This article appeared on Space.com
Note: The object referred to in this article as “Xena” is now officially named Eris.
The International Astronomical Union (IAU) has spoken on the status of Pluto. The only thing missing when they announced the decision at their press conference was the “Mission Accomplished” banners. Yes, I’m afraid this matter is about as settled as the Iraq war in 2003.
You’ve probably 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 own solar system, depending on how you count them and how hard we search for them with more powerful telescopes. But no matter how you cut it, this new definition takes away any pretense of Pluto being a member of the same elite, planetary club as Earth and Jupiter.
As a scientist, I think this was a pretty good outcome, though some of the justifications used to achieve it are dubious. But as a teacher, textbook writer, and builder of scale model solar systems, I have some reservations. In particular, 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 to ask: Should we now bulldoze Pluto?
A little background: Pluto was discovered in 1930, at a time when astronomers were searching for an object thought to be causing slight perturbations to the orbits of other planets around the Sun. Neptune itself had been discovered in just this way in 1846, after scientists used perturbations in Uranus’s orbit to predict the existence and location of an “eighth planet.” Neptune was found as soon as astronomers pointed telescopes to the calculated position, which is I like to say that Neptune was discovered with physics and mathematics, and only confirmed with a telescope.
Over the ensuing decades, a few scientists claimed to see ongoing orbital discrepancies and embarked on a search for a “ninth planet” that might be causing them. Pluto was found during this search, though about 12 full-moon-widths away from the predicted position. And though hailed as a planet upon discovery, its status gradually became suspect, as we learned that its orbit is much more tilted and elongated than that of any of the other planets, and that it has a mass much less than 1% that of Earth. Worse, reanalysis of past observations suggested that the claimed orbital discrepancies had simply been errors in measurement, making Pluto a solution to a nonexistent problem.
In 1951, by analyzing 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 technology caught up with this prediction in the 1990s, and astronomers soon confirmed the existence of vast numbers of “Kuiper belt objects” orbiting the Sun in the general vicinity of Pluto. Scientifically, it became obvious that Pluto was much more like a large member of this group than like a small version of any of the other 8 planets. But as long as Pluto was the largest Kuiper belt object, the IAU felt it acceptable to leave its planetary status intact.
Then, in July 2005, astronomer Mike Brown announced the discovery of a new Kuiper belt object, still nameless today but nicknamed “Xena” or “Planet X”, that is slightly larger than Pluto. This discovery was a fatal blow to the status quo of nine planets; after all, if Pluto is big enough to count as a planet, then Xena must belong to the club too. But what of the dozens of Kuiper belt objects only a little smaller than Pluto, and of future discoveries yet to be made? How would we draw the line between “planet” and “large Kuiper belt object?” Like all groups facing a tough decision, the IAU appointed a committee.
At first, the committee proposed roundness as a primary criterion for planethood, which would have admitted both Pluto and Xena to the club. However, their proposal would have also admitted Pluto’s moon Charon (for technical reasons), perhaps as 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 saying silly things like “The ninth planet, Neptune, was discovered in 1846, when it was mistakenly identified as the eight planet.”
The IAU members saw the light, and modified the definition to keep Ceres and Charon out of 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 returns to being a moon of Pluto. But Pluto loses its status too, because it is just one of many objects in the Kuiper belt, and in fact crosses over the orbital path of Neptune.
Some of my colleagues are quite upset, pointing out that no object has truly cleared it its orbit – that’s why collisions still sometimes occur, like the comet that smacked into Jupiter in 1994 or the asteroid that struck Siberia in 1908. Others wonder why we need to add the new term “dwarf planet,” when “large asteroid” or “large Kuiper belt object” can already describe objects like Ceres and Pluto and Xena quite well. And imagine the confusion if we someday discover a Mars- or Earth-size iceball in the Kuiper belt: officially, such an object would count as a dwarf planet, but it would be larger than some of the non-dwarfs. Indeed, many astronomers wonder why we need an official definition of “planet” at all. That’s why I doubt this debate is over, and it brings me back to my point: do we really need to take Pluto out of songs, place mats, mobiles, and model solar systems? I don’t think so.
The important thing to remember is that the new definition was established by a vote, making it politics, not science. The politics may yet change again, but Pluto will remain the same. I suspect it will stay a planet 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 not just any planet – it’s the only planet with a famous dog named after it, and some people may not take well to sending the dog home just because it didn’t turn out quite as big as originally hoped. So let’s keep the bulldozers away from the Pluto pedestals on the CU campus and the National Mall. In fact, if someone will provide the budget, I’d recommend adding a pedestal for Xena, if astronomers ever vote to give it a real name.