Pluto, the Un-Planet?

The world's astronomers finally voted today on the highly controversial issue of how to define a planet. The official definition means Pluto is no longer a planet. NPR's David Kestenbaum reports on the pandemonium in the convention halls of Prague, where the astronomers are meeting.

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ROBERT SIEGEL, host:

From NPR News, this is ALL THINGS CONSIDERED. I'm Robert Siegel.

If you have a little model of the solar system at home, you can break off the outermost planet. Astronomers today gave Pluto the heave ho, voting in an official definition for the word planet that leaves Pluto out. The term planet has been around since Ancient Greece, but coming to consensus on a precise definition today proved challenging, even for the world's top scientific minds.

NPR's David Kestenbaum reports.

DAVID KESTENBAUM reporting:

It's sometimes said that while democracy works, you don't want to see how the sausage is made. It was the same today in Prague, where the International Astronomical Union held its vote in a large auditorium. The wheels of democracy were grinding very slowly.

Ms. JOCELYN BELL BURNELL (Neutron Star Specialist): I think this microphone is working okay.

KESTENBAUM: Jocelyn Bell Burnell, normally a specialist in neutron stars, today her job - get everyone to vote on the definition.

Ms. BURNELL: You may have been given a white sheet with a resolution in English on it. That white sheet is wrong.

KESTENBAUM: The definition had been changed many times since the meeting began. Last week an official panel recommended keeping Pluto as a planet and adding at least three, perhaps 40 more small objects. But that proposal, which had been drafted in secret, led to an uproar and many, many revisions. Today Astronomer Richard Binzel stood at the podium and read the final proposal so it could be voted on.

Mr. RICHARD BINZEL (Astronomer): The word planet is strictly defined to be a body rounded by self-gravity, which has cleared its orbital zone.

KESTENBAUM: Tiny Pluto would not count. It doesn't have the mass to kick its neighbors out. The suburbs of the solar system are filled with small, icy objects, including Pluto. But astronomers continued to debate. Question, what about planets outside our solar system? Answer, this definition is just for our solar system. Question, and what about Neptune? It's not fully alone in it's orbit.

Unidentified Man: Is Neptune a planet, because everybody knows that Neptune's orbit is crossed by Pluto's.

KESTENBAUM: Answer?

Mr. BINZEL: Footnote One.

KESTENBAUM: The resolution had a footnote explicitly naming the eight planets. Well, someone said, why not drop the complicated definition and just leave the footnote? Everybody laughed. Then Michael Rowan-Robinson stood up to try to move things along. He's an astronomer and serves as the official representative of the United Kingdom.

Mr. MICHAEL ROWAN-ROBINSON (Astronomer): It would be disastrous for astronomy if we come away from the general assembly with nothing. We will be regarded as complete idiots.

KESTENBAUM: Time for the vote. President Ron Ekers, white-bearded and serious.

Mr. RON EKERS (International Astronomers Union): So will those members in favor hold up the yellow card?

KESTENBAUM: An enormous number did.

Mr. EKERS: Then I believe the resolution is clearly carried.

(Soundbite of applause)

KESTENBAUM: And Pluto was out. The resolution does make Pluto one of a number of dwarf planets, which confusingly are not officially planets. So there are eight planets in the solar system, not nine and not 10. Until today, Mike Brown at Cal Tech could claim credit for finding what was arguably the tenth planet. It's been dubbed Xena, and it's bigger than Pluto. It, too, is out.

KESTENBAUM: So Mr. dwarf planet discoverer.

Mr. MIKE BROWN (California Institute of Technology): The largest dwarf planet, please.

KESTENBAUM: Brown says he's actually happy about the decision today.

Mr. BROWN: This is great. This is the definition that we should have had all along. When people think of the word planet, they think of large, special objects, and when you tell people about Pluto and make them realize that Pluto really is quite small, people even start to realize well, okay, maybe Pluto shouldn't be called a planet.

KESTENBAUM: Part of him, though, is a little sad.

David Kestenbaum, NPR News.

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Pluto's Rocky Road

Percival Lowell

hide captionPercival Lowell died in 1916, unaware that his photographic plates had captured Pluto.

Lowell Observatory

1905: Percival Lowell starts the search for Planet X. The planets, including the newly discovered Neptune, didn't move around the sun in quite the way gravitational laws predicted, and Lowell proposes that an undiscovered planet must be the reason why. He never finds Planet X before his death in 1916.

Feb. 18, 1930: Clyde Tombaugh takes up the search in 1929 at Lowell's observatory and proves that discovering new planets is not glamorous work. For a year, he photographs the same section of sky several nights apart and then searches the images for any objects that move like a planet should. On Feb. 18 he looks at his photographic plates and knows right away that one of the dots is Planet X.

May 1930: A little girl in Britain interested in Greek and Roman mythology tells her grandfather over breakfast that the new planet should be named Pluto. He cables the Lowell Observatory, and they unanimously vote for the name because Pluto is the god of the underworld, which seems appropriate for such a cold and remote planet, and the first two letters of Pluto are Percival Lowell's initials.

June 22, 1978: The U.S. Naval Observatory's James Christy discovers that Pluto has a moon. He names it Charon, after the ferryman who take souls into the underworld in Greek mythology, but pronounces it Sharon because his wife's name is Charlene. The existence of Charon meant that scientists could get a better read on the mass of Pluto. They found that Pluto and Charon are actually pretty small. Together, they're smaller than Earth's moon, even.

Aug. 30, 1992: Pluto's tiny size didn't disqualify it from being a planet, but then David Jewitt, an astronomer at the University of Hawaii, and Jane Luu, a researcher at the University of California, Berkeley, discover Pluto isn't the only chunk of rock and out there in the Kuiper Belt beyond Neptune. Scientists have found hundreds of these objects since 1992, so some astronomers start to think that maybe Pluto isn't so special after all.

Feb. 3, 1999: Pluto's debated status as a planet gets publicity. The International Astronomical Union calms stargazers worried by recent media reports saying the IAU is planning to demote Pluto. Not so, they say in a press statement. They only want to include it in their numbering system for Kuiper Belt objects.

May 11, 2000: Scientists may debate whether Pluto is a planet, but it's place in the classical music canon gets secured. Composer Colin Matthews writes a movement for Pluto into Gustav Holst's The Planets. Although Pluto was discovered in Holst's lifetime, he declined to add it to his suite.

Feb. 19, 2000: The Rose Center for Earth and Space at the American Museum of Natural History in New York City opens — and sneakily omits Pluto from its list of planets. No one seems to notice until the next year, when the New York Times writes a front page article about it.

Jan. 5, 2005: Michael Brown, a planetary scientist at the California Institute of Technology, discovers what might be the 10th planet, Xena. He says it's rocky and icy like Pluto. When he announces his discovery on July 29, he forces astronomers to decide what makes a planet.

Oct. 31, 2005: The Hubble Space Telescope Pluto Companion Search Team discovers that Pluto has three moons, not just one. Moons don't qualify an object to be a planet, but having a couple moons doesn't hurt Pluto's case.

April 11, 2006: The Hubble Space Telescope finds that Xena is slightly larger than Pluto. Astronomers now have to make a decision: either Xena and Pluto are both planets or neither is a planet.

August 24, 2006: The International Astronomical Union strips Pluto of its planetary status. The group says a planet must, among other things, have "cleared the neighborhood around its orbit." Because Pluto's orbit overlaps Neptune's, Pluto is out. The celestial body formerly known as the ninth planet will be reclassified as a "dwarf planet."

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