Is Pluto a planet or not?
Alan Stern thinks Pluto bashers are just a vocal minority.
Alan Stern is a planetary scientist and director of the Southwest Research Institute’s (SwRI) Division of Space Science and Engineering in Boulder, Colorado. He is Principal Investigator for NASA’s mission to Pluto. Earth & Sky’s Abby Frank interviewed Stern in late 2005.
Frank: Is Pluto a planet?
Stern: Of course Pluto is a planet. And a Chihuahua is a dog.
Frank: Do you care to elaborate on that?
Stern: Sure, it’s all about genetics. It’s not about arbitrary classifications. It’s really that Pluto displays the hallmark of planethood, which is that it’s big enough that it knows it’s a planet. It’s become round under self–gravity. The same way that a Chihuahua knows it’s a dog: it has all the genes that make it a dog. And we don’t disqualify it just because it’s lower than some arbitrary size criteria.
Frank: What are the defining features of a planet?
Stern: Well I think that’s been evolving. To be honest, in my field, in planetary science, we didn’t have to grapple with that until recently.
But since the 1990s, we’ve seen so many discoveries, both in our solar system and others, of different types of planetary bodies that we never knew about before, like Jupiters in orbtis that are closer than Mercury, planets around pulsars, dwarf planters out in the Kuiper Belt, that we’ve had to really, for the first time, think about it in the same way that if you were just from a small town on a Pacific island, and you didn’t know of any other civilizations, you might define a town just one way. But then when you saw examples, cause you could travel the world, of New York cities and hamlets and towns and townships and all kinds of different expressions of what it is to be a town, you’d have to sit down and say “what’s the smallest thing we count as a town, and what’s the biggest thing we count as a town” and you know it could even jar your perceptions based upon what little you knew when you were growing up.
In the same way, we’ve been faced with that in planetary science now with “what is a planet?”
What we’ve come to the conclusion, I think this is pretty much a consensus, is that planets very simply should pass the, what is it called? A duck test. You know, if it looks like a duck and it walks like a duck, it’s a duck. We should have a definition that if you threw a picture of the object in front of a school kid, they should agree it’s a planet. They shouldn’t say “well that’s a rock or a boulder” or “that’s a star or a galaxy” or anything else.
And when we translated that into technical terms, we come up with an upper and a lower size limit. And it goes like this: it should not be so big that the object is doing what stars do for a living, that is doing fusion in their interiors. That is, it shouldn’t be so big that the central core temperature is so hot that the object ignites like a star. And it shouldn’t be so small that it’s a rock.
So the question is, what’s the boundary between a rock and a planet? Rocks have their shape controlled by chemistry, by the strength of chemical bonds. And that’s true of a boulder and a mountain, and even really big rocks miles across like Ayers Rock in Australia. But as you pour on more and more and more mass, eventually the gravity overwhelms the chemical bonds and the object will flow into a sphere atuomatically, spontaneously, just due to the gravity. And we can calculate the size where that occurs. It’s around 500 kilometers in radius. That’s the lower size limit for a planet.
Frank: So when we call something a planet, it is mainly a matter of size?
Stern: Correct. I mean, you know, we could be lawyerly and we could say, for example, it shouldn’t be made by human beings or other intelligent species, it should be a natural object. But without going that far, what the IAU is going to say essentially is that it has to be bigger than a lower size limit and smaller than an upper size limit, and the things in space that fit those two bounds are things we call planets.
Frank: If Pluto were discovered today, would it be considered a planet, given how small it is?
Stern: Yeah, because it’s much larger than that lower size bound. It’s about five times bigger than that.
Frank: Should 2003 UB313 be called a planet?
Stern: Yeah, because it’s even bigger than Pluto.
Frank: As you know, many people think that Pluto should be demoted…
Stern: I find that weird. I hear this in the press, but I never hear it in the astronomical community. Now I know there are a few astronomers who believe that, a few who people in the press go to a lot, like Brian Marsden and Neil Tyson. But it’s actually kind of laughable in scientific circles; it’s like the press only listens to the five naysayers. So, it’s a false impression.
Frank: So are they just a vocal minority?
Stern: That’s a good way to put it. And it gives the impression that there are a lot of people back there that want to make demotions and change this and change that. And you go to scientific meetings of planetary scientists and people are like “What are we talking about? That’s just a few guys over in the corner.”
