IAU Classifies Pluto & Eris As "Plutoids"

Kligat writes "The International Astronomical Union has decided that Pluto and Eris should be classified as "plutoids," alongside their 2006 classification as dwarf planets. Under the definition, the self-gravity of a plutoid is enough for it to achieve a near-spherical shape, but not enough for it to clear its orbit of its rocky neighbors, and the plutoid orbits the Sun beyond Neptune." Reader FiReaNGeL links to a similar story at e! Science News.

Comments

Re:Calimero

[sveard]

IMO classification can be a nice thing. It helps to reduce the clutter. However it seems that every now and then, something is discovered that 'refuses' to be classified.

Also, in Pluto's case, I think they should've made an exception for historical reasons. It should have remained the ninth planet while at the same time introducing a system for classifying objects. Every branch of science is riddled with exceptions, and it's nice for, for example, teachers to see their students get angry because there are so many exceptions to the rule. :D It makes learning something rather hard.

USAcentrism?

[papabob]

When Pluto lost its status of planet a couple of years ago I was shocked reading that the USA was lobying against that definition just because Pluto is the only planet discovered by an american scientist. Please, oh please, tell me that IAU hasn't produced this new denomination just for political reasons. It would be very sad...

What about Ceres?

[kfort]

I'm not sure why but they seem trying to purposefully exclude Ceres which is spherical in shape (able to overcome hydrostatic force) and exists in the asteroid belt

Whole debate seems bogus

[93 Escort Wagon]

Pluto isn't large enough to clear it's orbit of "rocky neighbors". Well, here's a news flash - neither Earth, nor Mars, nor Venus, nor Mercury have orbits that've been cleared of rocky neighbors. So apparently the bias only applies to the outer regions of our solar system?

For that matter, if you want to be REALLY pedantic - Pluto's orbit overlaps Neptune's, so Neptune apparently isn't large enough to clear it's orbit.

There! We've whittled it down to two planets total: Jupiter and Uranus. That'll be easy to remember...

Any excuse to make new words.

[Merls the Sneaky]

WTF is a plutoid? We already have a definition that could easily fit pluto and other celectial bodies like it
http://www.go-astronomy.com/glossary/astronomy-glossary-p.htm
"A large asteroid or other celestial body, also called a minor planet."

Call them planetoids. Therefore still remaining a planet but one that is not large enough to remove debris from its orbit. Then throw on mercury and mars and we can have a solar system of six planets and four planetoids (minor planets). This crap about removing debris from its orbit is farcical, how do they not know given another billion or two years it won't remove remaining debris?

Re:Other solar systems?

[Psion]

I don't think it was quite good at all. In fact, it wasn't even good. Calling it half-assed would be complementary. Consider:

...a "planet" is defined as a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit.

The first part suggest it is in orbit around the Sun. Not a sun. The Sun. Thus, there are no other planets in the universe outside of our Solar System. The second part is sorta-kinda okay, and I'll let it slide. But let's look at (c) ... Now look at this link. Does that orbit look clear to you? What about a billion years ago, when there was a lot more debris in the Solar System, was the Earth not yet a planet then? Even though it already supported life, had oceans and an atmosphere, etc?

In 1999, the IAU had its first brush with this issue when a proposal was made to consider Pluto a planet associated with the Kuiper belt. Astronomers then were strongly opposed to reducing the planet's status. But the discovery of 2003 UB313 (unofficially called "Xena" and it's satellite is "Gabrielle") might have caused this issue to be revisited. Slightly larger than Pluto, "Xena" was surely going to become our Solar System's tenth planet. But it's position in the Kuiper belt and the possibility for more similarly-sized objects caused some to worry that the number of official planets to grow dramatically. Might the Sun be home to 15, 20, 30 planets ultimately? Clearly, the matter needed to be pinned down. What is a planet after all?

