Pluto is tiny, icy, and used to be the ninth planet in the solar system. Used to be. So why isn’t Pluto a planet anymore?

Join Molly and co-host Momo as they learn Pluto’s history, talk to planetary scientist Kelsi Singer about NASA’s New Horizons mission, and even hear from the planet itself. All that plus a nifty new mystery sound!

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KID 1: You're listening to Brains On, where we're serious about being curious.

KID 2: Brains On is supported in part by a grant from the National Science Foundation.

STAR E. NIGHT: Welcome back. I'm Star E. Night, and this is Pop Planet, the premiere planet profile show in the galaxy. Today's guest is the planet that was. It's Pluto.

PLUTO: Hi, Star.

STAR E. NIGHT: You were once in the great planet lineup, but now, not so much. What happened?

PLUTO: First of all, rude. Second of all, I'm not one for labels. And third, that was so in the past.

STAR E. NIGHT: Ooh, mysterious. Are you like a Scorpio or something? You are giving cold and distant. Classic Scorpios.

PLUTO: So what if I am? Everybody always makes so many assumptions. But if people really got to know me, they'd realized I have a glittering personality and a wildly successful Etsy shop.

STAR E. NIGHT: You have an Etsy shop?

PLUTO: Only the most popular crocheted vest emporium in the game. It's called Yes to the Vest. It was featured in Oprah's favorite things last year, but no biggie.

STAR E. NIGHT: That's major.

PLUTO: I'll give you a coupon code, bestie with a vestie for 10% off your first order.

STAR E. NIGHT: And with that, it's time to pop off to commercial break. We'll be back with more Pop Planet in just a bit.

MOLY BLOOM: You're listening to Brains On from APM Studios. I'm your host Molly Bloom. And today, I'm with my co-host, Momo, from New York, New York. Welcome, Momo.

MOMO: Hi, Molly.

MOLY BLOOM: Today we're talking all about Pluto. We've gotten a lot of questions from our listeners.

ADRIANA: Hi, my name is Adriana. I'm from San Francisco, California. My question is, why is Pluto not a planet anymore?

AMARA: Hi, my name is Amara, I'm from Cary, North Carolilna. I was wondering why isn't Pluto considered a planet anymore? What does a dwarf planet mean?

MOLY BLOOM: So, Momo, have you heard about Pluto?

MOMO: Yes, I have.

MOLY BLOOM: What do you know about it?

MOMO: That it's considered a dwarf planet.


MOMO: And I know that it's very cold.

MOLY BLOOM: Yes, both very true. How do you feel about space?

MOMO: It scares me.


MOMO: Like, it just-- it's just so big and like you don't know what you could find out there. Like there could be aliens or we could be the aliens.

MOLY BLOOM: Whoa. So true.

MOMO: And some aliens could be like, Hey, guys. What brings you up here? Which scares me.

MOLY BLOOM: It hurts my brain like when I try to think about how big the solar system is or the universe. Like, wow, just my brain kind of hurts.

MOMO: Honestly, me too. My brain is hurting as you think about it.

CREW: B-B-B-B-B-B-B-B-B-B-Brains.

MOLY BLOOM: Well, just like you, our listeners have probably heard of Pluto. Maybe you've heard that it's a dwarf planet, or that it used to be the ninth planet in our solar system. Both are true.

MOMO: So what changed?

MOLY BLOOM: Well, nothing has changed about Pluto. It was here long before we were. But we have learned a lot more about space and what's changed is how we think about planets. Before we get into the history, let's learn a bit about Pluto itself.

Pluto is super small. Smaller than Earth's moon.

MOMO: Yeah, it's only about half as wide as the United States.

MOLY BLOOM: And not only is Pluto petite, but it's also very, very, very far away. 3.6 billion miles from the sun to be exact.

MOMO: It's hard to even wrap my head around how far that is.

MOLY BLOOM: Totally. You can think of it like this. Let's say Earth is the size of a basketball. Pluto would be the size of a golf ball.

MOMO: So tiny.

MOLY BLOOM: And in terms of distance, if Earth was here in the studio, Pluto would be about 50 miles away.

