Thanks to Staff Editor Tony Hoffman for this write-up! You can catch him on Gearlog Radio next week, discussing the asteroid he found and the demise of Pluto.
It's now been a week since the International Astronomical Union (IAU) stripped Pluto of its planethood. In choosing the more stringent of two competing definitions of the term planet, the IAU has booted Pluto into a new underclass of "dwarf planets", and seemingly capped the solar system's planet total at eight. Many scientists aren't pleased with the new solar-system order, saying it's imprecise and too restrictive. Meanwhile, Pluto continues to orbit out in the cold (literally, and now figuratively), oblivious to its demotion.
The ancient Greeks called the points of light that roved along the zodiac planetes: wanderers. But despite the discovery of several new planets since the invention of the telescope, astronomers never defined what a planet actually was. When Ceres, Pallas, Juno, and Vesta were discovered in the early 1800s, they were originally considered planets, but later reclassified as asteroids. Pluto, discovered by Clyde Tombaugh of Lowell Observatory in 1930, proved to be much smaller than originally thought, smaller than Earth's moon, leading some astronomers to call for its demotion. But it was Caltech astronomer Mike Brown's discovery of an ice-world slightly larger than Pluto, which he nicknamed "Xena", that brought the issue to a head—would Xena be classified as a planet, or a mere asteroid? And however Xena went, Pluto was likely to follow.
The initial definition proposed by an IAU committee in August defined a planet as an object that orbits the Sun and is large enough so that its gravity holds it in a near-spherical shape. Opponents claimed that this definition would permit tens or hundreds of puny poseurs to become planets (though the proposal took care to distinguish the eight "classical planets" from Pluto and the upstarts), and it would be confusing to students and the public. Almost no one has to memorize the entire periodic table of the elements, yet everyone knows oxygen, carbon, iron, and other key elements. So, kids would learn of "Xena" (whatever it is ultimately named), as well as far-ranging, reddish Sedna, several large, exotically named iceballs (Quaoar, Ixion, Orcus, and Varuna), a football-shaped world known only as 2003 EL61, and other worlds as yet undiscovered.
The struggle to define planets became a turf war between planetary geologists, who study the structure and composition of worlds, and orbital dynamicists, who are concerned with planetary motion. The latter group forced the inclusion of a provision that to be a planet, an object must have "cleared the neighborhood around [its] orbit". This rules out Pluto—which orbits amid a flock of similar (yet mostly smaller) iceballs in a region known as the Kuiper Belt—and Ceres, the largest asteroid; both are being reclassified as dwarf planets. But what of Neptune? It hasn't cleared away pesky Pluto, nor the Kuiper-belt objects that cross its orbit.
And by the new definition, Earth—which lies in a cosmic shooting gallery of tens of thousands of asteroids that potentially could collide with us—shouldn't be a planet. (As to whether there's intelligent life here, I'll leave that to a future discussion.)
The solar system was due for a reclassification, to keep up with new discoveries. It made sense for the IAU to call Pluto the prototype for a new type of "trans-Neptunian" object, but that and planethood need not be mutually exclusive. Any definition of planet is some-what arbitrary; it's not as if objects are hung with tags that say "I'm a planet!" Faced with competing definitions, both with some scientific merit, the IAU went the restrictive route. Even as telescopes reveal new and exotic denizens of the outer solar system, we've gone and shrunk the solar system. Pluto's gotten the boot, and dwarf planets by definition aren't true planets. (Perhaps a better name for them would be planettes or worldlets.) Even many of the astronomers who supported the new order admit a twinge of sadness over Pluto's demotion. The new system is not only imprecise, but it's demoralizing. People are inspired by the idea of new planets in a way that they're never likely to be for lesser solar system bodies.
*Pictured here is a stained glass panel created in honor of Pluto's discoverer, Clyde Tombaugh, which Hoffman photographed last year in New Mexico.
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August 31, 2006 3:55 PM
It is sad that the planet we all came to love in elementary and middle school has now been demoted. How could you be so mean to something with a name like "Pluto". On the other hand I am glad to see astronomers trying to come up with definitions to be used by everyone.
