What Is The Measure Of A Planet?

A few days ago, on August 15th, NASA released the news that their efforts to keep the planet-hunting Kepler spacecraft operational just weren't going to work. Of course, that depends on what level of "operational" you're talking about. It can send and receive messages from Mission Control just fine. Its solar panels are providing plenty of electricity. Its sensors are fully functional. But it's only got two control gyros left, one less than it needs to do the super-accurate pointing it really needs to do in order to see the tiny wiggles that betray a planet around a sun tens to hundreds of light-years away.

They're open to suggestions for other uses. If you've got a notion about how to use the last two gyros in concert with its thrusters to point it accurately, and with stability, they'd love to hear from you.

Kepler, to date, has found 134 fully-confirmed planets orbiting 75 different stars, along with 3,277 unconfirmed candidates. Not a bad haul for four years' work.

If only we had a good, universal definition of what a planet actually is.

Once upon a time, a good way to start a bar fight at any astronomers' convention would be to throw out the question, "Is Pluto a planet?" Did I say "once upon a time?" It's still a fairly contentious topic, seven years after the IAU formally demoted Pluto to "dwarf planet" status.

The current definition states that a planet is:

1) In orbit around the Sun,

2) Has sufficient mass to achieve hydrostatic equilibrium (a nearly round shape), and

3) Has "cleared the neighborhood" around its orbit.

I've had seven years to think it over, and I'm not sure I like the third part. No, scratch that, I'm sure I don't like it. My beef with this definition is that it's not universal enough to be of real use. How do we know any of the 940 confirmed exo-planets discovered by all means at our disposal are real, genuine, bona fide planets?

Well, I suppose we could add a fourth qualification. If it's detectable from at least ten light-years away, it ought to be good, right? Well, not really. That definition relies on how good your telescopes are. So that really doesn't work, either.

Here's the real problem, which caused the IAU to write the definition the way they did: using only (1) and (2) above would give us an enormous -- and possibly ever-expanding -- number of planets. Unwieldy lists aren't good or useful for anyone. So there had to be a third discriminant. The orbital mechanics weenies -- and I was one, part-time, back in grad school -- crafted the requirement to "clear the neighborhood", and called the problem solved.

My issue with the definition as written is that you can't apply that rule over interstellar distances. You just don't have enough information. There's no way, even in principle, to make your observations so precise. But you still need a third rule, so that the list can't grow without limit. There's gotta be a way to draw a metaphorical line, saying "You must be this tall to be a planet."

There are two ways to draw that line, by mass, or by radius. Or use both, allowing the candidate to qualify by one or the other. The nice thing about this discriminant is that it's universal. It doesn't depend on how the object moves, it depends on what the object is. You can apply it here, or around Alpha Centauri, or Epsilon Eridani. It works equally well everywhere.

And I don't especially care where you draw the line. I give not a rip if Pluto is above or below the cut-off. Set the limit at Pluto's size plus five percent, or minus five percent, I'm good with either one.

All I want is a rule that I can use wherever my attention wanders. And it wanders pretty damned wide.