MOMO: 50 miles?

MOLY BLOOM: It's true. And being so far away from the sun, that makes Pluto super cold. Temperatures can drop up to -400 degrees Farenheit.

MOMO: That's like five times colder than Antarctica in the winter.

MOLY BLOOM: Since it's so cold, Pluto's surface is made up of lots of icy mountains, valleys, plains, and craters.

MOMO: One day on Pluto is nearly one week on Earth.

MOLY BLOOM: And one year on Pluto is 248 years on Earth. That's almost as much time since the US signed the Declaration of Independence. Imagine how big a Pluto calendar would have to be. It'd be like a dictionary.

MOMO: And Pluto has five moons. The biggest one is called Charon, and it's nearly the size of Pluto itself.

MOLY BLOOM: So that's Pluto. And you're right, at one point, it was considered a planet.

MOMO: So what happened?

MOLY BLOOM: Well, it's a bit of a long story. So I asked our pal Ruby Guthrie to come help explain. Hi, Ruby. Awesome vest, by the way. Love the pink and red crochet.

RUBIE GUTHRIE: Oh, thanks. I got it from some Etsy shop. You know what they say, you can't go wrong with crochet. Anyways, I'm trying to give off like a grandma chic, kooky art teacher vibe. You know, Ms. Frizzle Realness.

MOMO: I totally see it.

RUBIE GUTHRIE: Wow. OK, as long as Momo says yes to my vest, that's all that matters. My impeccable wardrobe aside, I'm here to talk about Pluto. And not to be like back in my day, but back in my day, Pluto was the ninth planet in the solar system.

MOMO: Oh, yeah. In order from the sun, it goes Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, and Pluto, if you're including it, of course.

MOLY BLOOM: Yeah. In school, we used to make up sentences called mnemonic devices to help remember the order of the planets. We used this sentence to help us remember the order of the planets using the first letters so it went: My very excellent mother just served us nine pizzas.

MOMO: My, Mercury. Very, Venus. Excellent, Earth. Mother, Mars. Just, Jupiter. Served, Saturn. Us, Uranus. Nine, Neptune. Pizzas, Pluto. I get it.

RUBIE GUTHRIE: Ooh, yeah. That's a good one. Mine was: My very eager monkey just sat under Natalie's pillow.

MOMO: Who is Natalie, and why is your monkey under her pillow?

RUBIE GUTHRIE: I know, it's silly. And it really doesn't work if you drop the P for Pluto. My very eager monkey just sat under Natalie? That sounds dangerous. My poor fictitious monkey. But I'm getting way distracted here. Let's go to the beginning of Pluto.

Most planets in our solar system were discovered long, long ago, even before telescopes.

MOLY BLOOM: Right. That's because you can see a lot of planets with just your eyes, like Mars or Venus.

RUBIE GUTHRIE: Exactly. But once telescopes were invented, astronomers could find more planets and objects even further in space. That's how Uranus and Neptune were discovered in the late 1700s and 1800s, before rockets and even household electricity were a thing scientists. Had been gathering clues about a ninth planet for some time. But it wasn't until 1930 that the new planet was officially discovered.

The year is 1930. Swing and jazz music are the top tunes. Meanwhile, over half of US households own cars, and radios are becoming more and more popular in living rooms across the country. All the better to listen to news, like the discovery of a new planet.

REPORTER: Ninth planet on edge of solar system discovered, first in 84 years.

FEMALE REPORTER: Astronomers believe it's big and cold.

REPORTER: New world, larger than Earth.

RUBIE GUTHRIE: Well, they got the cold part right, but Pluto was not a giant. However, it was a giant discovery. Word traveled fast across the globe and the observatory started receiving submissions for names.

WOMAN: Kronos for the win.

MAN: It should be named Zeus.

WOMAN: Actually, Minerva is so much better.

MAN: Minerva?

RUBIE GUTHRIE: But the winning name actually came from an 11-year-old named Venetia Phair, who suggested Pluto, named after the Greek God of the underworld.

MOMO: So Pluto was named by a kid? Awesome.

