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Astronomers Offer New Vision of the Solar System

August 16, 2006 at 6:35 PM EST
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TOM CLARKE, ITV NEWS CORRESPONDENT: The classical Solar System that inspired Holst’s suite was a simple place: eight large spheres, including Earth, made up the local firmament, but it was soon to change. In 1930, using the latest telescope technology, astronomer Clyde Tombaugh discovered Pluto. It was immediately added to the pantheon of planets.

But to modern astronomers, Pluto has become a problem. After a decade of debate, the International Astronomical Union today proposed a radical fix.

RON EKERS, President, International Astronomical Union: The lack of a definition of a planet is becoming increasingly confusing. Now, what’s different in this case is this is something, the definition of a planet, which is not only something professional astronomers need, but something that extends beyond astronomy.

TOM CLARKE: Pluto’s not like the other planets. It’s on the edge of a swathe of cosmic rubble called the Kuiper Belt, and might just be part of that. And three years ago, a new object named Xena was discovered, bigger than Pluto.

Today’s announcement lays down strict scientific rules defining a planet. The role of gravity is key. It’s a bit complicated to explain, so the astronomers reached for the Plasticine.

RICHARD BINZEL, Massachusetts Institute of Technology: If an object is big enough and massive enough to have enough gravity to pull it into a spherical shape, we have a planet.

TOM CLARKE: The proposed new rules solve the Pluto problem but also mean an overhaul of nearly 80 years of planetary doctrine. It’s farewell to the traditional Solar System, with it’s nine planets. Now there’s 12 planets in the neighborhood.

In at number five, the planet formerly just part of the asteroid belt: Ceres.

Further out is a totally new group: the plutons. They’re deemed far too far away from the sun to be classic planets. They’re led by Pluto, but the new rules mean that what used to be thought of as Pluto’s moon, Charon, becomes a planet of its own. And the third member of the plutons, the newly discovered 2003 UB 313, or Xena for short.

The definition of a planet

Alan Stern
Southwest Research Institute
[T]he pluton category that's just been proposed just refers to the fact that these are objects with orbits of 200 years or longer and can have these very irregular orbits, as we find in the distant outer Solar System.

JIM LEHRER: Now, a Science Unit look at these planets questions, and to Jeffrey Brown.

JEFFREY BROWN: So 12 planets, maybe, and Pluto makes the cut, or does it? Astronomers meeting in Prague will vote on the planetary proposal next week. And we look at it now with Alan Stern, executive director of space, science and engineering at the Southwest Research Institute. He's also the principal investigator of NASA's New Horizons Mission that was launched earlier this year to explore Pluto and the Kuiper Belt.

And Michael Shara, curator of astrophysics at the American Museum of Natural History in New York.

Alan Stern, I'll start with you. You like the new guidelines. Tell us why.

ALAN STERN, Southwest Research Institute: Well, I like them because they recognize the number of discoveries that we've made in the solar system and in other solar systems in the last 15 years. And they really respond now in the scientific method to these expanded horizons, the fact that we really know that the panoply of planets is so much greater and richer than we ever expected.

JEFFREY BROWN: And this idea, staying with you, of Pluto as a planet but one that's called a pluton, explain the distinction for us and why that's important?

ALAN STERN: Well, you know, the pluton category that's just been proposed just refers to the fact that these are objects with orbits of 200 years or longer and can have these very irregular orbits, as we find in the distant outer Solar System.

JEFFREY BROWN: Dr. Shara, you have some problems with this proposal. Tell us about them.

MICHAEL SHARA, AMERICAN MUSEUM OF NATURAL HISTORY: Oh, I have some huge problems. I think it's a terrible proposal as it stands.

Actually, a strict reading of the current rules means that Saturn is no longer planet because Saturn isn't a perfect sphere. Saturn is about 10 percent off-round because it's spinning so fast. But that's a minor point.

The real reason I object to these new set of rules is that they don't explain all of the beautiful information that we have about the evolution of the Solar System. We understand far better than we did 20, 30, or 50 years ago how planets and stars came to be.

And, in fact, the plutons, and Ceres, and all these other bits of rubble, which are fascinating objects, are nothing but aborted planets. And the real planets of the Solar System are the ones that managed to sweep up all of the objects in their own neighborhood to become complete planets.

And under that definition, that much simpler definition, which really explains and takes into account all the evolution of the Solar System, we have eight planets in the Solar System. And Pluto, fabulous object and fascinating object that it is, is not a planet.

JEFFREY BROWN: So Pluto would not make the cut in your scenario?

MICHAEL SHARA: Absolutely not.

Picking apart our Solar System

Michael Shara
American Museum of Natural History
We ignore in the current set of definitions...the idea of accretion, the idea of an object that dominates all the others within its own gravitational range, is really the basic idea behind the way the Solar System formed, the way the planets formed.

JEFFREY BROWN: Dr. Stern, how many planets would there be under the new definition?

ALAN STERN: Well, it's open-ended.

