Astronomers Prepare to Fight Pluto Demotion Some astronomers are challenging the new planet definition that knocked Pluto out of the planetary line-up. Mark Sykes, director of the Planetary Science Institute, lays out his arguments.
NPR logo

Astronomers Prepare to Fight Pluto Demotion

  • Download
  • <iframe src="" width="100%" height="290" frameborder="0" scrolling="no" title="NPR embedded audio player">
  • Transcript
Astronomers Prepare to Fight Pluto Demotion

Astronomers Prepare to Fight Pluto Demotion

Astronomers Prepare to Fight Pluto Demotion

  • Download
  • <iframe src="" width="100%" height="290" frameborder="0" scrolling="no" title="NPR embedded audio player">
  • Transcript

Some astronomers are challenging the new planet definition that knocked Pluto out of the planetary line-up. Mark Sykes, director of the Planetary Science Institute, lays out his arguments.



We'll be talking about returning to the moon a little bit later. But first, we haven't heard the last about the ignominious vote last month to strip Pluto of its place in our solar system - kicked out of the planetary lineup by a group of astronomers because it doesn't fit the new definition of what a planet is. Well, now another group of astronomers, many of whom are in favor of keeping Pluto a planet, is challenging that new definition, saying it doesn't set clear criteria for what makes a planet. And one of the astronomers leading the challenge is here with us.

If you'd like to talk about, our number is 1-800-989-8255, 1-800-989-TALK. And as always, you can surf over to our Web site at Mark Sykes is the director of the Planetary Science Institute in Tucson, Arizona. He joins us today from NASA's Stennis Space Center in Mississippi. Welcome to the program, Director Sykes.

Mr. MARK SYKES (Planetary Science Institute): Hi, Ira.

FLATOW: Hi there. Did you put together this petition because you're pro-Pluto and you want the decision reversed?

Mr. SYKES: Well, I put together the petition with some others because when we looked at the definition that was passed, we couldn't quite understand how it would apply. The definition, in terms of its operational aspects, says that the planet has to have cleared the neighborhood of its orbit. And so after thinking about it for a bit, a lot of us were wondering, well, what does clear mean? Earth gets it by asteroids; does that mean Earth can't be a planet? The largest planet in the solar system, Jupiter, has these cloud of Trojan asteroids that straddle its orbit, so that would seem to violate a plain reading of the definition.

And when I talked to some dynamicists about - to get a little clearer explanation of it, it became apparent that in order to be a planet you had to be more and more massive the further and further away you got from the sun, which was also kind of odd because it doesn't really bear on the intrinsic - other intrinsic properties of things that we normally think of as planets.

FLATOW: So what kind of definition would you like to see?

Mr. SYKES: Well, what I'd like to see is a more open discussion that would lead to a definition that might capture the essence of the characteristics of things we call planetary bodies. You know, when we sent Cassini to Titan and sent a probe to beneath the cloudy - the cloud of its - in its atmosphere, and we see this surface with rivers and lakes even though they're, you know, methane and ice, you know, people are looking at it and thinking that this looks like a planetary body and it has properties that are very similar to things that we see on the Earth, even though it's not the same, of course, because we've got rock and water, which would be - the water would be rock on Titan because it's so cold. So we'd like to see - I think many of us would like to see a definition that would speak more to the properties of a body than how it affects others bodies around it.

FLATOW: Are you also unhappy with the way that that definition - the current definition was arrived - arrived at?

Mr. SYKES: Oh, oh yeah. Yeah. I mean, we live in a modern age of the Internet where communication is really no longer an issue and one shouldn't have to get on sailing ship and cross the ocean in order to discuss a matter that has such real broad interest as this with other scientists. And the IAU had setup a committee to come up with a definition, and it was meeting in secret, basically. And we didn't hear what their proposed definition was until the meeting actually occurred.

That's when there was the news about 12 planets in the solar system and possibly more, you know, coming. And then when people objected to that, that were in Prague, you know, the discussion was wholly within - within Prague, and the decision, you know, made there, leaving the vast majority of the astronomical community and the planetary community - the people that study planets - out in the cold.

FLATOW: So you say there's something like 10,000 people who study planets and only a few hundred who went to that IAU member's meeting.

Mr. SYKES: Right. Right. And actually, those were astronomers. I'm told that there were perhaps 50 or 60 planetary scientists actually present.

FLATOW: So you'd like to see a more democratic way of getting all the astronomers and the planetary scientists involved.

Mr. SYKES: Well, we have the means of opening up the discussion and getting all the different viewpoints, and so we're looking at setting up such a discussion initially online, in which people would talk about what are the characteristics of things that we call planets or planetary bodies, regardless of whether everybody agrees a definition of the outset. So have people talk about atmospheres, interiors, dynamics, and other processes that go on, on them - you know, on service processes, for instance, and then having discussed, you know, the things that we - the processes that we associate with these bodies to say, okay, now, can we - can we as a group articulate some definitions that would capture the essence of what we've been talking about?

In this day and age, you know, having a few thousand people engaging in a conversation isn't a bad thing.