August 31, 2006 5:21 PM
I know, it's really weird to think about, isn't it? The fact that Pluto is no longer considered a planet is so strange. And now textbook publishers will need to reissue books for the schools with the change. But, I guess this just goes to show that science isn't static; it does change. But it's almost like a little piece of my education is lost!! And catch Gearlog radio next week, Lupe. We mention your name! :)
August 31, 2006 6:07 PM
A few inconsistencies to clear up (Astronomy is a passion of mine, enough that I have a degree in it, even though I wound up studying something different in grad school and work in yet another different field, but anyway, long story) in this piece: Neptune does lie inside the orbit of the vast majority of the object that are considered the Kuiper Belt. In fact, the majority of objects in the Kuiper belt lie beyond the orbit of Pluto. Part of the problem is that Pluto has more similar characteristics with objects in the Belt than the gas giants that it lives so close to. Pluto's highly elliptical and off-degree orbit around the sun as giveaways; and the fact that it's the last rocky object past the realm of gas planets is another one. That and the fact that the Kuiper belt and asteroid-like objects that the Earth revolves around the sun among are very VERY different (the asteroids being mostly rocky and very small, most of them remnants of the planetoid that never congealed-ie the Asteroid Belt) wheras the objects in the Kuiper Belt are of varying sizes, but are generally larger then the inner system rocky objects, and are more ice and forzen gas than they are rock and debris, like the kind that falls to Earth during meteor showers. So using that as a distinction and the basis of the statement that Earth shouldn't technically be a planet either is woefully inaccurate. The real problem is that Pluto is more like the Kuiper Belt and the Oort Cloud than it is like it's closest neighbors like Neptune and Uranus, and the processes that formed the planets are relatively consistent-the rocky planets up front, and the gas giants out (back due to solar wind and pressure as the Sun formed) and small balls of gas, dust, and ice that was left over from planetary formation farther out and in a cloud around the solar system-Pluto falls so much more in line with those Solar leavings that were brushed out of the solar system over 4 billion years ago that it's impossible to really get away with classifying Pluto as more like its planetary siblings than not. I think the IAU was trying to take a middle road, where they wanted to acknowledge Pluto's status in history and through tradition, so they came up with the term "Plutons," to describe those dwarf planets like it and Ceres (Pluto being compositionally very different than Ceres, by the by) but still allow Pluto to be in the popular lexicon. Still while I see their point, I'm dismayed that Pluto got demoted as well. Still, it does show that science in general does what we're proud of it doing best-it evaluates, re-evaluates, and re-classifies; it's not static and it doesn't proclaim facts and then never revisit them. If proof comes up to say "no, that old theory is wrong and we shouldn't keep it just for tradition's sake," then the scientific community is capable of refuting it in the face of evidence. That's what I really really love about science. :)
September 1, 2006 4:53 AM
i think that its sad that Pluto is being removed... ever since elementary school we have learned the planets... pluto included... i guess, everything changes with time,... its just sad. how very, very sad. there will be a lot of new books to be purchased and that will really hurt the school funds... schools are in trouble...too much money... oh well... how sad...:(
September 1, 2006 7:45 AM
Does size really matter? I think not! Discovery Channel has our backs and put out a pin supporting everyone's favorite small planet. ;) http://www.zatznotfunny.com/i-love-pluto/
September 2, 2006 4:04 PM
Why should we let them tell us what is a planet and what is not. We all know its a planet I say ignore the never before known (IAU) and the US and other countries should declare it a planet. Disband the (IAU) we don't need them.
September 2, 2006 6:56 PM
Next Boulder will be renamed Little Rock.
September 2, 2006 11:15 PM
One would think these scientists could spend their time more wisely, dealing with more pressing and important matters RIGHT HERE ON OUR OWN ROCK!
September 4, 2006 4:08 PM
Wow...some of those last comments are just a whole other world of stupidity. The IAU has been doing work longer than most people have been alive-how about finding out what they do before trying to say you don't need them? And hello, they're ASTRONOMERS, their job involves things in space! If you want people to deal with things down to earth, go find a geologist. Wow. The moronity flows like water.
September 5, 2006 2:23 PM
Thanks, everyone, for responding. Phoenix, you do make some good points about the different populations (and you're right that most Kuiper-belt objects orbit farther out than Neptune or Pluto). People like Neil deGrasse Tyson had suggested even making further divisions of solar system objects: the small, rocky Terrestrial Planets (Mercury, Venus, Earth, and Mars), the Gas Giants (Jupiter, Saturn, asteroids (the small, rocky fragments), and then ice fragments in the outer solar system, and comets as well. It seems that the IAU gave a nod to that by suggesting a new class of trans-Neptunian object, of which Pluto is the prototype, separating rocky asteroids from at least the trans-Neptunian ones. Three words in particular ("the letter of"--as opposed to the spirit of) were edited out of my original piece in the sentence that follows, which makes it come across differently than I had intended: "And by [the letter of]] the new definition, Earthwhich lies in a cosmic shooting gallery of tens of thousands of asteroids that potentially could collide with usshouldn't be a planet." Clearly, nobody is actually suggesting that either Neptune or Earth won't be planets--but the definition isn't precise enough to exclude that. I am but an amateur astronomer, but many professional astronomers claim that. How it's actually applied is a different matter. (The implication of the new definition would seem to be gravitational dominance of the area in which the planet orbits.) And it's the gray areas, the borderline cases, that are always the real test. What if other largish objects (Mercury-size or larger) were discovered in the outer solar system, beyond Pluto and the Kuiper Belt? It might be very hard to determine whether they had swept their orbits clean, as smaller objects at that distance might be too faint to be visible. And what of large objects whose orbits are highly inclined to the ecliptic. Do you automatically exclude them as planets--in other words, should a planet only be something that orbits in the same plane as the other planets? I wonder if the IAU would have done better if they had involved a larger group of planetary scientists earlier on. (It's quite likely that they elicited feedback, and I'm sure they got much unsolicited, but the actual working committee was very small, just seven people.) That may have allowed them to at least work through the differences in hopes of reaching a consensus--if it didn't result in mass confusion. They were faced with a large order: to determine how to classify the objects in our solar system--something that's never been formally done before (at least with planets). There's talk of a conference of planetary scientists next year, and the IAU revisiting the subject in 2009. Ultimately, it's not about Pluto at all, but how we classify not only the objects that we know already, but those that await discovery, around other suns as well as our own. If it takes a bit more time to get a more satisfactory definition, it will presumably be worth it.
