What Defines a Planet?

That is still a good question even a couple of years after the big announcement that Pluto is not a planet. In fact, it’s the first of many other questions about the pieces and parts of our solar system and it’s also at the heart of how we classify things that exist here in our little niche of the galaxy. 

The system of classifying planets began as soon as the first planet was discovered moving across the sky. Well, perhaps even earlier than that, when Oogh and her friend Moogh stepped out of their upscale cave in the Dordogne or the Rift Valley (or wherever it was that the first stargazers set up shop) and saw the Sun and classified it as a shiny thing in the sky but different from the shiny thing they saw in the sky at night (and sometimes during the day) that we now know as the Moon).

The first five planets (derived from the Greek word “planetes” for “wanderers”) discovered (Mercury, Venus, Mars, Jupiter, and Saturn) were simply classified as bright things that weren’t stars, and they seemed to “wander” against the backdrop of stars. Okay, that’s simple enough. Everybody could buy that without too much disagreement.

That classification worked pretty well (aside from the problem of what the Moon was) for a while, until Galileo used his telescope to spot things that seemed to be orbiting Jupiter. What were those?  And, what about those other weird things he saw poking out from either side of Saturn?  Was it possible that wanderers had their own wanderers?

In the early 1600s, the solar system was already getting complex. From that point on, astronomers had to deal with finding a couple more big planets (Uranus and Neptune), and then the unimaginable discovery that there were moons around those planets. In addition, there were these pesky rings–which turned out to be, in essence, thousands of pieces of ice–orbiting Saturn. When telescopes got better, people started to find asteroids. And, of course, there were the comets. How to classify those?  Like it or not, classifying things in the solar system wasn’t easy anymore.

In recent times, we’ve extended our ability to see a whole class of small worlds that have orbits extending from the orbit of Neptune out to well beyond the orbit of Pluto (which was discovered early in the 20th century) and its companion Charon (discovered in 1970). How do you classify those? They’re bigger than asteroids and, we thought, smaller than moons.

It became pretty clear very fast that, for planets anyway, the old moniker of “planet” was being used to cover a lot of worlds that really needed better definitions. Calling everything of a certain size “planet” is sort of like calling everything that has wheels on it a “car” (which would then include tricycles, unicycles, bicycles, trucks, airplanes, trains, and so on).

Logically, it makes sense to define worlds by their size, for example. Which works to a certain extent, although then you have the specter of trying to figure out just what makes the cut size-wise and what doesn’t. This is where Pluto and its infamous “demotion” come in for attention.  It’s a tiny world — actually, it’s a double world with Charon–and some of the larger moons of the solar system are approaching its size. In fact, the recently discovered object Eris is actually bigger than Pluto.

As I’ve pointed out in my other articles about classification, the way we classify stars and galaxies is closely tied to their intrinsic properties and evolutionary histories (and futures). Those are useful categories that tell us something about the stars and galaxies.  We had to do the same thing for solar system objects.

So, when newer, bigger worlds were found that seemed to approach Pluto’s size, it was clear we needed to figure out what the term “planet” really referred to.  Astronomers did this with stars and galaxies by simply tweaking the categories they already had to reflect the increasing complexity of what they were finding in the sky.  Logically, planetary scientists needed to figure out what cases merited the term “planet” and which merited something else.

The problem is, everybody got attached to Pluto being a “planet.” And, we had these big, emotional arguments about “poor old Pluto” as if that world had feelings and we’d be ruining the character of the solar system by correctly classifying Pluto in its proper place. Those “debates” obscured a really cool fact: that the solar system is an amazingly complex place with structures that we were only just beginning to discover.

The fault, dear readers, was not with Pluto. It was with us, and our all-too-human propensity to turn planets into pets or cartoon characters.  It was also with our definition of “planet” and the way we applied it to certain solar system objects and not others.

And so, the International Astronomical Union took on the task of figuring out what “planet” means these days and which worlds got to be called by that name. It wasn’t easy, but they managed to come up with a more-or-less logical way of applying the word to solar system bodies that met certain criteria.  In particular, they invoked a dynamical basis for classifying planets into sub-categories such as dwarf planet.

Those categories are defined by answers to questions about the evolutionary history of a solar system object:  has it gotten big enough and does it have enough self-gravity to make itself more round?  does it orbit the Sun or some other object? Has it cleared out its own orbit?  Where does it exist? The answers for each world help define whether it’s a planet, a dwarf planet, and so on.

Now, the system is still far from perfect. In fact, it’s more complex and there’s a lot of debate about certain aspects of it. But, it’s a step in the right direction. And, since nothing is set in stone, I predict that there WILL be changes and tweaks to these definitions as time goes by.  This is not a bad thing. It is, in fact, one of the ways in which science works.

If you are truly fascinated with this topic, I refer you again to part one of Mike Brown’s excellent blog entry called “What’s in a Name?” (which I found after I started working on this three-part series). He has some good thoughts about this complex topic from the viewpoint of a scientist who’s been finding and naming worlds lately.

There’s also an event taking place starting tomorrow called the Great Planet Debate. It’s a meeting of planetary scientists who will discuss the latest solar system findings in an effort to further refine the answer to the question I posed above: what is a planet?  Interested folks can sign up to view a debate to be held on Thursday afternoon between Dr. Mark Sykes and Dr. Neil deGrasse Tyson. It should be very interesting!  The debate, moderated by National Public Radio “Science Friday” host Ira Flatow, is free and open to the public, and will be streamed live on the Web.

(NOTE: thanks to VagueofGodalming for pointing out that the debate is Thursday, not Friday. I misread the calendar.)