MICHELE NORRIS, host:

From NPR News, this is ALL THINGS CONSIDERED. I'm Michele Norris.

ROBERT SIEGEL, host:

I'm Robert Siegel, and now it's time for a puzzle, in this case, a planetary puzzle from NPR science correspondent Robert Krulwich.

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ROBERT KRULWICH: Maybe the first science thing you learn when you're a kid...

Unidentified Child: (Singing) (Unintelligible).

KRULWICH: Is that the sun is surrounded by planets, and you learn their order.

Unidentified Child: (Singing) Mercury, Venus, Earth and Mars...

KRULWICH: And then you learn there's a pattern. First, you get the rocky planets close to the sun, and then the normal progression is after rocky, rocky, rocky, rocky, you get gassy, gassy, gassy, gassy.

Mr. MIKE BROWN (Professor, California Institute of Technology): And then icy, icy, icy, icy, at the end of that. But when you say normal...

KRULWICH: What I mean by normal, I told Caltech astronomy professor Mike Brown is it's, you know, like our solar system.

Mr. BROWN: Exactly. So normal, this is if you had talked to me 20 years ago, I would have said, yes, not only is our planet our solar system that way, but that's the way all planetary systems that we find are going to be, and we understand why.

KRULWICH: Because 20 years ago, we thought there were rules about building solar systems, rules we understood, that you could you know, learn, say, in science films at school.

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KRULWICH: We knew that our solar system began with a cloud.

Mr. BROWN: There's this cloud of gas and dust in space.

KRULWICH: And in that dust, you'd find hydrogen and oxygen.

Mr. BROWN: H2O, water.

KRULWICH: Yeah, and because of gravity, all that water vapor and dust would kind of clump.

Mr. BROWN: So gravity is pulling everything together and pulling everything together.

KRULWICH: Creating in the hot zone near the sun first the rock planets.

Mr. BROWN: Mercury, Venus, Earth, Mars. But as you get out to about where Jupiter is, there's a place that's called, by astronomers it's called the ice line.

KRULWICH: And because the ice line was some distance from the sun, it's kind of cold out there.

Mr. BROWN: At the ice line, it gets to be below the freezing point of water. So all of that gaseous water that's circling the star freezes.

KRULWICH: And produces an orbiting snowball. Actually, it's more like an ice ball that as it gets bigger pulls in more and more water vapor.

Mr. BROWN: So you can start to get big.

KRULWICH: And the bigger it gets, the stronger its gravitational pull.

Mr. BROWN: And makes it go suddenly from being 15 times bigger than the Earth to 300 times bigger than the Earth.

KRULWICH: So the less that we learn from our Jupiter is that to create a big, gassy planet presumably anywhere in the universe, what you need to do is you start far from the sun, where it's cold, you form an ice ball, you grow, that's the theory.

Mr. BROWN: It's a beautifully worked out theory. When I first started teaching classes, this is exactly what I taught because it describes the solar system perfectly. There's beautiful logic to it. It explains all the features that we see.

KRULWICH: And then about 18 years ago, we discovered a planet that was not in our solar system.

Mr. BROWN: The very first planet that was discovered around another star was bigger than Jupiter, which is not surprising, it's easier to discover bigger things, and it was closer to its star than Mercury is to the sun.

KRULWICH: Oh, it's that close?

Mr. BROWN: It's right up next to the star.

KRULWICH: But wait. Didn't you just say that a Jupiter-sized planet forms far from stars, out where it's cold? See, so something's wrong.

Mr. BROWN: Okay, so when you find that first one, you just say, okay, that's weird. I don't know what happened.

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KRULWICH: But then they found a second one.

Mr. BROWN: The second one found, bigger than Jupiter, right next to the star. The third one found, bigger than Jupiter, right next to the star. It's crazy. It's probably two-thirds of them are this way.

