JOE PALCA, host:

This is TALK OF THE NATION/SCIENCE FRIDAY from NPR News. I'm Joe Palca.

In a few moments, I'll be joined by Steve Squyres to talk about the Mars rover missions and all the behind-the-scenes work that went into making those missions happen.

But first we start with another planet, or a possible planet. At the close of last week. Scientists from the California Institute of Technology announced that they had found an object larger than Pluto orbiting our sun about nine billion miles away from Earth. Like Pluto, it's called a Kuiper Belt object and also like Pluto there's considerable discussion about whether this new object really should be considered a planet. So joining me now to help sort that out is my guest Mike Brown. He's a professor of planetary astronomy at Caltech. He joins us by phone from his office.

Welcome back to SCIENCE FRIDAY, Dr. Brown. Thanks for talking with us.

Dr. MICHAEL BROWN (Professor of Planetary Astronomy; California Institute of Technology): Thank you.

PALCA: And if you'd like to join our conversation, our number is (800) 989-8255. That's (800) 989-TALK. If you want more information about what we'll be talking about this hour, go to our Web site: www.sciencefriday.com where you'll find links to our topics.

So maybe we should start and first of all talk about what the Kuiper Belt is so we're at least all starting at the same spot.

Dr. BROWN: Sure. The Kuiper Belt is a band of icy, rocky bodies out beyond Neptune. It's sort of like the asteroid belt between Mars and Jupiter, but the asteroid belt is so close to the sun that all the ice has melted away and it's just rocky. And out beyond Neptune, temperatures are very cold and you have this vast body of many, many thousands of objects out there. And until last Friday, we used to always say that Pluto was the largest member of the Kuiper Belt and now we have to say Pluto is the second largest member of the Kuiper Belt.

PALCA: OK. And how much larger is this new object, do you think?

Dr. BROWN: We know for certain that it's larger than Pluto and we don't have a very good upper-size of how big it could be. But our best guess is that it reflects the same amount of light as Pluto does and that would be about 25 percent larger than Pluto, but it could be a little bit larger still, too. We're trying some more observations over the coming months that will really tell us for sure what that size is.

PALCA: So, you know, if Clyde Tombaugh found Pluto--What is it, 75 years ago or something like that?...

Dr. BROWN: That's right.

PALCA: ...what took so long?

Dr. BROWN: That's a good question.

PALCA: What have you guys been doing up there or down there or over there?

Dr. BROWN: Astronomy has progressed greatly in 75 years, obviously. But the one thing that got worse from Clyde Tombaugh's time is that Clyde Tombaugh used these very large photographic plates which could cover huge areas of the sky at once and astronomers have gotten to use now these very fabulous CCD cameras, just like the ones in your digital camera, but they're very, very small. And as everybody knows, the same amount of money will buy a larger and larger CCD on your camera. And the same thing has happened in astronomy. We can now finally compete with--in size and coverage of the sky with what Clyde Tombaugh did 75 years ago.

PALCA: And so this--but this thing's also--it's not an--in an obvious spot, either. It's kind of out of the plain of the rest of the planets?

Dr. BROWN: Yeah. That's another good reason why it's stayed hidden for so long. It's tilted by 45 degrees from all the other planets. And you remember Pluto's tilted by about 19 degrees, so it's a little bit out of there, too. But this one is more than anyone expected. And so we've been looking for things like this for the past seven years and we started looking in the same plain as all the rest of the planets. That's why it's taken us even seven years to find it all the way up there.

PALCA: So, I mean, is this a valuable discussion at all to say, `Is it a planet? Is it not a planet?' Or are we just talking semantics here?

Dr. BROWN: It's--there are two answers to that one and there's the scientific answer, which I think it's incredibly valuable because it's important to describe the solar system correctly. And scientifically--this is the strange part, is that scientifically I'm going to tell you this thing should not be considered a planet.

PALCA: Mm-hmm.

Dr. BROWN: If you really were to consider the topic completely scientifically, you would say that there are eight planets. Pluto's not a planet; this is not a planet. This is a member of the Kuiper Belt. Things in the Kuiper Belt shouldn't be planets. And the reason that we have finally changed our minds and decided to say that this is a planet and Pluto is a planet is that we've been fighting this fight as astronomers ever since the Kuiper Belt was discovered. And I, personally, have been, ever since when we discovered Sedna a year and a half ago, and I kept on saying, `No, Pluto's not a planet. No, Pluto's not a planet'--and everybody loves Pluto. Nobody wants Pluto not to be a planet. And I finally realized that the word `planet' to most people is not a scientific word. It's a cultural word.

PALCA: Right.

Dr. BROWN: And for scientists to sit around and tell everybody what is and is not a planet, I suddenly realized is not the right thing. Culturally, Pluto's been a planet for 75 years. I don't think it's going to ever not be a planet. And so I think it's safe to say, `If Pluto's a planet, something that we find is bigger and further away, we're going to call that one a planet, too.'

PALCA: OK. Well, I had just one other question I wanted to put to you today, and now that I've got you on the spot here, I'm going to drill this home. So you announced this last Friday after SCIENCE FRIDAY went off the air. What was that all about? You're supposed to call here and--I mean, come on. No, no, seriously, I'm curious. What happened? It was sort of a hasty...

Dr. BROWN: Well, this is not my favorite story to tell, but I'll give you the quick version of it, is that we had intended to make the announcement in October and one of the main reasons--there were two main reasons; one is because I have a four-week-old baby at home and I don't have time for doing these sorts of announcements right now. But also, I like to do these when schools are in session. Schoolkids love this kind of stuff and the best time to bury a story like this, if you want no one to know about it, is late on a Friday afternoon and it's the last Friday in July.

But our Web sites that we had been using to have all of our information on where the objects were had been hacked--I don't know if `hacked' is the right word--broken into, figured out, something. People knew where these objects were suddenly. And if we didn't quickly announce it, then somebody else would have found it and done the same thing.

PALCA: Wow. It's a sad story but it has a happy ending. I think a lot of people heard the story anyway, so I'm sure it'll come up in the school this year, too.

Dr. BROWN: I hope so.

PALCA: Right. Thanks very much for talking with us. Mike Brown is a professor of planetary astronomy at Caltech. Thanks a lot.

Dr. BROWN: It's my pleasure.

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