Last weekend I attended the Astronomer’s Conjunction, an annual amateur event held in western Massachusetts. After listening to two teams of well-informed amateurs debate the Pluto-planet question, I asked the following hypothetical question: Let’s pretend that we wake up on Monday morning to the following exciting news. Astronomers using the Keck Telescope announce that they took a series of deep images of Proxima Centauri, the nearest star to the Sun. The pictures revealed an orbiting body that is well above the threshold to be spherical, it has three moons, and a spectrum shows that it has an atmosphere. Should we call it a planet? After nobody in the audience objected to this designation, I added, “Actually, I was just describing Pluto.”

We heard the news today that after 76 years, less than 400 members of the International Astronomical Union decided to boot Pluto out of the family of planets. This was an unfortunate decision for a number of reasons, not the least being the bad PR that such a tiny minority of self-appointed elites is ramming its arbitrary decision down the throats of the entire astronomical community. I predict that some of the members who voted for the new definition of “planet” will come to regret it, because it will fail to accommodate future discoveries. But I’m pleased that the IAU made a decision, and that the new official definition of “planet” is well motivated and based on sound scientific reasoning. Nobody is going to die or lose their life savings because Pluto will no longer be considered a full-fledged planet.

But there are several gaping holes in the IAU’s definition, including one that is so immediately obvious that it boggles my mind that intelligent people could actually vote for it. The definition is anti-Copernican in the sense that it defines planets as objects orbiting the Sun. In other words, only solar-system objects can be considered true planets. Excuse me, but even under the most liberal definition for “planet” that one could seriously propose, we know of about 50 planets in our solar system right now. We already know of more than 200 planets outside the solar system. And if the official definition were extended to include planets around other stars, the total in our Milky Way Galaxy alone must be something on the order of 100 billion to a trillion. In other words, the IAU definition completely ignores 99.99999999999 percent of the planets that exist in our galaxy. Huh?

Maybe this was prudent, because adding extrasolar planets to the resolution would have made it even harder to reach a verdict. Maybe a bunch of astronomers thought, “We need to learn more about planets outside the solar system before we can come up with a sensible definition.” But something inside of me says that the IAU wimped out, and that it might end up sweeping this problem under the rug for a very long time, just as it did with the Pluto question.

Perhaps certain members of the IAU leadership, who were responsible for the exoplanet omission, felt concern that future discoveries could throw a monkey wrench into the official definition. If that’s what they thought, their concern was justified. For example, one of the criteria is that a planet “has cleared the neighborhood around its orbit.” Not only is this statement rather vague when applied to our solar system, but I predict that in the next few years, we’ll find systems around other stars where it fails miserably. Astronomers who simulate the evolution of planetary systems on computers have shown that in some circumstances, interacting planets can leave behind a system with two Jupiter-mass planets that share the same orbit, with one 60 degrees ahead of the other. Such a “Trojan” configuration is stable for billions of years, so it almost certainly exists in nature. It’s only a matter of time until planet hunters such as Geoff Marcy and Paul Butler uncover this kind of system (it’s possible that several have already been found!). So if the IAU definition is applied to other stars, two very massive planets sharing an orbit would not be true planets, because neither one has cleared out its neighborhood.

Astronomers have also found that at least half of very young stars are surrounded by disks, and there’s good reason to think that many of these disks will spawn planets. It will take tens of millions of years for these planets to clear out their regions of space. Does that mean that a newborn 5-Earth-mass object that has not had time to clear out its zone is not a planet?

I also suspect we’ll find systems that through some kind of freak dynamical evolution, have left planet-mass objects in zones of rubble. Heck, even Earth shares its region of space with tens of thousands of asteroids of various sizes. One could counter that these bodies are in unstable orbits. But both Jupiter and Neptune harbor vast populations of Trojan asteroids that will remain in their orbits for billions of years. And even many trans-Neptunian bodies, such as Pluto, cross Neptune’s orbit. Under the IAU’s vague new rules, we could rule out Earth, Jupiter, and Neptune, and perhaps other planets as well.

And by not setting a specific size or mass requirement, the IAU has left itself wide open for criticism as soon as astronomers find Mercury-, Mars-, and perhaps even Earth-sized bodies in the distant solar system. Such discoveries are only a matter of time, given the limited nature of surveys to date.

I know it was really, really hard to come up with the definition that the IAU accepted today, and I also know it was impossible to come up with one that would satisfy everyone. The definition that was accepted could have been much worse. And while I hope the passage of this new definition calms down the debate for awhile, we have not heard the end of this story.


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