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Astronomers Debate Pluto’s Planetary Status

January 31, 2007 at 6:30 PM EDT
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BETTY ANN BOWSER, NewsHour Correspondent: Educators like Kris McCall, who runs the Sudekum Planetarium in Nashville, have a challenge.

KRIS MCCALL, Educator, Sudekum Planetarium: How many planets are there in our solar system, nine, eight? Anybody have a different answer?

BETTY ANN BOWSER: There was already a show there called “Nine Planets and Counting,” so McCall had to do a quick rewrite.

KRIS MCCALL: Why should Pluto be a planet?

PLANETARIUM VISITOR: Because, back in the ’50s, they told us it was. I was raised that way, and I’d hate to see it be gone now after all those years.

BETTY ANN BOWSER: The downgrading of Pluto to a new status called dwarf planet was decided by a vote of less than 400 of the International Astronomical Union’s 10,000 members at a meeting in Prague.

For the first time, they defined a planet, an object in the solar system that must be round, must orbit a star, and must clear out its neighborhood. In other words, it must not share its orbit around the sun with any other large objects. They said Pluto didn’t fit the bill because it had many other objects nearby.

The decision sparked a revolt among planetary experts, like these at a recent meeting of hundreds of astronomers and astrophysicists in Pasadena.

Planetary scientists react

ALAN STERN, Astrophysicist, Southwest Research Institute: They put together a slap-dash result, which is not the way we normally do science. They did it by politics, by voting, which is not the way we do science.

BETTY ANN BOWSER: Alan Stern was upset by the way the decision was reached.

ALAN STERN: Nobody ever voted on the theory of relativity. Nobody voted on whether DNA is the structure that encodes genetic information. When people put ideas out there, the ideas rise or fall based upon how well they fit the available data. But we don't actually sit down and vote; that's not the way it's done.

BETTY ANN BOWSER: Stern is principal investigator for the New Horizons spacecraft, currently on its way to the first close scientific encounter with Pluto. He was so upset by the decision he put a protest petition on the Internet. In four days, more than 300 scientists replied.

Astronomer Mark Sykes worked with Stern on the petition.

MARK SYKES, Astronomer, Planetary Science Institute: Every major discipline is reflected on the petition as saying, "You know, we have a problem with this. We can't use it. We need something better."

BETTY ANN BOWSER: Under the rules of the International Astronomical Union, it will be three years before a new planet definition can be considered. But that hasn't stopped the protests.

Stern is disturbed about a requirement that, to be a planet, an object must clear its neighborhood.

ALAN STERN: I think a planet is very simply an object that has grown to a size that it becomes spherical under its own self-gravity.

BETTY ANN BOWSER: That simple?

ALAN STERN: It's that simple.

BETTY ANN BOWSER: None of this "having to clear out the neighborhood" stuff?

ALAN STERN: I don't know who made that up

BETTY ANN BOWSER: You think that's silly?

ALAN STERN: None of that. I think it's unworkable. I think it's completely silly.

BETTY ANN BOWSER: One astronomer who signed the petition had already written a book entitled "Is Pluto a Planet?" David Weintraub thinks the new definition could confuse the status of other planets in the solar system.

DAVID WEINTRAUB, Astronomer, Vanderbilt University: Jupiter hasn't cleared its orbit, so perhaps Jupiter is not a planet. I don't think that's what they intended.

Neptune, pretty big object, it has this other object that crosses its orbit and is in a very stable orbit, and that object is named Pluto. So Neptune has not cleared its orbit, so I guess Neptune is not a planet, either.

Public response

BETTY ANN BOWSER: It's not just scientists who are unhappy. The public seems to have a special fondness for the only planet discovered by an American.

MARGIE GIFFORD, Castle Heights Upper Elementary School: Anybody hear about Pluto on the news lately?

BETTY ANN BOWSER: Margie Gifford's sixth-grade science club in Lebanon, Tennessee, has been having an intergalactic meltdown since all of this happened.

MARGIE GIFFORD: How many of you felt disappointed, upset, sad, or downright mad when you heard the news about Pluto?

Evan, what's so special about Pluto to you?

STUDENT: We've always called it a planet. We've learned about it as a planet. Why should we change it?

