10th Planet

  • Posted 01.10.06
  • NOVA scienceNOW

(This video is no longer available for streaming.) In 2005, astronomer Mike Brown and his team at Caltech discovered a new planet out beyond Neptune. Slightly larger than Pluto, the new object was nicknamed Xena (after the TV show "Xena: The Warrior Princess"). But is Xena a planet? For that matter, is Pluto a planet? Brown's discovery sparks a controversy over what exactly is meant by planethood.

Running Time: 05:00



PBS Airdate: January 10, 2006

ROBERT KRULWICH: Big news in the neighborhood. This is our solar system. And a couple of years ago, almost everybody would have told you that orbiting our Sun you would find one, two, three, four, five, six, seven, eight, nine planets; but, as of this past year, we've got a tenth. Or do we?

MIKE BROWN: This will absolutely rewrite the "History of Astronomy" textbooks. There is now a tenth planet out there.

ROBERT KRULWICH: When Mike Brown and his team at Caltech announced they'd discovered what looked like a new planet, they did not give it a name. Names are bestowed by an international commission of scientists. So, in the meantime, since they were looking at this little bright light for more than year, they gave it a nickname.

MIKE BROWN: It had no name at all, so we had to call it something, so when we talked to each other we'd know what we were talking about. So we always give things code names, nicknames, and this one we codenamed Xena.


MIKE BROWN: From, of course, the TV show, Xena, Warrior Princess.

Of course, Xena has a satellite, which we had no choice but to call Gabrielle, which is Xena's sidekick in the TV show.


MIKE BROWN: No choice.

ROBERT KRULWICH: But in proper scientific circles, when scientists talk about this object they use...

MIKE BROWN: ...the very unwieldy 2003UB313.

ROBERT KRULWICH: And it will stay 2003UB313 until the International Astronomical Union officially decides that this is, indeed, a planet—which by the way, they have not done. They're not even close.

MIKE BROWN: We're in committee limbo, international committee limbo, which is about the worst possible place you could imagine being.

ROBERT KRULWICH: But what exactly is the problem? It looks like a planet. I mean, it's round; planets are round.

MIKE BROWN: Nothing's really round. The Earth has got a bulge in it, and Saturn is actually quite squashed.

ROBERT KRULWICH: Okay, but Xena's round enough; and it's got a moon, which is very planet like; and it orbits the sun. But it turns out, this matter is still debatable because there's no precise scientific definition for "planet," and there hasn't been one for a long time.

NEIL deGRASSE TYSON: Did you know that in 1801 a new planet was discovered orbiting between Mars and Jupiter?

ROBERT KRULWICH: Neil deGrasse Tyson is director of New York's Hayden Planetarium.

NEIL deGRASSE TYSON: They called it Ceres. And they looked some more, and they found another planet, and another and another. The count of planets in the early 1800s was greater than it is today, thirteen planets in the solar system. And they kept looking, and the numbers kept growing. And they were running out of names, and they realized that, rather than counting new planets, they had discovered a new swath of real estate in the solar system called the asteroid belt.

ROBERT KRULWICH: So Ceres became an unplanet, and was re-designated to a new class. And it became the biggest asteroid.

NEIL deGRASSE TYSON: So we've been there before. We know how to demote something.

ROBERT KRULWICH: And that's exactly what he did to Pluto. When the Hayden Planetarium reopened in the year 2000, Neil Tyson decided to de-planetize Pluto. "In our opinion," he said, "Pluto is now an icy object that's different from the other planets."

NEIL deGRASSE TYSON: It's got enough ice, so that if you were to take Pluto and transport it to where Earth is right now, the heat from the Sun would evaporate the ice and it would grow a tail. And that's just no kind of behavior for a planet. We have words for things with tails, they're called comets.

ROBERT KRULWICH: Same for Xena, another icy body that lives in the Kuiper Belt out beyond Neptune. But a lot of people simply ignored Neil Tyson and the Hayden Planetarium and stuck with the list they'd learned in school. Remember those planets in the right order? Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto.

NEIL deGRASSE TYSON: My very educated mother just served us nine pizzas.

MIKE BROWN: Martha visits every, Martha visits every Monday—must be for Mars—and just stays until noon, period. So there are two stupid things there. There's an "and" which is dumb, except the "and" is right where the asteroid belt is. I don't know if that's an accident or not. And then, I remember at the time, being in third grade, thinking that there's something funny about Pluto if it's really just a period at the end of a sentence.

ROBERT KRULWICH: Sophisticated scientists and ordinary folks can't seem to get together on this one. And Mike Brown, even though he discovered what could be the tenth planet, has this advice for the scientists:

MIKE BROWN: What I favor is the "give up" approach, which is to say, the word planet is not scientific, and it doesn't need to be. And as astronomers, we just need to get over it.

NEIL deGRASSE TYSON: These are the Pluto files, okay? This is hate mail that I've gotten from elementary school children. Handwritten letters saying, "Please, Dr. Tyson, what are you doing with Pluto? What? That's our favorite planet!"

"You are missing Pluto. Please make a model of it. This is what it looks like. It is a planet. Turn to the other page. A picture of Pluto."

"Dear Dr. Tyson, I think that Pluto is a planet for a lot of reasons, but you treat it like nothing! So if you can, please leave it a planet. And if you don't, then I say it is. And have a good day."


10th Planet

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This material is based upon work supported by the National Science Foundation under Grant No. 0229297. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

© 2006 WGBH Educational Foundation

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Image credit: (Sedna) Courtesy NASA


Michael Brown
Geology and Planetary Science professor, Caltech
Neil deGrasse Tyson
Astrophysicist, American Museum of Natural History www.haydenplanetarium.org/tyson/profile/bio

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