ANDREW LIMBONG, HOST:
It's not exactly news to say that Pluto is no longer considered a planet. That made headlines almost 20 years ago. But what if I told you that decision at the time was controversial, not just among those of us who had grown up learning about Pluto, but also among scientists? Regina Barber of NPR's science podcast Short Wave has been looking into this controversial vote as part of the show's Space Camp series. She found that there are some planetary scientists like Wladimir Lyra, who are still making the case for Pluto years later, based on how it was formed.
WLADIMIR LYRA: Planetary sciences used to be part of astronomy. Now there is some mixing, of course. But by and large, we're talking about two different communities.
REGINA BARBER, BYLINE: So this - the fate of a planet was being decided by people who don't study planets - mostly.
LYRA: And that was and is still one of the criticisms about the vote, that astronomers were voting the definition of planets. And who study planets are planetary scientists.
BARBER: Regina zoomed in on the definition that sealed Pluto's fate and led to its demotion from planet to dwarf planet. She takes the story from here.
BARBER: In 2006, at the International Astronomical Union, 424 members representing over a thousand scientists changed Pluto's fate forever. They decided that in order to be a planet in our solar system, an object needed to meet three criteria. One, it had to orbit the sun. Two, it had to be big enough to assume hydrostatic equilibrium, which is a fancy way to say it needed to be round. And three, it had, to, quote, "clear the neighborhood around itself," basically to have a strong enough gravitational pull that there wasn't anything left immediately around it, like no asteroids or other small bits of rock or ice. And that third requirement? That's what disqualified Pluto.
Wladimir Lyra is a computational astrophysicist, and he says that an unintended consequence of this vote is that people don't understand the value of studying the dwarf planet anymore, when really, planet or not, it's a key piece of understanding our solar system and how we got here.
LYRA: Pluto holds many clues to understand how planets in general form. So as a scientist who studies how planets form, for me, Pluto is a brick that helps me understand the building.
BARBER: In fact, a great irony to the story is that there was several discoveries of large objects in the solar system that scientists excitedly called planets when they were found, and astronomers would later point to those discoveries when deciding how to differentiate the eight planets we still have today from the other objects in space. The big two are Ceres and Eris. The realization that Ceres exists came first in the early 1800s. It was born out of this search for a planet in between Mars and Jupiter. Then there was the dwarf planet Eris.
LYRA: When Eris was found, there was the catalyst to declassify Pluto because all of a sudden, you've found an object that was more massive than Pluto, right? So either you also call that object a planet or you have to rethink the definition of planet.
BARBER: Yeah. No. That totally makes sense. So let's just step back for a second, though, and, like, think about and talk about, how do planets and dwarf planets like Pluto form in the first place?
LYRA: Right. So the way that planets form is you start from a cloud of gas and dust. And modern astronomy has found these disks of gas orbiting young stars. And what we see is that once you have dust in a disk of gas, coagulation will take place. So you build larger grains out of the dust that is in this disk, a bit like if you don't clean your room often enough, you're going to get dust bunnies, right? Now imagine that you don't clean your room for 10 million years, just how big those dust bunnies will get, right? So with that...
BARBER: It would take the whole room.
LYRA: Yeah. So with that, you form the first grains - right? - the first dust grains. And then there are mechanisms happening in this disc to help concentrate these dust grains so that you get an object of the size of asteroids. We call these bodies planetesimals, which is a portmanteau of a planet plus infinitesimal, so a very small part of a planet. So asteroids and comets, they are these so-called planetesimals, or as I like to call them, the building blocks of planets, right? And then once you build it that size, gravity comes into play. And then they can keep growing to the size of the Earth or bigger. And at some point, they get so massive that they attract gas from the disk...
BARBER: That's so cool.
LYRA: ...And then form a gas planet like Jupiter.
BARBER: OK. So what happened to our friend Ceres, though? It's got a lot of asteroids near it. How come they haven't combined together and, like, turned into a bigger planet?
BARBER: So the asteroid belt did not become a big planet because of the presence of Jupiter.
BARBER: Really? OK.