RUBIE GUTHRIE: Right? So that's how Pluto became a planet. As scientists studied the planet more, they realized it wasn't actually giant as first predicted, but teeny tiny. And the more they researched, the more they learned. Like about its many moons or the fact that it has a stretched out orbit.

MOLY BLOOM: Yeah. Instead of having a circular orbit, Pluto's is more stretched like an oval.

MOMO: Funky.

RUBIE GUTHRIE: Yeah, super funky. They also learned that Pluto was floating in a disk of icy asteroids and comets called the Kuiper Belt.

MOLY BLOOM: Like the asteroid belt, but freezing.

RUBIE GUTHRIE: Exactly. So Pluto joined the ranks with the other planets. Kids learned about Pluto in school, museums included it in their exhibits, Mickey Mouse had a dog named after the planet, and Icelandic music icon Bjork even made a whole song about it. But that was all about to change.

MOMO: I'm on the edge of my seat.

MOLY BLOOM: Me too. But first, another mystery this time for your ears. It's time for the--

MOMO: Mystery Sound.

MOLY BLOOM: Momo, are you ready to hear the mystery sound?

MOMO: Yeah.

MOLY BLOOM: Wonderful. Here it is.


What do you think it could be?

MOMO: Honestly, my first thought is a constipated person. Like after coming home from traveling and being like, oh, dang it, I got to poop. Like that was the first thing I thought.

MOLY BLOOM: Yeah, you know, that is a really fair guess. I'm not going to lie, I thought a similar thing. Do you have any alternative guesses? Should we hear it again and see if anything else pops in your head?

MOMO: Yeah.


Oh, someone putting a bunch of fruit in a blender full of some sort of liquid.

MOLY BLOOM: Great. Excellent. Excellent guess. Well, we will hear it again, get another chance to guess, and find out the answer after the credits. So stick around.

We are working on an episode about how we grow. And we want to know, if you had the power to either grow super tall or shrink super small, which would you choose and why? So, Momo, would you rather be able to grow super tall or shrink super small?

MOMO: I would rather shrink super small.


MOMO: Because then it may be like a lot easier to travel. Or if like I'm too tired to walk, I can just shrink down and like sit on my dad's shoulder or like my mom can just like hold me in her parm.

MOLY BLOOM: Oh, that is a great idea. I think I might choose to grow super tall just because there's a surprising number of things I want from the top shelf in the grocery store and I can never reach and I have to wait for a nice tall person to come by and get it for me. So it would be nice to be able to just grow when I want it and grab that thing off the top shelf. Well, listeners, send us your answer at And while you're there, you can send us mystery sounds, drawings, and questions.

MOMO: Like this one.

KID: My question is, do flies get itchy?

MOLY BLOOM: Again, that's

MOMO: And keep listening.

CREW: Brains On. On. On.

STAR E. KNIGHT: I'm Star E. Night, and we're back with Pop Planet, your preferred planet profile show. Today we're here with Pluto and it's time for a little Q&A. We have tons of listeners' questions. How does that sound?

PLUTO: I'm game.

STAR E. KNIGHT: OK, first off, @Ziggystarcrust wants to know, who's your best friend?

PLUTO: My best friend's name is Chiron. They're technically my moon, but first and foremost, they're myr ride or die. I can't imagine doing anything without them by my side. They've even nicknamed us the "Double Planet."

STAR E. KNIGHT: Oh, bestie goals. OK, next up, @Rocketfullofsunshine wants to know, what music have you been listening to lately?

PLUTO: OK, I'm a Bjork stan, but I've really been into Olivia Rodrigo lately. I really connect to her sense of angst and betrayal. I think we'd have a fun time together.

STAR E. KNIGHT: Truly, a pop punk prodigy. And we have time for one last question. @UFOOMG asks, do you ever wish you were a planet again?

PLUTO: Look, planet, dwarf planet, Janet, I don't care what humans call me. Just don't call me boring.

STAR E. KNIGHT: Boring? I couldn't ever. Best guest ever. Wow, I could rhyme, forever. Thanks for answering those questions, Pluto. That's all for today's show. Catch us next week with a brand new episode of Pop Planet.

CREW: Brains On.