STUDENT: We've never actually had a real definition of a planet, and so we shouldn't change it just because we don't think that we should have a real definition now. Why didn't we have one earlier in the years?

MARGIE GIFFORD: So what do you think about this definition?

STUDENT: I think the definition would have to be...

BETTY ANN BOWSER: As part of their protest, the sixth-graders are producing a program on the Pluto controversy to run on the school's TV station.

Addressing the critics

Even comedian Stephen Colbert has joined the fray by grilling astrophysicist Neil Tyson, whose Hayden Planetarium in New York City took Pluto out of its planet display years ago.

NEIL DEGRASSE TYSON, American Museum of Natural History: I never wanted to kick Pluto out of the solar system. I just wanted to group it with its brethren.

STEPHEN COLBERT, Host, "The Colbert Report": It sure sounded -- a funny way of showing it.

NEIL DEGRASSE TYSON: With its icy brethren in the outer solar system, and that's all we did at the Rose Center for Earth and Space.

STEPHEN COLBERT: So, sure, you can be anything you want, just not here, OK? Not in my backyard? That's what you said. "Not in my backyard" to Pluto.

NEIL DEGRASSE TYSON: It's got family where I think it's happier there, because it's one of the biggest of the ice planets.

STEPHEN COLBERT: Are you saying Pluto should be with its own kind, separate but equal?

NEIL DEGRASSE TYSON: Yes, I guess that it comes out that way, doesn't it?

STEPHEN COLBERT: Doesn't it?

BETTY ANN BOWSER: Tyson decided Pluto did not belong with the planets, because, as a giant ice ball, it had very little in common with them.

NEIL DEGRASSE TYSON: I got hate mail from third-graders. Well, it's not so much hate mail, but anger mail, anger mail. "Please, Dr. Tyson, don't demote our favorite planet. And here's a picture of it so you can make a model of it and put it in your exhibit."

And it's in whole packages, with cover letters from science teachers, that they're like egging them on.

BETTY ANN BOWSER: Tyson thinks the traditional academic business of counting planets, no matter how many there are, is an outdated way to look at the skies.

NEIL DEGRASSE TYSON: To believe that just counting them and memorizing them in order from the sun, to believe that that's an interesting exercise, misrepresents what we've actually discovered about the solar system.

And here in this facility, we group objects according to properties that are kind of interesting, like some are large and gaseous, others are small and rocky, others are icy. Some have storms. Let's talk about that.

Defining a planet

BETTY ANN BOWSER: And if Pluto is a planet, then how to name other large objects being discovered near it in a faraway region called the Kuiper Belt? That was the quandary California Institute of Technology scientist Michael Brown faced when he discovered an object he named Eris.

MICHAEL BROWN, Astronomer, California Institute of Technology: What this is, is a little postage stamp of the sky, and three pictures in a row -- one, two, three -- and each of these little dots is a star, very, very far away. And right in the center, this one right here, you can see is barely moving across the sky. So over three hours, you can actually see that this thing has moved.

BETTY ANN BOWSER: He did not call it a planet. Instead...

MICHAEL BROWN: ... Kuiper Belt object is what I would call it.

BETTY ANN BOWSER: Brown says, as planetary scientists discover more and more objects, the new definition of planet will be very useful.

MICHAEL BROWN: I think it's absolutely great. It needed to happen for a long time, and it's the right definition. You'll hear complaints. People like to nit pick and say, "Well, but this, but that, but this, but that."

Astronomers don't deal very frequently in definitions. And so, when they come up with a definition, they're not used to having people pick at it and figure out all the little problems.

BETTY ANN BOWSER: Tyson says the scientific community will eventually come together on a planet definition, whether it includes Pluto or not.

NEIL DEGRASSE TYSON: The real issue here is, it's not a fight about what Pluto is. Pluto hasn't changed. Pluto's just Pluto. It's this ice thing out there with a really funky orbit. The real problem is the word "planet" has not been defined since ancient Greece, and if you don't have a definition, of course you're going to have arguments.

BETTY ANN BOWSER: Stern and Sykes already have a Web site up inviting experts from all over the world who are unhappy with the definition to a conference to discuss the science of planets. And organizers say there will be no voting.