Will Pluto prevail?
Posted: Friday, August 24, 2007 8:10 PM by Alan Boyle
It's been exactly a year since the International Astronomical Union busted Pluto down a rank, from one of the solar system's nine major planets to one of potentially thousands of dwarf planets. Scientifically speaking, the debate over planethood for Pluto (and other denizens of deep space) will go on for years. But when it's time to buy that glow-in-the-dark planetary mobile, you're increasingly likely to get eight planets, plus an explanation.
If Alan Stern has his way, the makers of toy planets shouldn't be too quick to toss out their Pluto mold. Stern, one of the underdog planet's biggest proponents, is the principal investigator for NASA's Pluto-bound New Horizons probe as well as NASA's associate administrator for space science.
NASA / ESA / JHUAPL / SwRI |
A Hubble photo shows Pluto with its largest moon,
Charon, as well as two moonlets, Nix and Hydra.
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Stern thinks the scientific tide has actually turned in favor of Pluto's planethood over the past year: "Many people just refuse to use the IAU definition," he told me this week. "Although a lot of teachers think the IAU [decision] is a done deal, people are slowly coming to realize, 'Not so fast.'"
Even at the time that the definition of planethood was hammered out in Prague, the IAU faced a storm of criticism over some of the clauses in that definition - for example, the rule that a planet had to have "cleared the neighborhood around its orbit." That definition could conceivably allow someone to argue that Jupiter wasn't a planet, due to the asteroid belt or even perhaps the Oort cloud.
"It's like saying a cow is not a cow unless it's on a particular kind of ranch," Stern said.
The planethood debate arose in the first place because some of the worlds discovered on the solar system's icy edge were close to or even exceeded Pluto's size. That forced astronomers to choose between adding to the list of planets and subtracting from it. The IAU chose subtraction - but others have different ideas. Stern even noted that some experts have referred to Ceres, the biggest object in the asteroid belt, as a planet. Maybe there should be 12 planets rather than just eight or nine.
The fact that some scientific societies still haven't settled on the IAU's definition could leave the door open for Pluto's comeback.
Even if the "dwarf planet" designation sticks, Pluto still might hold its historical place of prominence - or at least that's Stern's hope. "A dwarf planet is still a planet, just like miniature dogs are still dogs, and dwarf people are still people," he said.
The IAU's defenders would agree that nothing about Pluto itself has changed, and that the plucky little world is still worthy of attention. It's not so much a question of Pluto, but of the pigeonhole you put it in. If you have only so much space in the pigeonhole, and only so much time to devote to the solar system in the classroom, where do you draw the line?
When you look at the issue that way, Pluto's chances for lingering on as the ninth planet aren't that great. Sure, you can still find the nine-planet set at most toy stores, but that might last only until the new stock comes in, said Carl Benoit, editorial director for Illinois-based Learning Resources, which sells educational supplies, toys and games.
"From what I've seen, it's eight planets - and then they will talk about Pluto being a dwarf planet," he told me today.
Anton Skorucak, chief executive officer for PhysLink, another online science store and Web portal, said he's seen mixed reaction to Pluto's demotion.
"Some still specifically want products that do have Pluto in the planets, particularly the older clientele," he told me. "A lot of people are buying products that still have Pluto left over because of the collectible value, because in five years or so, the products that have Pluto in them will probably be priced higher. It's very interesting."
The changeover from nine to eight planets depends on the product cycle: Web sites, for example, can change at the drop of a hat. Wikipedia already recognizes eight planets and at least three dwarf planets, including Eris and Ceres as well as Pluto. The widely respected "Nine Planets" Web site has had its logo virtually spray-painted with the number 8, and although it's accessible via nineplanets.org, you can also get to it through eightplanets.org.
When it comes to physical products, it's easier to change a planetary poster or place mat than a solar-system planetarium kit with more than two dozen parts. Skorucak admits that people sometimes ask for a solar-system poster that has Pluto as a planet. "But it's too late - the manufacturer has already changed its stock," he said.
San Francisco-based Great Explorations currently sells a nine-planet set as well as an eight-planet set, but program manager Amy Rosen said the company hasn't heard strong feedback either way. "I don't think consumers think about whether there are eight or nine in the box," she said. "They just take what's available."
Most companies, like most scientists and educators, see Pluto's predicament as a teachable moment. "We do have several games and mobiles and puzzle pieces that have Pluto in there, but we're just using the opportunity to teach that things change in science, and that this is one of those times when Pluto has been declassified from a planet to a dwarf planet," Benoit said.
Maybe it's not so bad being a dwarf planet - considering that they'll be having their day in the sun in the years ahead. Next month, NASA is scheduled to launch the Dawn spacecraft toward Ceres and its smaller sister in the asteroid belt, Vesta. Then there's New Horizons, which is due to fly by Pluto and Charon in 2015 and perhaps see other icy worlds as well.
New Horizons famously flew past Jupiter earlier this year, and since then the spacecraft has covered half the distance between Jupiter's orbit and Saturn's, Stern said. The craft is due to go through a course correction next month and run through some instrument calibrations before going back to sleep.
As it makes its way into the outer solar system, New Horizons will periodically wake up to make observations - for instance, measuring the solar wind and taking pictures of Pluto ahead. "There are a half-dozen things that we're doing, and we're always on the lookout for something we can get relatively close to," Stern said. "If something pops up in our path, we're going to go after it."
Stern admitted that it can be hard to keep the public interested in a mission that will take nine years to reach its main destination. To humanize New Horizons' progress, Stern came up with the idea of selecting five kids who were born on the day New Horizons was launched - Jan. 19, 2006 - as well as five more who turned 10 on that date.
The first group of "Pluto Pals" made their Web debut this month, and the 10-year-olds (who have by now turned 11) will be selected this fall. The New Horizons Web site will check in on the kids annually through 2016 to find out how they're doing, and compare their milestones with those of the spacecraft.
Will the debate over eight vs. nine planets still be simmering when those Pluto Pal infants turn 10? Or will we instead be wondering how to classify hundreds of planetoids around distant stars as well as our own? Stern trusts that the scientific debate will eventually settle on the right answer - about Pluto, and about the worlds to come.
"Things that don't work fall by the wayside," he said. "Things that do work are the ones that we keep."