LYRA: Jupiter is a very massive planet. So the tides from Jupiter end up exciting the orbits, so that it is much more likely that when two planetesimals in the asteroid belt collide, they're going to fragment.
BARBER: Wait. So just - so that I understand. So you're saying that because Jupiter is so big, and because its gravitational pull is so influential on all of these asteroids, they make them move faster, so that when they do collide, they smash instead of moving slower and just coalescing.
LYRA: Correct. Yeah. We call it a dynamically hot population. Right.
BARBER: And is the Kuiper Belt similar? Like, is it also - dynamically hot population or is there something else going on?
LYRA: So the Kuiper Belt is different, though, especially at the region where Pluto is. The number of objects per volume of space is just so low that they can go ages without finding another object. So these bodies just didn't grow large because they formed so far away from the sun. And also, the density of other objects nearby is so small that they never meet each other, so they couldn't really grow, right?
BARBER: Then how did Pluto get made then?
LYRA: Right.
BARBER: It's pretty big.
LYRA: That's another point. Pluto is very big. Pluto did not form where Pluto is. Pluto formed at about half the distance where it is now and was put in its orbit by Neptune.
BARBER: What? So this is kind of blowing my mind because you're saying that the definition of a dwarf planet is that it orbits the sun, that it's round, basically, and that it cleared its path. But you're telling me that at one point, maybe Pluto did have a cleared path. Maybe it wasn't in this belt.
LYRA: Right. There is a very good point. And that's one thing that prompted...
BARBER: Getting goosebumps.
LYRA: ...Planetary scientists, in fact, should use another definition of planet that they call the geophysical definition of planet that depends only on the mass, right? So in that case, a planet is an object that has enough mass to be round. So that's only the second part of the IAU definition. And that makes sense for some astronomers, including me. Because, to me - right? - ...
(LAUGHTER)
LYRA: ...And to others too, it doesn't really make sense to define a planet based on one location. If you take Pluto and put it where Mercury is, Pluto will clear the orbit. So then Pluto and Mercury orbit would be a planet. You take Mercury, put it where where Pluto is, it is not...
BARBER: A planet anymore.
LYRA: Yeah.
BARBER: What?
LYRA: So the geophysical definition of planets looks only at the intrinsic characteristics, right? And it boils down to just mass. That means that you have rocks, right? There are like asteroids, things that are not massive enough to be round. And then you have planets, which is anything that has mass enough to be round but is not fusing inside. And then once you get big enough, you become a star, right? So there's rocks, planets and stars.
BARBER: This is so amazing. I had no idea. OK. I kind of want to keep going because this is kind of fascinating.
LYRA: Oh, please do. Yeah. I can talk about Pluto all day long.
BARBER: Then what's next? Like, what do - what else can we find out about Pluto? What does that mean for its designation? Like, what's the future hold for Pluto?
LYRA: Well, the IAU vote that was held in 2006, I don't know if it's being challenged, but definitely what is happening is that some people are not comfortable with the dynamic part of the definition, the clearing of the orbit. And planetary scientists have been advocating for a purely geophysical definition. In this case, Pluto is a planet. The moon is a planet, right? And one of the arguments that is being given by that is that, oh, if we do that, then there's going to be too many planets. How are kids supposed to remember the name of all of the planets if we have so many? It's like, can you tell me the lineup of the U.S. women's soccer team?
BARBER: No.
LYRA: I don't know all names there, right?
BARBER: Yeah, I don't either. But they're still pro soccer players.
LYRA: Yeah. And so this argument, I think that doesn't hold much water. So classification wise, I am completely comfortable calling Pluto a planet, the moon a planet.
BARBER: Europa, one of Jupiter's moons, is a planet?
LYRA: Europa is a planet. Yes, exactly. If you put Europa in an independent orbit around the sun, you would call it a planet.
BARBER: A hundred percent, I would.
LIMBONG: That was Regina Barber from NPR's science podcast Short Wave. You can listen to Short Wave wherever you get your podcast. A special shout-out to our friends at the U.S. Space and Rocket Center, home of Space Camp. Transcript provided by NPR, Copyright NPR.
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