JEFFREY BROWN: I know there's 12, but they're talking about possibly more under the definition.

ALAN STERN: Right. It could grow. And we're not going to legislate the number of planets. We're going to see what falls within the physical boundaries of objects that share these common attributes that we call planets.

By the way, what Dr. Shara just told you is incorrect. In fact, the definition doesn't have to do with whether an object is round; it's whether it's large enough for gravity to dominate over its material strength to make it flow into a sphere. So Saturn certainly qualifies. I think he should read it a little closer.

JEFFREY BROWN: Well, Dr. Shara, come back. Go ahead.

MICHAEL SHARA: The spherical symmetry is specifically called out, as is hydrostatic equilibrium. And then the question becomes, well, how round does an object have to be in order to be considered a planet? But that's a minor point.

The real weakness of all of this is that there is no fundamental physics. An object will either be round or not or in hydrostatic equilibrium or not to some extent, depending on what it's made up of, how quickly it's spinning, how massive it, not how it got to be what it is.

We ignore in the current set of definitions the evolution of the Solar System. And the idea of accretion, the idea of an object that dominates all the others within its own gravitational range, is really the basic idea behind the way the Solar System formed, the way the planets formed. And that is, I think, a far better way of defining what a planet is.

Changing our cosmic understanding

Alan Stern
Southwest Research Institute
It is going to rewrite the textbooks, and it's very exciting, because as I said earlier it reflects the fact that we know so much more about the range of planetary types that there are.

JEFFREY BROWN: Both, help us understand, the layman, understand the consequences here. Dr. Stern, starting with you, what would all this mean? Is it rewriting textbooks? Do school kids everywhere have to learn 12 and, as you said, who knows how many more planetary names in the future?

ALAN STERN: It is going to rewrite the textbooks, and it's very exciting, because as I said earlier it reflects the fact that we know so much more about the range of planetary types that there are. But I don't expect that schoolchildren will have to learn the names of additional planets.

You know, when I was a kid -- I think when most of us were taught, there were a lot of rivers on the Earth. We weren't expected to memorize the names of all the rivers, just the really big ones, the important ones. And I think something similar will take place now in astronomy. We don't make schoolchildren memorize the names of all the stars, and yet we know there are billions.

JEFFREY BROWN: And yet I know that people take this very seriously. Dr. Shara, I was reading about the flack that your museum took a few years ago. In an exhibition, you left out Pluto. So people do care.

MICHAEL SHARA: Well, they certainly do care. To get a death threat from a third grader -- not quite, but certainly very unhappy third-graders -- is a remarkable thing. Now, as long as kids are taught, not just the names of planets or whether there are eight, or ten, or 12, which doesn't really matter at all, but the processes which form planets, that's really what we're after. We want them to understand the evolution of the Solar System, and I think that, on that point, Dr. Stern and I agree.

A process beyond rules

Michael Shara
American Museum of Natural History
[T]he planets that formed completely, we think there are eight of them, the inner planets... the outer ones... did in the early history of the Solar System sweep up all of the surrounding material. And so we consider them completely formed planets.

JEFFREY BROWN: Well, tell us a little bit more, though, because the third grader -- explain us to. I mean, I see why this debate matters to you two, but why should it matter to the rest of us?

MICHAEL SHARA: Well, the planets that formed completely, we think there are eight of them, the inner planets, Mercury, Venus, Earth, Mars, the outer ones, Jupiter, Saturn, Uranus and Neptune, did in the early history of the Solar System sweep up all of the surrounding material. And so we consider them completely formed planets.

The leftover debris, things like Ceres, we consider to be essentially aborted planets -- aborted fetuses, if you will -- things that didn't make it to the final stage of planethood. It is true that they may have managed to gather enough material to become round, to become spheres or nearly spheres, but there are things out there that are 80 percent spherical, 70 percent spherical, 60 percent spherical. Where do you draw a cut-off line?

That has nothing to do with the evolution, with the birth process of the Solar System. And that really is what the science is all about. That's what astrophysics is all about, and that's what we want the third- or eighth- or tenth-graders to understand, that there's an evolutionary process, that there is evolution actively at work here.

JEFFREY BROWN: Dr. Stern, what's your answer to why the rest of us should care?

ALAN STERN: We care because we live on planets. I should say that I think the IAU very wisely recognized that our notions of planetary evolution are very naive. They've changed a lot in the last 10 years, radically in the last 20. They probably will again as we learn more. So to craft a definition around how something evolves is something that would probably be very short-lived.

By contrast, this kind of definition is very powerful. It's based upon very deep physics, and it's very easy to apply. So I'm very happy with it. And I think, as evidenced by the division of planetary sciences, the largest organization of planetary scientists today who came out in favor of this, it's going to be widely accepted.

JEFFREY BROWN: All right. We'll see what happens with the vote next week and beyond. Alan Stern and Michael Shara, thank you both very much.

ALAN STERN: Thank you.