Pluto Isn't A Planet - But It Gives Us Clues For How The Solar System Formed

This episode explores why Pluto is classified as a dwarf planet and its implications for understanding planetary formation.

1. Pluto was reclassified in 2006 due to not meeting all the newly defined criteria for planets.
2. The episode discusses the historical and scientific context of planetary classification.
3. Insights into the formation of planets and the role of celestial mechanics are provided.
4. There's an ongoing debate about the criteria for planet classification, emphasizing geophysical properties over orbital dynamics.
5. The discussion extends to other celestial bodies, suggesting a broader reevaluation of what constitutes a planet.

1: Introduction

Regina Barber introduces the topic and the context of Pluto’s reclassification. Regina Barber: "Today we delve into why Pluto isn't a planet and what it tells us about our solar system's history."

2: The Vote That Changed Pluto's Fate

Detailing the 2006 IAU vote that redefined planet criteria. Virginia Trimble: "I was there...counting the votes."

3: Planetary Formation Insights

Exploring theories of how planets form from cosmic dust and gas. John Erickson: "You start from a cloud of gas and dust...imagine not cleaning your room for 10 million years."

4: Definitions and Controversies

Discussion on the implications of Pluto's reclassification and the criteria used. John Erickson: "If you take Pluto and put it where Mercury is, Pluto will clear the orbit."

5: Closing Remarks

Summarizing the episode's insights and teasing the next installment of the space camp series. Regina Barber: "Thank you for joining us on this cosmic journey through our solar system's architecture."

• Stay Curious: Continue exploring and learning about space and planetary sciences.
• Engage in Discussions: Participate in debates about space and science to expand understanding.
• Follow Updates: Keep up with the latest discoveries and changes in scientific classifications.
• Visit Planetariums: Experience space-themed exhibits to enhance learning.
• Read Widely: Dive into books and articles on astronomy and planetary science to broaden knowledge.

Pluto hasn't been a planet for almost 20 years. In the early 2000s, scientists discovered several objects of a similar size to Pluto. So, during the summer of 2006, members of the International Astronomical Union convened in Prague to reconsider what counts as a planet in our solar system. IAU members decided that there were three criteria to be a planet — and Pluto didn't meet all of them.

But planetary scientist Wladymir Lyra says that even though it was downgraded to a dwarf planet, Pluto still has much to teach us about planet formation. This episode, he also lays out his case for Pluto — and many other objects in the solar system — to be considered a planet.

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Regina Barber, Virginia Trimble, John Erickson

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Regina Barber
You're listening to short wave from NPR.

Hey, short wavers. So as part of our shortwave space camp series, we're dropping every Tuesday. We recently asked you to tell us what's your favorite planet? And you called in from all over.

John Erickson
This is John Erickson, a retired planetarium.

Mom
Operator from Richmond, and I live in Wheat Ridge, Colorado. Mom, my name is, and I live in little Georgia.

Regina Barber
You named a lot of good ones.

John Erickson
Saturn is a favorite planet of mine. It's got bright rings, awesome moons, and atmospheric waves that make a hexagon around the north pole.

Mom
My favorite planet is Earth because we live here and everyone I love is here. It is the only planet with dogs, and I have two. My favorite planet is Uranus because it's been sideways.

Regina Barber
But the one you didn't name was Pluto. That's because Pluto isn't a planet. It's now a dwarf planet. In 2006, a meeting in Prague changed its fate forever. At the International Astronomical Union, 424 members representing over 1000 scientists passed a resolution to decide what the word planet would actually mean in our solar system. And what kind of scientist would you call yourself?

Virginia Trimble
Old.

Regina Barber
That's Virginia trimble. She's an astronomer and also studies the history of science. And she was there in Prague when the vote happened.

Virginia Trimble
I was there as an ordinary IAU member. I also skippered the team of tellers who counted the votes. Everyone who was entitled to vote had a yellow card, and you voted by holding your yellow card up.

And so the tellers came down the aisles, counting the number of yellow cards were being held up in the row and doing basic arithmetic to get the total numbers.

Regina Barber
Leading up to the meeting, multiple objects that were around the same size as Pluto had been found. So the IAU decided to reopen the question of what makes a planet a planet. 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, cleared 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.

John Erickson
Pluto at the time. It felt that something had to be done about Pluto. And with that, I mean, that, yeah, Pluto was always the oddball among the planet.

Regina Barber
That's Vladimir Lyra. He's a computational astrophysicist. And he says even though scientists made up the decision back in 2006, it's still kind of controversial. Now, depending on who you ask, planetary.

John Erickson
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
So the fate of a planet was being decided by people who don't study planets mostly.

John Erickson
And that was and is still one of the criticisms about the vote, that astronomers were voting on the definition of planet.

And who study planets are planetary scientists.

Regina Barber
So today on the show, the case for Pluto, what it can tell us about how planets were created and why are some planetary scientists still Pluto defenders almost 20 years later? I'm Regina Barbour, and you're listening to short wave from NPR.

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Regina Barber
Okay, Vlad, let's get into these discoveries that ultimately led to the vote in 2006 to demote Pluto. Like first of all, there was Ceres in the early 18 hundreds, and it was born out of this search for a planet in between Mars and Jupiter, which scientists kind of focused on because, like, the four planets closest to the sun are somewhat evenly spaced out until you get to Mars. And then there's this, like, big gap between Mars and Jupiter where there's this asteroid belt where there's no planet. And this perplexed scientists.

John Erickson
So the idea that there should be something there was already in the mind of astronomers. They found Ceres. It was herodoted as a planet, but then they kept looking and kept finding more stuff. They found more and more objects. Nothing as big as Ceres, but Ceres was sharing its orbit with many other objects, right?

