TheInternational Astronomical Union (IAU) has spoken on the status of Pluto. Theonly thing missing when they announced the decision at their press conferencewas the "Mission Accomplished" banner. Yes, I'm afraid this matter is about assettled as the Iraq war in 2003.
You'veprobably heard the basics: Pluto is no longer to be considered a "real" planet,but will instead be part of a new class of objects called "dwarf planets."These midgets may number anywhere between a handful and hundreds in our ownsolar system, depending on how you count them and how hard we search for themwith more powerful telescopes. But no matter how you cut it, this newdefinition takes away any pretense of Pluto being a member of the same elite,planetary club as Earth and Jupiter.
As ascientist, I think this was a pretty good outcome, though some of thejustifications used to achieve it are dubious. But as a teacher, textbookwriter, and builder of scale model solar systems, I have some reservations. Inparticular, for the model solar systems I've helped develop on the University of Colorado Boulder campus and on the National Mall in Washington, DC, I have toask: Should we now bulldoze Pluto?
A littlebackground: Pluto was discovered in 1930, at a time when astronomers weresearching for an object thought to be causing slight perturbations to theorbits of other planets around the Sun. Neptune itself had been discovered injust this way in 1846, after scientists used perturbations in Uranus's orbit topredict the existence and location of an "eighth planet." Neptune was found assoon as astronomers pointed telescopes to the calculated position, which is Ilike to say that Neptune was discovered with physics and mathematics, and onlyconfirmed with a telescope.
Over theensuing decades, a few scientists claimed to see ongoing orbital discrepanciesand embarked on a search for a "ninth planet" that might be causing them. Plutowas found during this search, though about 12 full-moon-widths away from thepredicted position. And though hailed as a planet upon discovery, its statusgradually became suspect, as we learned that its orbit is much more tilted andelongated than that of any of the other planets, and that it has a mass muchless than 1% that of Earth. Worse, reanalysis of past observations suggestedthat the claimed orbital discrepancies had simply been errors in measurement,making Pluto a solution to a nonexistent problem.
In 1951, byanalyzing comet orbits, astronomer Gerard Kuiper predicted the existence of a"comet belt" beyond Neptune -- now officially named the Kuiper belt --analogous to the asteroid belt between Mars and Jupiter. Telescope technologycaught up with this prediction in the 1990s, and astronomers soon confirmed theexistence of vast numbers of "Kuiper belt objects" orbiting the Sun in thegeneral vicinity of Pluto. Scientifically, it became obvious that Pluto wasmuch more like a large member of this group than like a small version of any ofthe other 8 planets. But as long as Pluto was the largest Kuiper beltobject, the IAU felt it acceptable to leave its planetary status intact.
Then, inJuly 2005, astronomer Mike Brown announced the discovery of a new Kuiper beltobject -- still nameless today but nicknamed "Xena" or "Planet X" -- that isslightly larger than Pluto. This discovery was a fatal blow to the status quoof nine planets; after all, if Pluto is big enough to count as a planet, thenXena must belong to the club too. But what of the dozens of Kuiper belt objectsonly a little smaller than Pluto, and of future discoveries yet to be made? Howwould we draw the line between "planet" and "large Kuiper belt object?" Likeall groups facing a tough decision, the IAU appointed a committee.
At first,the committee proposed roundness as a primary criterion for planethood, whichwould have admitted both Pluto and Xena to the club. However, their proposalwould have also admitted Pluto's moon Charon (for technical reasons), perhapsas many as 40 or more other Kuiper belt objects, and even the asteroid Ceres.Given that Ceres was discovered in 1801, we would have had to start sayingsilly things like "The ninth planet, Neptune, was discovered in 1846, when itwas mistakenly identified as the eight planet."
The IAUmembers saw the light, and modified the definition to keep Ceres and Charon outof the club by adding that a planet must not only be round, but must also have"cleared the neighborhood around its orbit." Ceres fails this criterion,because it is just one of many asteroids in the asteroid belt. Charon returnsto being a moon of Pluto. But Pluto loses its status too, because it is justone of many objects in the Kuiper belt, and in fact crosses over the orbitalpath of Neptune.
Some of my colleagues are quite upset, pointing out that noobject has truly cleared it its orbit -- that's why collisions still sometimesoccur, like the comet that smacked into Jupiter in 1994 or the asteroid thatstruck Siberia in 1908. Others wonder why we need to add the new term "dwarfplanet," when "large asteroid" or "large Kuiper belt object" can alreadydescribe objects like Ceres and Pluto and Xena quite well. And imagine theconfusion if we someday discover a Mars- or Earth-size ice ball in the Kuiperbelt: officially, such an object would count as a dwarf planet, but it would belarger than some of the non-dwarfs. Indeed, many astronomers wonder why we needan official definition of "planet" at all. That's why I doubt this debate isover, and it brings me back to my point: do we really need to take Pluto out ofsongs, place mats, mobiles, and model solar systems? I don't think so.
The important thing to remember is that the new definitionwas established by a vote, making it politics, not science. The politics mayyet change again, but Pluto will remain the same. I suspect it will stay aplanet in hearts and minds no matter what the IAU says, much as Europe and Asia remain separate continents to everyone except the geologists. Pluto, after all, is notjust any planet -- it's the only planet with a famous dog namedafter it, and some people may not take well to sending the dog home justbecause it didn't turn out quite as big as originally hoped. So let's keep thebulldozers away from the Pluto pedestals on the CU campus and the NationalMall. In fact, if someone will provide the budget, I'd recommend adding apedestal for Xena, if astronomers ever vote to give it a real name.
- BLOG: The Right Decision
- Pluto Demoted: No Longer a Planet in Highly Controversial Definition
- 3rd Proposal 'Shot Down in Flames'
- Details Emerge on Plan to Demote Pluto
- Pluto May Get Demoted After All
- One Astronomer Says 'It's All About the Atmosphere'
- Earth's Moon Could Become a Planet
- Public Laughs and Shrugs at 12-Planet Proposal
- Astronomers Sharply Divided on New Planet Definition
- Adding Planets Means New Textbooks, Toys
- Nine Planets Become 12 with Controversial New Definition
- Image Gallery: The 12 "Planets"
Defining Moments: The Saga's History
- Pluto's Fate to be Decided by 'Scientific and Simple' Planet Definition
- JUNE: Definition of 'Planet' Expected in September
- 2005: Definition Debate: Planets May Soon Get Adjectives
- 2003: Controversial Proposal Would Boost Solar System's Planet Tally to 12
- 2000: What is a Planet? Debate Forces New Definition
JeffreyBennett is the author of more than a dozen books, including Life in theUniverse (with Seth Shostak), Max Goes to the Moon, Max Goes toMars, and the forthcoming Beyond UFOs.