The International Astronomical Union (IAU) released a draft resolution today on the definition of a planet. The seven-member panel agreed unanimously (some say miraculously so) on the following verbage:
A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.[*]If this definition is adopted, there would be twelve (12) planets in the solar system: the familiar nine plus Ceres (the largest of the Asteroids), 2003 UB313 (provisional designation for the larger-than-pluto object announced last year sometimes referred to as "Xena" or "Lila"), and perhaps most surprisingly Charon (Pluto's first "moon" discovered in 1978 would be elevated to make Pluto-Charon the first double-planet system--with additional two small moons.) The 26th General Assembly of the IAU will consider this question at its meeting in Prague on August 24th, 2006.
*For two or more objects comprising a multiple object system... a secondary object satisfying these conditions is also designated a planet if the system barycentre resides outside the primary.
It should be noted that the world Ceres (pardon the pun) was originally designated a planet when it was discovered in 1801; it was demoted about fifty years later when it became clear that there were millions of asteroids between Mars and Jupiter. A few years ago, with the discovery of thousands of trans-Neptunian objects (also called Kuiper Belt Objects), Pluto looked ready to suffer the same fate. Instead, the IAU working group decided that it was an error to require that planets have unique orbital characteristics.
I applaud this move because it applies a quantitative test and also recognizes that the solar system is not an empty Copernican orrery of concentric circles but rather a complex mish-mash of millions of bodies on all kinds of orbits. We know now that even the classical planets share their orbits: Jupiter shares its path with two clumps of "Trojan" asteroids, and it is a curious fact that the Moon's orbit is always concave to the Sun, which exerts twice the gravitational force on the Moon as the Earth. (But the barycenter of the Moon's orbit lies within the Earth, so it still counts as a satellite.)
Science should not stand still, and should be willing to change outmoded definitions regardless of their popularity. Now we just need to get a decent name for 2003 UB313 and its satellite.
31 comments:
What about the "if it's a sphere, it's a planet" standard? Isn't that being tossed around?
// posted by RBR
I think that's the standard, RBR.
// posted by LTG
Let's see: Ceres, UB313, Charon, Xena... What about Quaoar?
I thought it was still a matter for debate. For example Pluto is a sphere but there are those who want to demote it. Charon is a sphere but not considered a planet yet. Right?
// posted by RBR
I mean, I think that is exactly the standard the IAU is now endorsing, at least tentatively.
// posted by LTG
LTG is exactly right. The standard RbR asks about is precisely the standard that the IAU is now recommending (as mentioned in the original post.)
Quaoar, Ixion, and Varuna are possible candidates, but currently considered too small to be spheres.
Quaoar is bigger than Charon AND Ceres.
// posted by LTG
Quaoar is not necessarily bigger than Charon... The best estimate for Charon's diamaeter is 1207 ±3 km, but the best current estimate for Quaoar is 1260 ±190 km. While Quaoar is certainly bigger than Ceres (about 950 km) Quaoar is made of much lighter material than Ceres and may not have undergone the heating process necessary to reach a spherical shape.
Still, it is certainly a candidate, as are a few other bodies, such as Orcus, Varuna, Sedna, and 2005 FY9. We could end up with 16 planets quite soon.
Incidentally, the moons of Pluto-Charon are now named Nix and Hydra.
What will Astrologers do with 16 planets? Won't that upset the charts? Oh dear. But maybe I can get out of work by saying that it's a tough week, my Sedna is in Libra.
// posted by LTG
Of course, this being a political blog, we should be reminded that the real issue here is historical revisionism about Pluto by left-wing academic bodies. Pluto is and always was a planet, and there are only nine planets, just like it says in the bible (somewhere...).
Thanks for reminding me, LTG, about the point I wanted to make about science and politics. Conservative and progressive forces vie within the scientific community as well. The old (and not all that funny) joke is that the only way for new science to take root is for the old scientists to die off.
The battle over Pluto--joined by the battles over 2003 UB313 and extrasolar planets--has been an arena in which these forces have clashed out in the open. The recommendation by the IAU working group to clarify the definition with a quantitative test is, in my opinion, a triumph for progressive forces. If the IAU accepts the recommendation next week (and I hope they do!) then the question will no longer be, "is it a planet" but "what kind of planet is it?"
We will have then 5 terrestrial planets (Mercury, Venus, Earth, Mars, and Ceres) four gas giants (Jupiter, Saturn, Uranus, Neptune) and three in a newly-christened category of 'plutons' (Pluto-Charon, 2003 UB313). As LTG has pointed out, the plutons group may well be agumented to eight members should Quaoar, Orcus, Varuna, Sedna, and 2005 FY9 all be found to qualify. And that number may well increase as we discover more.