MOMO: You're listening to Brains On. I'm Momo.

MOLY BLOOM: And I'm Molly.

RUBIE GUTHRIE: And I'm Ruby. And where did we leave off?

MOMO: Pluto had just been discovered in 1930.

MOLY BLOOM: It was the ninth planet in the solar system after Neptune.

MOMO: It was named after the Greek God of the Underworld.

MOLY BLOOM: And marveled at by all.

RUBIE GUTHRIE: Wow, you two are great at recaps. And you're right, for a while, Pluto was the cutest little planet there ever was. It was a part of school curriculums, it was in museums, and researchers kept looking into it. And then came 2005.

YouTube had just gotten its start. The new Harry Potter books and movies were all the rage. And Momo wasn't even born yet.

MOMO: Like not even close.

RUBIE GUTHRIE: Bonkers. And the love for Pluto was still going strong. In fact, NASA was about to launch New Horizons, a spacecraft that would travel all the way to Pluto to observe the planet and beyond. But just a few months before the launch, scientists made a new discovery. A Pluto-sized object that was even farther from the sun. They named it Eris, and it started a whole big debate.

MAN 1: Should we even have a tenth planet?

MAN 2: Pluto shouldn't even be a planet. It's too small.

RUBIE GUTHRIE: And this made everyone rethink some things, because historically, there hadn't been an official definition of what made a planet a planet. So after a year of brainstorming, a group of astronomers came together and made a list of three rules.

First rule. For a planet to be a planet, it must orbit the sun.

MOLY BLOOM: Pluto does that just, like Earth. Check.

RUBIE GUTHRIE: Rule two. A planet must have a nearly round shape, which separates it from objects like asteroids that can have all sorts of wonky, irregular shapes.

MOMO: Pluto is round. Check.

RUBIE GUTHRIE: And last but not least, the planet must be gravitationally dominant and clear the neighborhood around its orbit.

MOMO: Translation please.

RUBIE GUTHRIE: Basically, it means that the planet has to be the biggest object in its orbit, with no other planet-sized objects nearby. Except for moons. Because according to this rule, those don't count.

MOLY BLOOM: So for a planet to be official, it can't be like a pop group. It has to be a solo superstar, dominating the space around it and leaving no room for others except its posse of moons, of course.

MOMO: Uh-oh.

RUBIE GUTHRIE: Yeah, that third rule, that's where they got Pluto. There were too many other Pluto-sized objects orbiting in space nearby, which actually affected Pluto's orbit.

MOLY BLOOM: Ooh, like Eris.

RUBIE GUTHRIE: Exactly. So instead, the researchers decided to classify Pluto in a new way, calling it a dwarf planet because it only met two out of their three rules.

MOMO: Dang, that's rough for Pluto.

RUBIE GUTHRIE: Yeah. After the decision, a lot of people were upset-- both scientists and the general public. It caused such a hubbub that the word of the year in 2006 was "plutoed", meaning to downgrade or to devalue something.

GIRL: I can't believe mom moved me from dishwasher duty to cleaning the bathroom.

BOY: Dude, you just got plutoed.

RUBIE GUTHRIE: It might have seemed strange or dramatic for Pluto to no longer be a planet, or like the scientists were just making up rules out of nowhere. And remember, it's not that Pluto changed, we just changed the way we categorized it. That's just a part of the scientific process. The more we ask questions and the more we learn, the more we might change how we define something.

MOLY BLOOM: It's true. The way we understand our world is constantly changing. So it's natural to make up new definitions and rules along the way.

RUBIE GUTHRIE: Couldn't have said it better myself. And just because Pluto isn't a planet doesn't mean it's not important. Scientists are still super interested in studying Pluto and the Kuiper Belt, that disk of icy asteroids, to learn more about the formation of our universe.

MOMO: But just so we're clear, Pluto, by any other classification, is still as sweet.

MOLY BLOOM: Something we'll learn more about in just a bit.

RUBIE GUTHRIE: Momo, Molly, it's been real, but I've got a blast so I don't miss the latest episode of Pop Planet. See ya.

MOMO: Thanks for sharing with us, Ruby.


