What Defines a Planet? In 2006, astronomers voted to remove Pluto from the list of planets in Earth's solar system — continuing an ongoing controversy over what exactly defines a planet. Planetary scientist Mark Sykes argues in the journal Science that a planet is simply "a round object orbiting a star."
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What Defines a Planet?

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What Defines a Planet?

What Defines a Planet?

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IRA FLATOW, host:

This is TALK OF THE NATION: SCIENCE FRIDAY. I'm Ira Flatow.

A bit later in the hour, we'll be talking about the science of baseball. But leading off: the latest in the Pluto planet debate. When we last checked in on Pluto, the last planet to join the solar system had been ejected from the planetary lineup after a group of astronomers from the International Astronomical Union rewrote the definition of a planet. And since Pluto did not qualify under the new definition, we were left with a solar system of eight planets, and Pluto became a dwarf planet.

Well, soon after that, hundreds of scientists protested the change, signing a petition, saying that the new definition should be set aside. But so far, it has not been.

Joining us today is one of the petition's organizers. And in today's issue of the journal, Science, he is proposing a new definition of a planet, one, he says, will be more useful for astronomers and school children alike, and will bring Pluto back into the lineup. Mark Sykes is the director of the Planetary Science Institute in Tucson.

Welcome back to SCIENCE FRIDAY.

Dr. MARK SYKES (Director, Planetary Science Institute): Hi, Ira. Welcome. It's good to be back.

FLATOW: Good to have you. Tell us what your new definition would be?

Dr. SYKES: Well, it's not so much a new definition, but it's rather a -understanding why it works. The new definition is that objects that are round, that who's - who are massive enough that the gravity crashes them into a round shape, and orbit a star, should be a planet. And the reason why that's works well is that when we look at all the objects of the solar system that we've seen over the last 40 years with spacecraft, we see processes that - on many objects that we've likened to processes we see on the Earth — atmospheric processes, volcanism, tectonics.

And if you group all the objects together which share these geophysical-type processes - sharing some or all of them - we find that all of them are round.

FLATOW: Is Pluto round?

Dr. SYKES: Pluto is round.

FLATOW: Mm-hmm.

Dr. SYKES: And there's other objects that are smaller than Pluto, like Ceres, which is between Mars and Jupiter, which is also round. It was something we discovered just a few years ago using the Hubble Space Telescope.

FLATOW: Mm-hmm.

Dr. SYKES: Objects that don't exhibit any of these terrestrial-like processes are small and irregular objects like asteroids and comets.

FLATOW: Mm-hmm.

Dr. SYKES: And so when we are trying to understand, you know, where we should point our telescopes to study these geophysical processes, or send spacecraft to understand these processes better than, you know, organizing things on their physical basis - and it makes more sense.

FLATOW: So, you're saying that by their roundness, they are a planet? By the fact that they have…

Dr. SYKES: By their roundness and the fact that they orbit the sun or a star - another star, that would make them a planet in this context. And actually, the IAU planets would be a subset of these geophysical planets. But the way IAU defines things, which says you have to clear your orbit over the edge of the solar system, and you have to orbit our Sun, the smaller, round objects don't fall into their scheme.

FLATOW: You're not thinking of the moons making them - some of them all round, moons, making them planets.

Dr. SYKES: Well, the moon - if the moon was orbiting the sun by itself, then it would fall into the planet category.

FLATOW: Yeah.

Dr. SYKES: But, you know, certainly not into the IAU definition.

FLATOW: Mm-hmm. Why is roundness a more useful way to think about a planet?

Dr. SYKES: Well, you know, it's interesting that once you get big enough to be round, you know, once you've reached that point, you get hot enough in your interior to cause something called differentiation. That means that the interior, that the material that makes up the object — because it's kind of plastic - and you get heavy stuff moving towards the center, lighter stuff moving towards the outside. And so your volatiles, your lighter stuff that might be water, carbon dioxide, oxygen, nitrogen, they move towards the surface.

And if your object is big enough, they'd be retained as oceans and atmosphere. The under - mantel underneath the surface is warm enough that it's what's called it's convecting. And that gives rise to processes that we observe as volcanism and tectonics. And so that reaching that round shape seems to be the trigger for giving rise to all of these familiar processes that we fly around the solar system and study.

FLATOW: Mm-hmm. But according to your definition, as you say, there would be — not only would Pluto be back in the lineup, but there would be other bodies in our solar system which would qualify for the first time.

Dr. SYKES: Yeah. Immediately, well - immediately, there would be Ceres and -which is between Mars and Jupiter. And when it was first discovered, it was hailed as a planet — as a missing planet, in fact — but then it became the first of the, quote, "asteroids." But as we studied Ceres - here is an object that is not only round, it's covered with clay. And models suggest that it may have a subsurface ocean.

And so, when the - which raises the question, could there be life there on this object or, I should say beneath, the surface of this object? We have a spacecraft that's going to be visiting Ceres in 2015, the Dawn mission, which I'm a co-investigator on. And one of the things we'll be looking for is evidence of an atmosphere…

FLATOW: Mm-hmm.

Dr. SYKES: …which may have been detected from an early satellite. And also, whether there's evidence of communication between the subsurface ocean and the surface of the object. I mean, where did this clay come from?

FLATOW: Mm-hmm.

