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NASA / ESA / SwRI / U. of Md.
The way some scientists see it, the asteroid Ceres (on the left) would be a planet
while the asteroid Vesta (on the right) would not. The difference? Roundness.

Does Pluto deserve a place among our solar system's main planets, or were astronomers right to demote it to second-class status? Two years ago, poor Pluto's plight touched off the dispute over the how you define a planet, but now it's about much more than one little icy world. The Great Planet Debate rises to a whole new level this week, and thanks to the Internet, you can join in the debate yourself.

The long-awaited debate takes center stage at Johns Hopkins University's Applied Physics Laboratory, which is the base of science operations for NASA's New Horizons mission to Pluto and other denizens of the solar system's outer regions.

The main event comes at 4:30 p.m. ET Thursday, when Mark Sykes of the Arizona-based Planetary Science Institute faces off against Neil deGrasse Tyson of New York's Hayden Planetarium. You can register to watch the Sykes-Tyson debate as well as other "Great Planet Debate" presentations via streaming video. The event's organizers also hope to pass along some questions for the debaters from the online audience.

Pro-Pluto? Anti-Pluto?
Tyson is often typecast as a "Pluto-hater," while Sykes is characterized as a "Pluto-hugger." However, their real views about planets are more complex - and Sykes said that the greatest benefit of the Great Planet Debate may be the opportunity to show the general public how scientists deal with such complex disagreements. It's a process that is applied to other controversies as well, ranging from climate change to particle physics.

"People get to see that there really is this process that goes on, and the process doesn't really result in a winner or a loser," Sykes told me this week. Instead, he said scientists and the general public should ideally "gain an understanding of why we think the way we do."

In Sykes' view, that is 180 degrees opposite to the process that resulted in Pluto being dubbed a dwarf planet by majority vote at the International Astronomical Union's general meeting in 2006. Sykes said scientific questions should be decided by discussion and the data, not by taking a vote. And for that reason, you shouldn't expect scorekeepers to declare a winner after Thursday's matchup.

"Even though I think I would win, I wouldn't let there be a vote," Sykes said.

12 or more planets?
So where does Sykes stand? He would get rid of the IAU's idea that a true planet would have to "clear the neighborhood around its orbit," and instead go with this seemingly simple definition: "A planet is a round object (in hydrostatic equilibrium) orbiting a star."

Why is being round a big deal? "Roundness is just an indicator that this object has undergone evolution, and that it will exhibit geological processes," Sykes explained.

Objects big enough to be gravitationally compressed into a roundish shape will usually have differentiated layers in their interiors, and could exhibit other features such as volcanism or an atmosphere. Pluto, for example, is thought to have a thin atmosphere - and its largest moon, Charon, might have ice volcanoes.

Syke's definition would put Pluto back on the list of planets that existed before the IAU's decision, but it would also add the asteroid Ceres and the recently discovered ice world Eris (which is thought to be bigger than Pluto). It would even add Charon to the list, because the two worlds trace orbits around each other even as they both orbit the sun.

Such a lineup was initially proposed to the IAU by a panel of experts but never saw the light of day - in part because of that Pluto-Charon issue. Sykes, however, didn't see that as a problem. "Why can't we have double planets?" he asked. "That's actually pretty cool."

The definition raises other tricky questions. For example, what about the not-quite-round asteroid Vesta, which is due to be studied up close along with Ceres during NASA's Dawn mission? In Sykes' view, Vesta might well have been a planet billions of years ago - but lost that status after a cosmic collision gouged a huge crater in the rock, ruining its roundness.

"It was a planet, but then it evolved," Sykes said.

Still more planets could be added to the 12 as astronomers take a closer look at the edges of our solar system. And a wealth of worlds beyond the solar system would fit the definition as well.

For more of Sykes' perspective, check out his recent article in the journal Science.

Are 'planets' passé?
Tyson is a little cagier about his strategy for Thursday's debate: "I have no platform, so what I will end up saying will depend largely on what Mark Sykes says," he told me in an e-mail.

However, Tyson pointed to an article he wrote last year for the American Astronomical Society's Spark newsletter, titled "Pluto's Requiem," as an indication of where he would land - "if I were to land anywhere," he added.