KRULWICH: At this point, we have seen more than 400 planets out there in space, most of them big and gassy, and overwhelmingly, they are not where they are supposed to be. Instead of being out in the cold like our Jupiter, scientists now figure they formed in the cold, and then very early on, they pushed through the dust and the other baby planets to move close to their stars.

So many gassy planets formed and then moved next to stars that Mike Brown wonders:

Mr. BROWN: Why didn't Jupiter?

KRULWICH: Yeah, why didn't our Jupiter do what all the other guys did? It's an important question because if our Jupiter had moved in toward our sun, things would be very different today.

Mr. BROWN: If you take Jupiter from where it is now, and you were to move it inward, you will destroy anything that's inside there. There will be no Earth-like planets that are left after you've moved Jupiter in.

KRULWICH: So if our Jupiter had moved in?

Mr. BROWN: We would have never been here.

KRULWICH: Uh-oh. So what if it turns out that most solar systems in the universe have gassy planets that move in? Does that mean we're going to find fewer Earths or different Earths? What do we now think is normal out there?

Mr. BROWN: We have no idea. Every planetary system that we get to see in detail appears different from anything that I would've expected.

KRULWICH: Yeah, but still, there's got to be something you can predict. I mean, you do teach astronomy at Caltech. So...

Mr. BROWN: I've given up. I have no idea what to expect.

KRULWICH: But haven't you told me that other astronomers were searching for solar systems like ours for, I dont know, like five years, 10 years, 12 years?

Mr. BROWN: Fifteen years, probably more than it's probably even 20 years by now, but certainly 15.

KRULWICH: And the method they've been using to search for planets is you have to wait for the possible planet to circle its star.

Mr. BROWN: Yes.

KRULWICH: So for a planet like Jupiter...

Mr. BROWN: You would have to wait 12 years.

KRULWICH: Okay, but since we've been looking for 12 or 15 or 20 years, it's right around now, if there are lots of solar systems like ours out there, we should start to see them, or we should have seen, or we're about to should have seen them?

Mr. BROWN: Or we should have seen a few, and when I look at the list of everything that's been discovered, there is one that maybe fits our picture.

KRULWICH: Okay, so that's a start.

Mr. BROWN: It is a start. It was discovered a couple of years ago, and there's one.

KRULWICH: One, and you're now looking for two.

Mr. BROWN: Two would be good. A few more would be better.

KRULWICH: I should say there are scientists, planet-hunting scientists, who think Mike's a little picky and that actually, there's several solar systems now that look kind of like ours, and they do expect to find more. Well, maybe, says Mike.

Mr. BROWN: But we are at the stage now where we have had that time. We're starting to have enough time to have been able to see these things.

KRULWICH: And now every month it goes by when we don't find solar systems like ours out there in the universe, we have to consider the possibility that what we call home may be a surprisingly uncommon place.

Mr. BROWN: And now I'm wondering if it's going to the point where we're not uncommon, we are a rare exception.

KRULWICH: Well, I think it's a little early to get that worried.

Mr. BROWN: I'm not quite worried yet, but if things don't happen in the next year or so, I think it's time to get very worried.

Unidentified Child: (Singing) Mercury, Venus, Earth and Mars.

KRULWICH: After all, Mike wants to think that what he has taught for years at school is still true, that solar systems like ours are ordinary and everywhere.

Mr. BROWN: You want to be representative of the universe.

KRULWICH: Like the other guys.

Mr. BROWN: Yeah.

KRULWICH: And if it turns out that what we call home, our solar system, is a very rare thing, that's unsettling.

Mr. BROWN: I think that's right, and I would like deep down inside, I think I would like to see other things like this solar system out there, and it would make me feel at home.

KRULWICH: Robert Krulwich, NPR News.

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SIEGEL: Speaking of other solar systems out there, if you go to our Web site, Robert has arranged a trip across the galaxy to see a solar system literally being born. That's at npr.org. Transcript provided by NPR, Copyright NPR.

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