Amazingly, until August 2006, there was no official astronomical definition for a planet. The IAU settled the problem by arguing over several different definitions, ultimately selecting one that kicked Pluto out of the club after two years of frustrating efforts. Indeed, the first committee to look at the problem, a group of astronomers, could not agree on a scientifically accurate definition. A second committee comprised of historians and educators was formed a few months ago to look at the problem from a fresh perspective.

Alan Stern, the head of NASA's New Horizons mission to Pluto, was outraged that less than five percent of the world's astronomers had a vote on the issue. Others think that the new category for Pluto is awkward when it says "A dwarf planet isn't a planet." Ultimately, I think the definition needs to be refined. Read it again. Then look at this chart. By the official definition currently accepted by the IAU, the Earth itself is not a planet. The existing definition not only places Earth on the questionable list, it also will present problems when we begin to identify planets in younger solar systems orbiting other stars. Inevitably, the issue will have to be revisited (probably in August 2009 at the next IAU General Assembly meeting), and at that time Pluto may be restored to planetary status.

Begging the question.

[Estanislao Mart�nez]

Classyfing Pluto as a Plutoid is not silly. Classifying Jupiter as a gas giant is not silly. Classying Mars as a telluric planet is not silly, either. This is exactly what you pointed out when you said that classifications were useful to catalog objects orbiting other stars.

You and GP are begging the question. Yes, classifications are "useful" to catalog objects orbiting other stars. But, what is the use of cataloguing objects orbiting stars, in the first place? What does it tell us? Does the classification of an object predict any properties of it that beyond those that were required to successfully classify it?

Two subpoints here:

1. You're committing a very common philosophical error, that I'll call ontological essentialism: the belief that there exists such a thing as a context-independent "correct" classification of things according to a given scheme. This error is leading you to think that there really must be a truth of the matter as to whether Pluto, as a thing in itself, is a "planet" or not.

   The response to this is that classifications aren't properties of things in themselves, but rather, are context- and purpose-dependent distinctions that people impose on them.

2. Astronomy is a natural science. Natural science is concerned with making predictions. The most natural use of classifications in natural science is, therefore, as a predictive apparatus: a classification has predictive value if, when you observe the properties of an object required to classify it correctly, you can use the classification to predict further properties that you did not observe.

   I've not seen anybody come even close to doing this for "planet." Once you observe all the things you need to observe to decide whether a celestial body is a planet or not, you're not in a position to predict anything else about the object.

   This doesn't mean that scientists can't use non-predictive classifications for genuinely useful means; non-predictive classifications can be quite useful for communicating with other people (if somebody says "planet," it may not allow you to predict a lot about the object, but it helps you guess what the other person may be talking about). But usually, those classifications don't really need to be very precise.

3. In any science, it pays to be skeptical about the validity of received vocabulary and classifications. I like the way one of my professors puts it: when faced with terms like "planet," it is often valuable to step back and, instead of seeing them as the names for distinct kinds of things, to see them as the names of distinct kinds of problems that the people who came up with the term were trying to solve.

   In this case, the problem is pretty simple. The ancients charted the movements of the lights in the night sky, and were concerned with formulating laws to explain their motion. The problem you hit right away when you start doing this is that a handful of those lights move in a manner that's very different from the vast majority of the others. Those weird, "wandering" ones are the so-called "planets," in the original sense. This goes back to point (1): the classification of some celestial objects as "planets" responds to the purpose of formulating and solving this problem.

   Guess what? We're not the ancients. We don't have their problems in explaining the motion of those things. We have super-powerful telescopes that show us all sorts of funny rocks in space that they could never hope to see, moving in all sorts of weird trajectories. We have a theory of Newtonian mechanics that explains their trajectories as a specific case of more general laws, without having to formulate laws of weird-space-rock-motion. Why are we keen at all to try to get a precise fit between what we see and their vocabulary? The reason we have problems with deciding whether something like Eris is a "planet" is because we know a lot more than the ancients did. Insisting too eagerly on the classification just demonstrates a failure to appreciate how very different and superior our understanding is.