In that article, Tyson says the focus on defining the word planet to the satisfaction of scientists and students has held "an irrational sway over our hearts and minds." It would be better to group celestial objects in multiple ways - for instance, studying the cyclones of Earth and Jupiter, or weighing the prospects for life on Europa and Enceladus, or comparing ring systems, or magnetic fields, or orbital characteristics.

"These classifications say much more about an object's identity than whether its self-gravity made it round, or whether it is the only one of its kind in the neighborhood," Tyson writes. "Why not rethink the solar system as multiple, overlapping families of objects? Then, the way you organize the properties is up to you. The fuss over Pluto doesn't have to play out as a death in the neighborhood. It could mark instead the birth of a whole new way of thinking about our cosmic backyard."

Is this a Solomonic solution to a scientific problem? Or is this just a way to talk around the planethood problem without solving it? The debate doesn't end on Thursday: Scientists and educators will be meeting into the weekend, and I have a feeling the issue won't be resolved in one meeting - just as it wasn't really resolved two years ago.

Feel free to add your own perspective on Pluto, planets and the scientific process as comments below - and then tune in for Thursday's debate.

Update for 2 a.m. ET Aug. 14: I want to apologize to all the commenters whose words sat in online limbo while I've been traveling. I underestimated the time and the trouble it would take me to get online in the midst of a California vacation. You may continue to see long lag times this week between your posting of a comment and my approval (and resulting publication) of that comment.

You'll find lots of great comments below, including observations from Alan Stern, the principal investigator for the New Horizons mission (and a principal instigator for today's debate); and from Dave Mosher, the science writer who hangs out at Discovery.com's Space Disco. (It looks as if the Space Telescope Science Institute's Ray Villard will be liveblogging the debate for Discovery.)

To answer one of the questions raised by commenters: Yes, in Sykes' view, the world that was recently named Makemake would rate as a planet in his book, bringing the current count to 13. Here's a news release that provides the details.

Will 13 planets (including Pluto and Charon, Ceres, Eris and Makemake) bring more fortune than eight (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune)? That's something worth musing over during this numerology-conscious Olympics.

Update for 2:30 a.m. ET Aug. 15: So the Great Planet Debate is finished ... or is it? Dave Mosher ended up doing the liveblogging for Discovery.com, and Ray Villard weighed in as well. Nature's Eric Hand summarized the debate for The Great Beyond.

Johns Hopkins University's Applied Physics Laboratory says the archived video of the debate will be available sometime in the next couple of weeks.

I'm still on vacation, but when I read the descriptions of the proceedings, I couldn't help thinking of a movie titled "The Englishman Who Went Up a Hill but Came Down a Mountain." The film was about villagers who tried piling more dirt on their local promontory so it could retain its mountain status. Similarly, it sometimes sounds as if the planetary pecking order is based on the volume of piled-up dirt (or gas, when we're talking about giant planets).

I'd be OK with roundness serving as some sort of threshold for the definition. Those spaceballs (even if they're as small as Ceres) would be more interesting to the fictional Captain Kirk as well as real-life planetary scientists. Astronomers are used to dealing with such size thresholds, even if they're a bit arbitrary. For instance, they're more interested in near-Earth asteroids that are more than a kilometer wide, because those are the biggest potential killers.

I also think scientists could figure out a rule of thumb to distinguish between planets and moons. Just as there are double-star systems, there could theoretically be double-planet systems - and perhaps the Pluto-Charon system will be the first on the list. However, that doesn't mean every world we consider a moon today (such as Pluto's Hydra and Nix) would have to be upgraded to planet status.

Because of Pluto's historical significance, I'd be OK with putting it back onto the list of nine "classical planets," even though astronomers will almost certainly continue to find bigger iceballs on the solar system's edge. Does that sound like a wishy-washy solution? Maybe so. But I do think this would give educators a teachable moment - that is, an opportunity to explain how scientists wrestle with the kinds of issues that came up during Thursday's debate.

Were you swayed by any of the arguments aired over the past few days? Have you changed your position on the Great Planet Debate? Or are you more certain than ever that Pluto and its ilk should (or should not) be lumped together with Earth and Jupiter? As always, feel free to weigh in with your own views below.

Update for 2:30 a.m. ET Aug. 16: Additional perspectives on the debate are in from New Scientist and Space.com, among others.