Regina Barber
And initially, they were calling a bunch of things planets. It seemed at the time, there wasn't really a super clear distinction between planets and asteroids, right?

John Erickson
So that led then to astronomers to rethink what these minor planets were, and then they were called the asteroids.

Regina Barber
So I guess there's, like, precedent for Pluto being, like, demoted.

John Erickson
As I said before, Pluto was always the oddball, right? It was seen that Pluto just didn't fit among the eight planets, and that.

Regina Barber
Was partly because of the dwarf planet Aries. Like, it's in the Kuiper belt with Pluto towards the end of the solar system, right?

John Erickson
Yes. When Neres was found, that was the catalyst to declassify Pluto, because all of a sudden, you found. You 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.

Regina Barber
Okay, so let's talk about Pluto for a little bit. It was discovered by an american in 1830, not even 100 years ago. And after that, I. You vote it's not even a planet anymore. So why do you think Pluto is so fascinating if you think Pluto is fascinating?

John Erickson
Oh, I think Pluto is absolutely fascinating, for sure.

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.

Regina Barber
Yeah, no, that totally makes sense. So let's just step back for a second, though, and think about. And talk about how do planets and dwarf planets like Pluto form in the first place?

John Erickson
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 gonna 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?

Regina Barber
You're gonna take the whole room.

John Erickson
Yeah. So with that, you form the first grains. Right? The first dust grains. And then there are mechanisms happening in these discs that help concentrate these dust grains so that you catch an object of the size of asteroids. We call these bodies planetesimos, 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 that size, gravity comes into play, and then they can keep growing to the size of the earth.

Regina Barber
That's so cool.

John Erickson
Or bigger. And at some point, it gets so massive that they attract gas from the disk and then form a gas planet like Jupiter.

Regina Barber
Okay, which brings us back to our friend Ceres. Right? The dwarf planet in the asteroid belt between Jupiter and Mars. It's got all these asteroids around it. Like, how come they haven't combined together and turned into a bigger planet?

John Erickson
So the azore belt did not become a big planet because of the presence of Jupiter.

Regina Barber
Really? Okay.

John Erickson
Jupiter is a very massive planet, so the tides from Jupiter end up exciting the orbits. So that is much more likely that when two planetesimals in the Azerob collide that are going to fragment. Right.

So that is the main reason why the acrobat did not become a planet.

Regina 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?

John Erickson
Correct. Yeah. We call it a dynamically hot population. Right.

Regina Barber
And is the Kuiper belt similar? Like, is it also dynamically hot population, or is there something else going on?

John Erickson
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, so small that they never meeted each other. So they couldn't really grow. Right.

Regina Barber
Then how did Pluto get made then?

John Erickson
Right. That's another thing. 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.

Regina Barber
What?

So this is kind of blowing my mind. Cause 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.

John Erickson
Right. There is a very good point. And that's one thing that prompted getting goosebumps. Planetary scientists, in fact, to use another definition of planet that they call the geophysical definition 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. And to others, too, it doesn't really make sense to define a planet based on one location. So if a planet has to clear the orbit to be defined as a planet, if you take Pluto and put it where Mercury is, Pluto will clear the orbit. So then Pluto, a Mercury orbit, would be a planet. You take Mercury, put it where Pluto is, it is not a planet anymore. Yeah. So the geophysical definition of planets looks only at the intrinsic characteristic, 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 massive enough to be around but is not fusing inside. And then once you get big enough, you become a star. Right. So they're rocks, planets and stars.

Regina Barber
This is so amazing. I had no idea. Okay, I kind of want to keep going because this is kind of fascinating.

John Erickson
I can't talk about Pluto all day long.

Regina Barber
Then what's next? Like, what else can we find out about Pluto? What does that mean for its designation? Like, what's the future hold for Pluto?

John Erickson
What it means for the designation?

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 dynamical 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 have so many? It's like, can you tell me the lineup of the us women's soccer team? No, I don't know all names there.

Regina Barber
Yeah, I don't either, but they're still pro soccer players.

John Erickson
Yeah. And so this argument, I think, that doesn't, doesn't hold much water. So classification wise, I am completely comfortable calling Pluto a planet. The moon a planet.

Regina Barber
Europa, one of Jupiter's moons, is a planet.

John Erickson
Europa is a planet. Yes, exactly. If you put Europa in an independent orbit around the sun, you would call it a planet.

Regina Barber
100%. I would. Okay, well, thank you so much, Vlad, for talking to me today and making me really excited about Pluto.

John Erickson
My pleasure.

Regina Barber
Thank you so much again. We'll be back tomorrow with our regular shortwave and back Tuesday with our next installment of the space Camp series. And I have the sneak preview from one of our experts.

Doctor Serafina Nance
Hey, short wavers. It's Doctor Serafina Nance, your supernova guide back on Earth. I hear you're getting to leave our solar system and are on your way to visit stars other than the sun. One of my favorites is Betelgeuse, and I'm so jealous that you might get to see it explode from space.

In case you didn't know, Betelgeuse is a red supergiant fated to explode any day now. Take a selfie with beetlejuice for me. Thanks, and have a blast.

Regina Barber
This episode was produced by Hannah Chin, edited by our showrunner Rebecca Ramirez, and fact checked by me. The audio engineer was Gilly Moon, Julia Carney is our project manager, Beth Donovan is our senior director, and Colin Campbell is our senior vice president of podcasting strategy. I'm Regina Barber. Thank you for listening to Space Camp, a special space science summer series from NPR.

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