Dr. SYKES: Another object which would fall on the planet category would be, Eris, formerly Xena Warrior asteroid…

(Soundbite of laughter)

Dr. SYKES: …when it was first discovered, it had a, kind of, a secret name. And it orbits - if Pluto is at about 40 astronomical union - units from the sun, and astronomical unit being the distance of the Earth from the sun. So, Pluto is 40 times as far. Eris is about 68 times as far from the sun. And it's just slightly larger than Pluto. But it's also one of the brightest objects in the solar system. It reflects about 90 percent of the light that hits it.

And the reason for that is that it's covered with these frozen atmospheric ices, like frozen nitrogen. And so, that means - that's the evidence that Eris has undergone this differentiation process…

FLATOW: Mm-hmm.

Dr. SYKES: …like Pluto has, like Ceres has. In fact, Pluto is - going back to Pluto, that's interesting because Charon is also - it's an erstwhile moon - is large enough to be round. But we would characterize Pluto as a double planet because the point about which both objects rotate or orbit is in between the two of them.

FLATOW: Right.

Dr. SYKES: You know, if you imagine dumbbells…

FLATOW: Right.

Dr. SYKES: …spinning in space.

FLATOW: I think that's cool.

Dr. SYKES: Yeah.

FLATOW: I mean, you can have a double sun, why not a double planet?

Dr. SYKES: Yeah. And, of course, this opens the door for many other planets being discovered in our own solar system in the future. We're not locked to 1846…

FLATOW: Yeah.

Dr. SYKES: …which is when Neptune was discovered. And there's an interesting paper that's coming out next month in Astronomical Journal by two scientists, Lykawka and Mukai from Kobe University in Japan. They predict an object out at about 100 A.U. that may be bigger than the Earth.

FLATOW: Hmm.

Dr. SYKES: At a high inclination like 20 to 40 degrees, that in their simulations reproduces the structure of that second asteroid belt way beyond Pluto, they said the Kuiper belt and the trans-Neptunian objects. And so - but under the IAU definition, because you have to be bigger and bigger and bigger the further you get from the sun in order to clear your orbit over the age of the solar system, this object, which could be bigger than the Earth, though, it'll only be, say, two-thirds its mass because it's supposedly be lower density, would not be classified as a planet by an IAU.

And that's fine for people that are interested in what objects are gravitationally dominant in context. But for people that, you know, such as myself and other planetary scientists who are interested in the physical characteristics of these objects, and how are they alike, and what gives a rise…

FLATOW: And these are all good points for teachers to teach about astronomy.

Dr. SYKES: Absolutely. Absolutely.

FLATOW: How, you know, investigate it on your own and, you know, as an…

Dr. SYKES: Right.

FLATOW: …assignment during class.

Dr. SYKES: Who knows, you may discover something down the road.

FLATOW: Yeah. And understand what makes our solar system what it is.

Dr. SYKES: Mm-hmm.

FLATOW: And the whole debate is just, I would think, be interesting for students.

Dr. SYKES: Well, debate is interesting because unfortunately science was kind of misrepresented, I think, in this process that the IAU went through. People get the impression that our science ideas comes from majority rules/votes in a backroom some place. And it's not the case. It's a very dynamic process in which there's a lot of discussion, and three people are throwing their theories against the wall to see if they stick, and categorization, how we organize things, is a part of that process.

And how we categorize things is not governed so much by, again, majority rules or votes by an authoritative body or a special elite, but rather what works, what's useful. And that's the debate that's going on right now within the community because people are publishing papers that are ignoring the IAU definition.

Some people, you know, because it's not very useful to them; other people, you know, use their nomenclature. And - but what's valuable, I think, for teachers is the fact that it's not a list of facts. It's not like, is it eight or is it 12. It's rather that — what are the definitions that people are using? Why are they useful in different contexts? And to introduce science to the process to students using a topic that they're very interested in.

FLATOW: Yeah. I got about a minute left. Do you think you're going to be raising this issue in your definition formally in science communities?

Dr. SYKES: Well, that's why I published this paper in Science. And actually, the definition itself has been out there previously. It was actually proposed with the IAU by their planet-definition working group. But it just wasn't supported by any argument like this. And what I'm providing is the argument that motivates and supports that particular definition.

FLATOW: Mm-hmm.

Dr. SYKES: And so, yes. So that's why I wrote the paper and that's in Science today. And there will be more discussion and more arguments.

FLATOW: I understand that there's going to be a new poster with 12 planets coming out. Is that your doing?

Dr. SYKES: My institute will be putting out a poster later this year that has the geophysical view of the solar system, and we hope to be giving it broad distribution. We just had a very nice debate up at the Adler Planetarium, myself and Brother Guy Consolmagno of the Vatican Observatory. It was a very lively - I learned about your stuff, too, and it was - so I think, there's value in the discussion itself for people and particularly teachers.

FLATOW: You have to send us one of those posters when it comes out.

Dr. SYKES: Absolutely.

FLATOW: All right. Thank you very much, Dr. Sykes.

Dr. SYKES: You bet.

FLATOW: Mark Sykes is director of the Planetary Science Institute in Tucson, Arizona, maybe the Don Quixote of the planet Pluto, trying to tilt it - who knows, they may be winning. We'll never know. He's got 12 planets with a new definition. We'll just throw it into the debate and see what happens. This goes on for awhile, and we like it because it shows that science is an organic and changing process.

We're going to take a break and when we come back, we're going to talk about one of my favorite subjects — thank goodness, it's baseball season. But we're going to talk about the science of baseball and some interesting new ideas about, especially one idea about how to prolong the life of pitchers in the leagues by lowering the mound even more.

So, stay with us. We'll be right back after a short break.

(Soundbite of music)

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

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