, Uranus, Neptune -- belong in one category, the four terrestrial planets -- Mercury, Venus, Earth, Mars -- belong in one category, and everything else belongs in one or maybe more categories. You wouldn't lump the largest asteroid -- Ceres -- in with the planets, you would group it with the other asteroids. Likewise you wouldn't group the largest object in the vast swarm of objects beyond Neptune (the "Kuiper belt") with anything other than the Kuiper belt. The previous nine (or ten) "planets" encompassed the group of giant planets and the group of terrestrial planets and then awkwardly ventured out into the Kuiper belt to take in one or two of the largest of those objects. Using the word in this way makes no scientific sense whatsoever.

Two solutions to the problem of Pluto and 2003 UB313

Leave no iceball behind
    Astronomers were faced with two options for a scientific definition of the word planet. One option was to say that what makes a planet a planet is simply the fact that it is large, round, and orbits a star. Ignore everything else that you know and concentrate on that. Why round? If you place a boulder in space it will just stay whatever irregular shape it is. If you add more boulders to it you can still have an irregular pile. But if you add enough boulders to the pile they will eventually pull themselves into a round shape. This transition from irregularly shaped to round objects is important in the solar system, and, in some ways, marks the transition from an object without and with interesting geological and planetary processes occuring (there are many many other transitions that are equally important, however, a fact that tends to be ignored in these discussions).

Of course, to truly talk only about the object in isolation and to ignore everything else you know you should also ignore the fact that the object is in orbit around another planet. It is hard to make a consistent argument that a 400-km iceball should count as a planet because it might have interesting geology, while a 5000-km satellite with a massive atmosphere, methane lakes, and dramatic storms (Titan) shouldn't be put into the same category, whatever you call it. For most people, considering round satellites (including our Moon) "planets" violates the idea of what a planet is.

The other difficulty with this definition is that it instantly makes 50 planets in the solar system with the likelihood of hundreds coming soon. Such a huge change in the number of planets is a big hint that this definition is a huge change in what we commonly think the word "planet" means. While most astronomers would agree that round vs. non-round is an important distinction in the solar system, most appear to feel that forcing the word "planet" to be the word that describes this transition is incorrect. This radical re-definition was initially proposed by the IAU but was met by heated opposition and had to be dropped.

Consider the circumstances
The other scientific definition that makes sense is to acknowledge that by any classification scheme that considers circumstances -- where the object is, what else is in orbit near by, whether an object is a satellite -- the first eight planets are clearly in a class of their own. There are many ways to attempt to state this definition: a planet is by far the dominant mass in its region of space, a planet has cleared its neighborhood of all other significant masses, a planet is the accumulation of most of the material in its orbital vicinity. While none of these definitions can be stated with rigorous precision, they are precise enough for the case of the Solar System where the division between objects with have and have not mostly cleared their regions of space is enormous.

One of the best ways to view this definition is to consider some of the distinct regions of space. The asteroid belt, for example, is a collection of small rocky bodies between Mars and Jupiter with many millions of members. The largest asteroid Ceres is not nearly massive enough to have accumulated all of the other asteroids, nor is it massive enough to shove the asteroids out of the solar system. It is not a dominant mass within the asteroid belt. The exact same could be said of Pluto and 2003 UB313 (which are essentially the same size and both in the Kuiper belt along with millions of other bodies). Every one of the eight planets easily passes this test though. The eight planets were created from an accumulation of most of the material that remained in their vicinity. They are the dominant bodies in their regions of space.

This view is the one officially adopted by the International Astronomical Union. Because of the relatively chaotic process that occured before reaching this very rational decision the actual wording of the definition is not as precise as it might have been, giving people room to quibble and to say that the definition is unclear. The important point to remember, however, is that the difference between the eight planets and everything else known in the solar system is so huge that even a definition with a lot of wiggle room will not make any difference. If you are trying to define the difference between North America and Europe, for example, the exact position of the line that you draw in the middle of the Atlantic Ocean does not matter much. The precise definition in the IAU resolution may be a tad unclear, but the concept is absolutely rock solid with absolutely no room for doubt about which objects do and do not belong.

Here are some of the issues that have come up:
What about Pluto crossing Neptune's orbit?
    Partly this issue has come up from an incorrect statement in an AP wire story which says that Pluto is autmatically disqualified because it crosses the orbit of Neptune. Untrue. Pluto is disqualified because it is in the Kuiper belt but has not cleared out the Kuiper belt nor accumulated most of the mass in the asteroid belt, nor does it dominante the Kuiper belt. Pluto is part of a vast population and is rightly classified with that population where it belongs.
    But surely this means Neptune has not cleared out Pluto and thus is not a planet, right? No. The problem here is simply with the hasty way in which the final definition was drafted, not with the concept, which is quite solid. And the concept is more important than a lawyerly reading of the definition. Neptune has a mass more than 8000 times greater than that of Pluto, and, in fact, totally dominates Pluto's region of the Kuiper belt. Much of the material in the Kuiper belt has indeed been tossed aside or accumulated by Neptune, but a very special region ("the Plutinos") have actually been captured by Neptune instead. We now know that Neptune formed much closer to the sun than where it was today, and, as Neptune moved out, it pushed these Plutinos out with it while forcing them into a peculariar orbit where they orbit the sun precisely twice for every three orbits of Neptune. Pluto is the largest of the Plutinos, and it and the others only exist where they do because of the dominance of Neptune. While a lawyer could make a case that Pluto has not been cleared by Neptune, the concept and intent of the definition is sound, and Neptune's total domination of Pluto's dynamics is actually an excellent demonstration of precisely the concept the definition is meant to convey.

What about Jupiter and the Trojan asteroids?
Jupiter (and now also Neptune) is known  to have asteroids in orbits that are almost identical except 60 degrees ahead or 60 degrees behind the planet, in what are known as the Lagrange points. Jupiter exceeds the mass of these Trojan asteroids by a factor of many millions. Like the case of Pluto and the Plutinos above, the Trojan asteroids are in fact captured by Jupiter and only exist where they do because Jupiter totally dominates their dynamics. Jupiter is so totally dominant in this region that it even prevented the asteroid belt from accumulating into a planet.

What about near-earth asteroids?
There are asteroids and comets strewn throughout the solar system that don't fit nicely into the asteroid belt or into the Kuiper belt. Some of these are, for example, the near-earth asteroids. Again, a lawyerly reading of the definition might try to argue that there are therefore no planets in the solar system whatsoever! The concept of clearing and dominance is still sound. All of these extra bodies flying around the solar system are on unstable orbits and will eventually get ejected from the solar system or collide with a planet. This process, in fact, is precisely what astronomers refer to when they talk about "clearing." Much like at home, the process never actually ends. But the planets have mostly cleared their regions, even if that process always continues.

What about the moon?
The earth hasn't cleared the moon, so why is the earth a planet? Like the arguments about Pluto and Trojan asteroids above, the Earth totally dominates the orbit of the Moon. And the pair totally dominate everything else around. Ergo, by the concept, planets.

What about the astronomers who say this is a poor definition?
Astronomers might quibble with the definition, but no rational astronomer is going to disagree with the concept. The precise wording of the definition might need to be fixed still, but the hugely important astronomical concept of what now separates planets from non-planets should be clear to all.

Disclaimer: I didn't make the definition or even participate in the vote, not being a member of the IAU and all. I'm just trying to explain it. While I think it is the best possible scientific definition we could have had, I am still in mourning.

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