September 5, 2006 3:08 PM
Dusty, you might as well say "why don't baseball players become diplomats?" Actually, NASA has taken the study of the Earth (specifically the words "to understand and protect our home planet") out of its mission statement, which I feel sets a bad precedent (even if it has no immediate effect on operations).
September 5, 2006 9:52 PM
Thank you, Tony, for revisiting this topic, continuing the discussion, and fleshing out some of your points. I found the discussion interesting, if not a bit tounge-in-cheek when I think I might have missed the sarcasm and taken it for seriousness, so for that I apologize at least. I may have spent years studying the stars, but something tells me you've clocked more hours in front of a telescope or in front of photo plates than I have. I'm particularly impressed with your proposal of a more unclear definition-if we suddenly find a Mercury-sized object in the Kuiper belt, is it not a planet? I think that's a question that Planetary Astronomers will be struggling with for years to come, and again, I agree with your assertion that I, for one, would rather see more time and investigation taken to get a precise and clear definition, one that comes with the solid knowledge of what objects exist in our solar system and how they relate to one another. My particular contention comes from the physical makeup of the objects and their orbital plane, which makes definition a bit easier than perhaps it really should be. Either way, I think your perspective warrants greater attention, especially in the vein of the IAU taking coments and criticism from external bodies and groups of planetary astronomers. You raise the excellent point I had all but forgotten: what happens as we discover more and more planetary systems beyond our solar system? Will they, given that they could have formed under different circumstances as our own solar system, fit in nicely with our preconceived notions of "planethood," even those that astrophysically would be considered consistent across the stars-eg orbital plane, system sweep-out of stellar matter during the formative eons, rocky planets interior/gas giants exterior, etc. As we find more and more examples and systems that test our physical knowledge of stellar formation and the challenge our concept of traditional astrophysics, I wonder if we'll be able at all to stick to the narrow definitions that we have been able to up to this point. Thanks again, Tony!
September 5, 2006 9:55 PM
By the by-I'm very very glad you brought this up, Tony! It's incredibly sad that not only has NASA removed its study of our home planet from its mission statement, but its also gone to great lengths to fire scientists for daring to practice science, if it conflicts with the moon/mars "vision" set forth in the past several years. As NASA's actual science budget gets slimmer and slimmer in the face of big ticket projects like massive manned-mission projects, I worry that studies of important topics like climate change, potential asteroid collision, environmental studies, and global mapping might suffer-real science, that might falter in the wake of expensive projects that might stir the spirit but return exceptionally little to the tome of human knowledge.
September 6, 2006 1:56 PM
Phoenix, Thanks for your kind words, and I can understand your reaction to the tongue-in-cheek nature of some of my remarks. It's funny how the whole situation seems to have inspired more passion, controversy, and opinion than probably any astronomy-related issue in memory--presumably because we, ourselves, live on a planet, so the subject really hits home for us, and helps to define our place in the cosmos. What does Earth have in common with, say, Jupiter, an enormous ball of hydrogen and helium (with small amounts of methane, ammonia, and other compounds that help make its clouds so colorful) which contains most of the mass of the solar system (outside of the Sun) but has no solid surface, rather layers of liquid hydrogen and metallic hydrogen that surround a rocky core? They're both clearly planets, sure, but it makes some sense to further classify them as terrestrial planet and gas giant, respectively. With other solar systems being discovered--only large worlds so far--we may be able to get a clearer idea of what separates a gas giant world from a brown dwarf, or what lies between terrestrial planets and gas giants. We live in a fascinating time, when our telescopes are revealing so much, yet so much of it is at the fringes of detectability that we have a long way to go to fully understand it. The diversity of the new solar systems being discovered has caught many astronomers by surprise, and doubtless the surprises have only begun.
September 6, 2006 2:01 PM
One of the tragedies of NASA's shift in priorities and budget cuts is that the Terrestrial Planet Finder mission (http://planetquest.jpl.nasa.gov/TPF/tpf_index.cfm), which would detect smaller worlds around other stars, has been put on indefinite hold.