ROBERT SIEGEL, HOST:
Tomorrow, a NASA spacecraft will fly past Pluto. Here's how Alice Bowman, the mission's operations manager, puts it.
ALICE BOWMAN: We are actually, as a nation, funding a spacecraft to go explore Pluto, which is the last unexplored planet.
SIEGEL: The last unexplored planet - that's actually a controversial thing to say because nine years ago, Pluto was officially reclassified as a dwarf planet. But Bowman steadfastly ignores that change of stature.
BOWMAN: My teachers taught me when I was growing up Pluto is a planet, and that's the way I will always think of it.
SIEGEL: NPR's Geoff Brumfiel has this report on the scientists who refuse to call Pluto a dwarf.
GEOFF BRUMFIEL, BYLINE: This story about Pluto begins not with an observation through a telescope, but with a vote by members of the International Astronomical Union, the world's leading group of astronomers.
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UNIDENTIFIED MAN: We will now take the vote on Resolution 5A.
BRUMFIEL: Resolution 5A was a new definition of the word planet. Astronomers voted overwhelmingly yes.
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UNIDENTIFIED MAN: Then I believe the resolution is clearly carried.
BRUMFIEL: And with that, Pluto was kicked out of the planetary club. Alan Stern, the principal investigator of this current mission to Pluto, thinks that was a bad decision. He says the redefinition came because astronomers started to find more and more planets beyond Pluto at the edge of our solar system.
ALAN STERN: They reacted to the new data that there are many more planets in a rather unscientific way by trying to legislate what is and isn't a planet to keep the numbers small.
BRUMFIEL: And to keep planets as special objects we can all remember.
STERN: It's kind of so 20th century - just think of planets as something that you should be able to name all of them.
BRUMFIEL: There are now three requirements for something to be a planet in the solar system. First, it must orbit our star, the sun. Second, a planet has to be round. Stern says that actually makes some sense.
STERN: You know, all the planets you've ever seen in science and in science-fiction are always round. That's for a physical reason because gravity shapes them.
BRUMFIEL: But it's the third rule that's controversial. Resolution 5A says planets must be big enough that their gravity can clear debris like asteroids out of their path.
STERN: And that's flawed for a couple of reasons.
BRUMFIEL: Jupiter and Earth both have asteroids in their orbital zones. The other reason Stern hates this rule is that Pluto has a much bigger orbit than any other planet, so there's way more debris to clear. It's not fair, but the International Astronomical Union is unmoved.
THIERRY MONTMERLE: Pluto is a dwarf planet. So be it. It's a classification. It's not a demotion or promotion.
BRUMFIEL: That's Thierry Montmerle, the group's general secretary. He says Pluto clearly doesn't fit with the other planets.
MONTMERLE: It is a very, very small planet. It's smaller than the moon, so it's really, really small.
BRUMFIEL: Pluto also orbits at a funny angle compared to the other planets, and there are at least a dozen other Pluto-like things cluttering up the outer reaches of the solar system. Anyway, Montmerle says, the term dwarf planet has stuck. These days, textbooks, museums and planetariums all use the diminutive title. Even NASA does it, officially, at least.
MONTMERLE: You may accept it or not accept it, but it's there. It'll last. I mean, there's no doubt about that.
BRUMFIEL: But Pluto mission leader, Alan Stern, thinks still thinks the definition is wrong. All the Pluto-like planets at the edge of our solar system that were shut out, they should be let back in.
STERN: The variety of and richness of nature is just spectacular. We're learning that there were many more kinds of planets than we knew when we had very little data. Let's let the data keep pouring in. Let's see how this works out from a classification standpoint.
BRUMFIEL: His spacecraft will swoop past Pluto tomorrow morning after 3 billion miles and almost a decade of travel. And while most Earthlings now know Pluto as a dwarf, in Stern's eyes, it will remain a planet. Geoff Brumfiel, NPR News.
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