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Astronomers from around the world are meeting in Prague. This morning they were handed a proposal for a new definition of the word planet. As we reported last week, Pluto would qualify under the new rules despite its small size. The proposal is getting mixed reactions however. Some say it could open the door to 40 or more new planets.

NPR's David Kestenbaum reports.

DAVID KESTENBAUM reporting:

If there is an epicenter for the debate over Pluto's planetness, it might be the office of Neil deGrasse Tyson, director of the Hayden Planetarium in New York, which does not refer to Pluto as a planet.

Mr. NEIL DEGRASSE TYSON (Director, Hayden Planetarium): I have a fat file full of hate mail from elementary school children because they read that we have somehow kicked Pluto out of the solar system.

KESTENBAUM: There is no accepted definition for a planet. The word came from the ancient Greek term for wanderer, something that moved differently from the stars. Today, astronomers are considering a formal definition, and it says a planet has to do more than circle the sun.

Mr. RICHARD BINZEL (Planetary Scientist, MIT): We're defining a planet to be an object that is round, basically. It has enough gravity to pull it into a round shape.

KESTENBAUM: Richard Binzel is a planetary scientist at MIT and helped write the proposal. Gravity explains why the earth is round.

Mr. BINZEL: There is a limit to how high you can build a mountain on the Earth before gravity pulls it back down. And, in fact, Everest is right at that limit.

KESTENBAUM: So tiny Pluto, smaller than our moon?

Mr. BINZEL: Pluto is a planet.

KESTENBAUM: And it will have some friends.

Mr. BINZEL: If this resolution passes, there will be 12 planets in our solar system.

KESTENBAUM: Instead of the current nine. One new member, a big asteroid called Ceres discovered in 1801. Pluto would stay a planet, but a special type of planet, which the proposal dubs plutons. These would include Pluto, Pluto's moon Charon, and a recently discovered object known as Sedna(ph).

Mr. BINZEL: The definition of a pluton is that its orbital period is greater than 200 years.

KESTENBAUM: In other words, it takes more than 200 years to go around the sun, so basically anything further out than Uranus. Binzel admits this is not terribly scientific.

Mr. BINZEL: You know, you could have made it 198.1342862 years - so, yes, 200 years is an arbitrary number.

KESTENBAUM: Scientists at the International Astronomical Union meeting in Prague will vote on the proposal next week. Mike Brown, who helped discover Sedna, says if he were there, he would probably vote no. Brown is an astronomer at Caltech.

Mr. MIKE BROWN (Astronomer, Caltech University): It just doesn't quite work. If they had just basically had a one-line definition, anything round is a planet, I think that would've been much better. But the goal should've been to simplify things rather than complicate things, and I think they've just kind of made a mess.

KESTENBAUM: There's the tricky issue of moons, for instance; they're round, but are they orbiting the sun or just their own planet? The proposal counts Pluto's moon as a planet, because it's more like a twin than a moon. Earth's moon would not count. And Brown says the definition would mean adding lots more planets to the solar system; he calls it No Ice Ball Left Behind.

Mr. BROWN: I did a quick count, and right now there are 53 objects like that in the outer solar system that we know of. And the 44 new ones are so small that they could all fit inside the Earth's moon with room to rattle around.

KESTENBAUM: Many are icy objects. He says gravity makes even small ones - 250 miles across - round. The prospect of 40 new planets does not bother Neil deGrasse Tyson at the Hayden Planetarium.

Mr. TYSON: It's about time we had the word planet defined. It hasn't been defined in 2,500 years. So, sure, if it's round, let's do it. I don't have any problems with that.

KESTENBAUM: Tyson thinks this whole debate misses the point. Learning about the rich variety of things in the solar system is good. How many planets are there? He says that's possibly the least interesting question to ask.

David Kestenbaum, NPR News.

MONTAGNE: And to get vital statistics on would-be planets, go to npr.org. This is NPR News.

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