FLATOW: Well, what kind of new definition, I mean, can you come up with? When you have a committee, aren't you going to always offend some people?

Mr. SYKES: Yeah, there will probably always be, you know, a minority, and perhaps the result would be that there is no single definition that everybody would agree on as having general usefulness that there would be one definition for one type of work, another definition for another type of work. Of course that basically the question of what people should be using in schools when teaching kids.

FLATOW: What do you say to people who think that this is just a bunch of sour grapes to astronomers who didn't get their way? You know?

Mr. SYKES: Well, you know, I think that if there had been a more open discussion with greater participation, that maybe a comment like that would have some merits, but you know, there's a great community of people on the outside who study planets who were not even a part of the process.

FLATOW: Right. 1-800-989-8255 is our number. Let's see if we can get a comment or two from our listeners. Maury in - where is it (unintelligible) Washington?

MAURY (Caller): Yes.

FLATOW: Hi. Go ahead.

MAURY: Yes. Years ago I heard a definition for a planet that had to do with its atmosphere. If a body as big enough to maintain an atmosphere - even a very light one - then it's a planet. And I'm wondering, my question is, whatever happened to that definition? And I'll take my answer off the air.

FLATOW: Good question, Maury.

Mr. SYKES: Well, people come up with various ways of defining things, and certainly that's, you know, one way that one could do it. And you look at planets like Mercury that have very, very thin atmosphere - similar to Pluto's atmosphere, and it's a different situation then for the Earth or Jupiter, which is substantially atmosphere. So I think that with any of these definitions, they become - they become more accepted as they become used more. In other words, people find them useful. And so that was a definition that somebody put out, but not too many people have made use of, because it hasn't had much in the way of applicability.

In the case of the IAU definition, you have a definition which instead of having evolved through, you know, decades of discussion and use or not - or disuse - is more imposed upon everyone, and that's why there's more controversy today.

FLATOW: What happens with your petition, and what kind of mechanism process does it have to move through to gain acceptance or to get the discussion going again?

Mr. SYKES: Well, the I.U.'s got its hands tied by its own rules. It can't make any more decisions on the matter for three more years. But fortunately scientists are not so restricted and so there's a group of us that are looking at setting up an independent process, more of a grassroots discussion, and collecting people's thoughts, giving them an opportunity - whether you're in the United States, Europe or Asia - to, you know, put in comments and see where that leads us.

And then, you know, three years from now the I.U. can catch up. But part of the problem with the definition - I mean, as an example - is that if you look at Mars - if Mars - if we discovered a Mars-sized object out beyond the orbit of Pluto, it couldn't be a planet, because over the age of the solar system it wouldn't have time to clear its orbit.

And when we look at objects that we want to group into one category, you know, if they're like each other, then, you know, we feel that they should have - that they should be lumped together. Whereas in this case the definition leads to - seems to violate common sense.

FLATOW: What do you tell all these kids who are studying planets in school now?

Mr. SYKES: Well...

FLATOW: How does this appear to them? Does it appear like science is just a mish-mash of here today, we'll change the definition next time?

Mr. SYKES: Well, I think that what should be taught is not what the new list is, because that depends on your perspective. I think that what should be taught is perhaps the issues that are framing the discussion and why some people want to have a definition that leaves you with eight planets while other people are happy with, say, a geological definition that leaves you with 12, and perhaps more in the future, because, you know, then you get into more of the discussion.

I don't think the kids are well served when we teach them science as a collection of some kind of immutable facts, because that's not what science is all about. Science is a lot about argument and process and seeing what works, and more importantly, seeing where our explanations and models for things break, because that's where the interesting - that's where the interesting discoveries and future ideas and perspectives are.

FLATOW: If nothing else, it gets everybody to talk about the solar system.

Mr. SYKES: Oh yeah. Yeah. In fact, I'm seeing that this discussion that we're talking about - the international discussion on planetary characteristics - will have wonderful opportunities for the public to, you know, watch and to learn about what we know about our own solar system, what we know about planetary bodies within the solar system. And not just within our own solar system, but other solar systems, which this definition specifically excludes. And we have more than 200 planets around other stars that we know about today, and also about what we may expect to discover in the future in our own solar system.

FLATOW: All right, we're going to have to stop our conversation, pick it up again next time. Thank you...

Mr. SYKES: Sure. Thank you.

FLATOW: ...Dr. Sykes, for taking time to be with us.

Mr. SYKES: Okay.

FLATOW: Mark Sykes, director of the Planetary Science Institute in Tucson. We're going to take a short break and when we come back we're going to switch from Mars, Jupiter - where you want to go? We're going to the moon. So we're going to talk about returning to the moon when we get right back. Stay with us.

I'm Ira Flatow and this is TALK OF THE NATION: SCIENCE FRIDAY from NPR News.

Copyright © 2006 NPR. All rights reserved. Visit our website terms of use and permissions pages at for further information.

NPR transcripts are created on a rush deadline by Verb8tm, Inc., an NPR contractor, and produced using a proprietary transcription process developed with NPR. This text may not be in its final form and may be updated or revised in the future. Accuracy and availability may vary. The authoritative record of NPR’s programming is the audio record.