The roll call of discoverers of planets in our solar system would then return one demoted icon and add a few more: Herschel (Uranus, 1781), Piazzi (Ceres, 1801), Adams/Leverrier (Neptune, 1846), Tombaugh (Pluto, 1930), Christy (Charon, 1978), and Brown/Trujillo/Rabinowitz (provisionally named 2003 UB313).
Good stuff for the trivia meisters!
btw, the Brown/Trujillo/Rabinowitz group could soon have five planetary jewels in their collective crown: in addition to the acknowledged candidate 2003 UB313, they also discovered four of the five other potential candidates mentioned on this blog: Quaoar, Orcus, Sedna, and 2005 FY9! (The other one, Varuna, was found by R. McMillan and Spacewatch in 2000.)
Personally I just love what this will do to the astrologers...
Dr S, any thoughts on the detractors to the definition? My understanding is that the most reasoned criticism is that the naming convention does not include "planets" that that originate from outside the solar system (interstellar planet) or are ejected from the star system (rogue planet). This is different to the definition of a moon. Also isn't there an issue with brown dwarfs?
// posted by Numbat o Love
For UB 313 I recommend the anglicized "Nidhogg" - the dragon who gnaws away at Yggdrasil, the world tree. Also spelled Nithhoeggr (with special symbols) in Norse. Why? Because this may be the particular Trans-Pluto Object that brought down a whole world (Pluto) and forced Astronomy to reconceive of planets.
For good reasons, Cronus, Loki, Prosperina, and Persephone are also all popular. Loki would be best of these for ease of pronunciation.
// posted by Anonymous
I'd vote for Loki!
// posted by Raised By Republicans
As for the detractors, here are my thoughts:
a) brown dwarfs. By defining a planet as "not a star", they leave open the question of whether brown dwarfs qualify. I believe brown dwarfs will be classified as a type of star.
b) rogue/interstellar planets. The definition of a planet (or moon) is not merely the type of body, but what it orbits. A body that orbits nothing (except the center of the galaxy, perhaps) deserves a different name, even if it is planet-like. After all, Ganymede is planet-like, but gets a different name because of its environment.
c) consistency with satellite definition. A satellite is not just anything that goes around a planet; likewise, a planet is not just anything that goes around a star. For planets, we now have a defined lower limit, as opposed to the nebulous one we used to have. The problem is not with the planetary definition, but with that of a satellite definition: the satellite has no clear lower limit, but some things (e.g. the particles orbiting Saturn in rings) are not considered moons.
Why classify at all? Because it helps to give us a map. We label somethings as cities, others as towns; some things as lakes, others as inland seas; some things as rivers, others as creeks; some as hills, others as mountains. It is useful for understanding relative scales in the names. We are fortunate that we can make a clear distinction based on science for a "planet" definition--something we cannot do for any of the others I listed.
Incidentally, the discoverers of 2003 UB313, Quaoar, Varuna, Orcus, etc. believe that the proposed IAU definition (which they dubbed, "leave no ice ball behind") would actually make about 53 currently known bodies into planets (once the details of them are better known) and ultimately, hundreds.
This is because they take 400 km to be the minimum possible size of a round body, whereas the IAU used 800 km as their suggested cutoff. Alas, the quantitative definition may not be as quantitative as I had hoped. Food for thought.
To be honest, I don't have a huge problem with our little neighborhood having dozens or even hundreds of tiny planets floating around in it. I kind of like the idea of a fuller universe.
But many seem bothered by this perfunctory planetary proliferation. Is there any reason why there can't be one definiation for scientific classification and another for daily usage by the "laity?" Such is the case in many fields. A number of fairly specific and refined concepts used in the social sciences have alternative definitions in daily usage that are much looser: rationality, end game, liberal, fascist, market forces, free trade, socialism, democracy etc etc.
// posted by Raised By Republicans
RbR: you are quite right that the "official" scientific definition of a term often differs from popular usage. Astronmers are fortunate, however, that there exists a single body (the IAU) that is recognized by the entire astronomy community as the rightful arbiter of nomenclature (whether this is fair or not).
Unless I am mistaken, there is no equivalent body for political science or economics--so in a sense, there is no "official" definition for many of the terms you listed. In fact, don't different schools of thought disagree on the definitions of "socialism" etc.?
Assuming that that having consistent terminology is of value, we should support the IAU's efforts to arrive at a common (if not consensus) definition for "planet." And when the scientific community agrees on a single definition, it is probably more likely that the "laity" will use it than if there were no official definition.
I suspect the solution for everyone will be to speak of *classes* of planets: terrestrial, gas giant, and the new "pluton" class as defined by the working group. Rogue and interstellar planets might also be added as new categories later on.
Let me clarify that I mean no disrespect to the social sciences when I say (as I believe is true) that there are no universally recognized organizations in those fields that have promulgated universally accepted definitions for the sorts of terms RbR mentioned.
