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Planet debate shifts focus

A map of Pluto's surface,
based on brightness.

The main players in the planethood debate gathered together this week to look back at Pluto's woes - and look ahead to fresh discoveries on the solar system's farthest frontiers.

Among the speakers at Tuesday's forum, held at the American Astronomical Society's summer meeting in Pasadena, Calif., were Neil deGrasse Tyson, the astrophysicist and planetarium director whose Pluto-less planetary display contributed to the controversy, and Alan Stern, the planetary scientist who heads NASA's New Horizon science mission to Pluto and its neighbors on the solar system's edge.

In all, seven speakers revisited the International Astronomical Union's decision three years ago to issue a definition of planethood that excluded Pluto because it hadn't "cleared its orbit" of other objects close to its size.

Pluto and other round objects beyond Neptune were instead classified as "dwarf planets," and later as "plutoids" - sparking protests from Pluto's planetary defenders.

It's safe to say that no one's mind was changed up on the speakers' platform: Tyson traced Pluto's troubled past - including a progressive downsizing of size estimates that led astronomers to joke during the 1970s that the planet would disappear completely by 1984. As it happened, more objects like Pluto were found starting in 1992, climaxing with the 2005 discovery of another ice world that is bigger than Pluto. (The uncertain status of the newfound object, now known as a dwarf planet named Eris, is what sparked the IAU's decision a year later).

Moving on?
In Tyson's view, it's time to move on. He noted that the solar system contained a wide diversity of objects that could be sliced and diced into categories ranging from shape and composition to geology and suitability for life.

"This is what we should be thinking about now, not arguing over the fricking definition of a planet," Tyson wisecracked.

Stern agreed that the solar system's diversity should be the focus of the debate. But he said the way we think of planets should reflect that diversity. The way he sees the issue, objects like Pluto make up the most numerous class of planets in the solar system - and objects like Earth should be considered the true oddballs. "I think there's a bit of a Copernican revolution," Stern observed.

Other speakers added their own particular perspectives:

  • Charles Beichman, executive director of NASA's Exoplanet Science Institute, noted the difficulties in using any single criterion for defining planets. The IAU backed off from setting up a standard for extrasolar planets three years ago, and Beichman joked that the standard once cited by a Supreme Court justice for obscenity might have to be used: "I know it when I see it."
  • The University of Arizona's Renu Malhotra noted that Pluto ended up sparking a revolution in the way scientists thought about planets. Today, Malhotra and other astronomers say Pluto and its kin were pushed outward in a grand migration of the solar system's outer planets. "The planets did not form where we find them today," she said.
  • Mark Sykes, director of the Planetary Science Institute, argued that a broad definition for planets - that is, a definition that accepts anything that is massive enough to crush itself into a round shape due to self-gravity - should win out because it's easier to understand and leads to more logical groupings of celestial objects. He sketched out a Venn diagram categorizing objects that had geological processes, atmospheres and the potential for life. "It makes sense to group all these things together as planets," he said.
  • Jean-Luc Margot, an astronomer at the University of California at Los Angeles, took a fresh approach to the whole definition question: Stick to the IAU's view on planethood, and set aside the word "world" to apply to round objects like Pluto as well as satellites like Saturn's Titan and Earth's moon. The world definition also would apply to free-floating objects that don't orbit any star. "Some worlds are planets, others are not," he said.
  • Vanderbilt University's David Weintraub traced the twists and turns in the concept of planethood over three centuries, noting that scientists often got it wrong. (For example, by thinking that planets had to follow a distance pattern known as the Titius-Bode rule.) Weintraub - author of the book "Is Pluto a Planet?" - said he'd like to see a sensible definition of the word "but I'm not sure we're anywhere close to being able to do that."

Tuesday's forum was remarkably free of rancor. In fact, the loudest exchange came between Tyson and Bill Nye (the Science Guy), who was sitting in the second row of the hall and went on a mock rant over the term "trans-Neptunian object," which scientists use to refer to solar system objects even if they don't cross Neptune's orbit.

"'This is not complicated," Nye said "'Trans' means across, 'ultra' means beyond!"

The question on many people's minds was whether there would be an effort to modify or overturn the IAU's definition at the organization's next meeting, set for August in Rio de Janeiro. Based on the views aired on Tuesday, neither side sounds anxious to have IAU officials revisit the issue in Rio.

"I think they should quit while they're behind," Stern said.

The last word
If the IAU's deliberations are being left behind, what's ahead? Caltech astronomer Mike Brown, whose discovery of Eris kicked the whole debate into high gear, said today during a different AAS session that he tries to avoid talking about whether or not Pluto should be a planet. (After the 2006 decision, he declared himself satisfied with the outcome.)

Now Brown is focusing on Pluto's kin at the solar system's edge - fellow dwarf planets such as the fast-spinning, football-shaped world named Haumea, and a mysterious far-flung object called Sedna.

Sedna inhabits what appears to be something of a no-man's land between the solar system's Kuiper Belt and the cometary Oort Cloud. However, statistical analyses suggest that there should be about 40 objects out there that are at least as big as Sedna. Some of those objects may even be big enough to reignite the "What is a Planet" debate yet again.

After his talk, Brown told me he's looking forward to the Hubble Space Telescope's return to service this summer, because he has several observational opportunities coming up that could shed more light on the solar system's farthest, dimmest frontier. He gave Hubble a high rating on the telescope scale. "It's the coolest one out there," he said.

I've written a whole book about this planethood issue, titled "The Case for Pluto," It should be out in November. I mention this only because I feel as if I'm short-changing you when it comes to the whole discussion that took place this week. To make partial amends, I've put together a bare-bones Web page that combines all of my Twitter updates from the planet forums (and the updates I would have made if I hadn't violated Twitter's tweet-per-hour limit).

You can watch some of Tuesday night's action via Ustream's video archive.

Join the Cosmic Log corps by signing up as my Facebook friend or hooking up on Twitter. If you really want to be friendly, give some thought to "The Case for Pluto."