The concept of "socialism" (for example) is rather more complicated, and cuts across many more fields of knowledge, than the basic concept of a "planet"--which is why we teach the latter to kindergartners and not the former.
I believe that elementary school teachers are already saying, "There are eight major planets..." That's how this will be left with the public.
// posted by LTG
Dr. S. is mostly right that there aren't official definitions in political science. But for most of the terms I mentioned, there are definitions that are acknowledged by nearly all political scientists that use them on a regular basis in their research.
There is of course a national organization - the American Political Science Association. It is de facto the global organization as well because the discipline is far more extensive and advanced in the US than in the rest of the world. 90% of the cutting edge research is done in American universities by US trained political scientists. Much of the good research done over seas is done by people trained in the US. In that, political science is different from Astronomy as well I suspect.
(by "cutting edge" and "good" I mean research based on careful definitions, attempts at valid and precise measures, hypothesis testing etc. As opposed to impressionistic speculation of the type we all do on this blog as a hobby or some "old fashioned" political scientists do professionally)
// posted by Raised By Republicans
The IAU just killed the proposal. They are now going to opt for 8 planets only. Pluto will be demoted shortly, if the new proposal is adopted. Maybe it's time. But I think the other objects would be well called "minor planets" rather than Trans-Kuiper Objects or something hypertechnical.
// posted by LTG
The issue remains in flux as of this morning. The vote is scheduled for this afternoon. The proposed additional criteria is "orbital dominance"--essentially, a planet must be large enough to have swept away almost everything from its orbit. This gets prevents the possibility of co-orbital planets or double planets. We'll see what they do... but at the moment, yes, the "eight planet" conservatives are winning out over the "ice ball" rebels :-)
I don't see what's wrong with saying, "Pluto is a planet, but it's an exception" Of course, I'm a lawyer, so "grandfathering" Pluto is nothing new or surprising to me.
// posted by LTG
Biologists had similar problems with Linnean taxonomy until DNA analysis permitted them to be far more precise about how closely organisms are related. They still wrestle with issues, but they have a framework. Astronomers are still working on the framework to categorize and understand the cosmic zoo. As we are in our infancy in classifying the cosmos, it is critical to attempt to draw distinctions without arbitrary exceptions; the issue is about not about how we label the universe but how we understand it.
To make a biological analogy, I would argue astronomers have recognized four main "kingdoms": star, planet, asteroid, and exotic (quasars, black holes, etc.) But Pluto-Charon and 2003 UB313 challenge the planet/asteroid boundary. The "ice ball rebels" see the possibility of double planets, co-orbital planets, and extra-solar planets, and conclude that a broader definition of planet would be more robust. The "eight is enough" group see the bewildering array of planet-like objects and conclude that a narrower definition of planet would be more robust. (They may create a new "order" for the ice balls in the catch-all asteroid kingdom.)
Arguments can be made for both. I side with the ice-ball rebels on the theory that, since the planet kingdom already includes such diverse orders as "gas giant" and "terrestrial" (different states of matter!) then it was OK to create a third order in the kingdom for the smaller bodies.
I guess we'll just have to wait and see which way the solar wind blows at the IAU today.
The draft resolutions for voting have been released. They reflect the "eight is enough" point of view. Though I disagree, I am not a professional astronomer... and perhaps at this point it would be preferable to accept this resolution than to leave the issue unresolved--even though it would mean bye bye to Pluto.
The first resolution would declare a planet to be the working group definition EXCEPT that it must also have swept away most everything in along its orbit. A "dwarf planet" would be a body that has not cleared its orbit but otherwise fits the definition. Contrary to the original working group, this resolution specifically only applies to planets in our solar system. (No provision is made for extra-solar planets.)
The second resolution makes a teensy change, adding "classical" before the word "planet" in the definition of what would be "classical planets."
The third resolution clarifies that Pluto is to be classified as a dwarf planet of a new class of objects.
Finally, a fourth resolution names this new class the "plutonian objects" for now and calls for a committee to come up with a final name.
NEWSFLASH: It's been done and voted on: there are only eight planets.
There's an interesting political lesson here. Before the IAU released its initial draft resolution going the opposite way - permitting a dozen or even dozens of planets - I was upset that Pluto might lose its status. I thought the whole thing was dumb. Why change? Then, in the wake of the potential cosmic chaos created by up to 53(?) planets, a consensus gelled among astronomers, with which I and probably many others, now agree: keep it at 8. Proposals can create consensus around their alternatives. It's interesting bit of negotiation.
// posted by LTG
Yep, you can get people to agree to just about anything if you convince them that the alternative is crappy enough.
// posted by RBR
That's only part of it the lesson. The lesson is also that sometimes a consensus can only emerge after the door closes for endless debate. Think about Iraq in this context: does the US need to announce it's going to leave in order for Iraqi politicians to finally get down to brass tacks about putting their country together or dividing it?
// posted by LTG
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