CREW: Brains On.

MOLY BLOOM: We've learned about the New Horizons program. That's the spacecraft that's dedicated to observing Pluto.

MOMO: It was launched in 2006, before I was born. And it wasn't until nine years later in 2015 that it made it to Pluto.

MOLY BLOOM: Once there, it took pictures, made observations, and studied everything from the surface to the atmosphere. Turns out there's a lot going on on Pluto.

MOMO: Yeah, we spoke to the lead scientist of this mission.

KELSEY SINGER: I'm Kelsey Singer and I'm a planetary scientist, and I work on Pluto. There's so many exciting things about Pluto. I first got interested in Pluto because it's this amazing, icy world out in the far distant parts of our solar system. And it's got all these really unique geologic features on the surface and they're all made out of ice.

It's very, very cold on the surface of Pluto. And because of that, we have a bunch of different kinds of ice. We have the ice like we are all used to on Earth. Water ice, like an ice cube that you might have in your drink. But we also have other kinds of ice. We have nitrogen ice, we have methane ice. And those things that would be in our atmosphere that we breathe on Earth, they are an ice on the surface of Pluto. The ice on Pluto acts very much like rock because it is just so cold and the ice there is very hard. We also have ice on the surface of Pluto that flows like a glacier.

One particular aspect about Pluto that I really love is these very, very tall mounds that look like they could be volcanoes. But instead of being made out of rock, they are made out of ice.

MOLY BLOOM: Before New Horizons visited Pluto, this dwarf planet was still mostly a mystery.

MOMO: Kelsey says there are some photos taken by powerful telescopes, but they were blurry and not very detailed.

MOLY BLOOM: New Horizons was the first time we got up close and personal with Pluto.

KELSEY SINGER: And I always like to say that we were surprised by how much we were surprised. The surface of Pluto had so many really fascinating and beautiful geologic features that we didn't expect. We also found some surprising aspects about Pluto's thin atmosphere. And one of them was that it is blue. We did not know or expect that. And the other one is that it had all these layers in it that turned out to be layers of haze, and none of this was predicted beforehand.

MOMO: After checking out Pluto, New Horizons floated off to study icy objects floating in the Kuiper Belt.

MOLY BLOOM: Yeah. And Kelsey says part of what makes Pluto and the Kuiper Belt so fascinating is because they aren't planets. Instead, they're kind of like the raw ingredients planets are made of.

KELSEY SINGER: We like to think of them as the building blocks of the planets. But these particular objects that are still out there, they're leftovers and they didn't get made into the planets. But they can still tell us about all those early times when our planets were forming. It's kind of like getting a window into the past. It's sort of like the archeology of the solar system. So we really had a lot to learn by taking a spacecraft there.

MOLY BLOOM: Pluto used to be the ninth planet in our solar system, but now it's called a dwarf planet.

MOMO: It's really small, really cold, and has a really stretched orbit.

MOLY BLOOM: Pluto helped inspire us to change the way we define what makes a planet a planet.

MOMO: We changed the way we think of things as we learn more information. That's a part of the scientific process.

MOLY BLOOM: And there's still a lot scientists can learn by studying Pluto and other objects in the Kuiper Belt. That's it for this episode of Brains On.

MOMO: This episode was produced by Molly Bloom, Anna Goldfield, Molly Quilan Artwick, Rosie Dupont, Aron Woldeslassie, Anna Weggel, Nico Gonzalez Wisler, Ruby Guthrie, and Marc Sanchez.

MOLY BLOOM: Our editors are Sanden Totten and Shahla Farzan. We had engineering help from Jess Berg and Gary O'Keefe, with sound design by Rachel Brees and Marc Sanchez. Beth Perlman is our executive producer. The executives in charge of APM Studios are Chandra Kavati, Alex Schaffert, and Joanne Griffith. Special Thanks to Mishi Kobayashi, Lulu, and Coco.

MOMO: Brains On is a non-profit public radio program.

MOLY BLOOM: There are lots of ways to support the show. Head to

MOMO: While you're there, you can send in your mystery sounds, questions, and drawings.

MOLY BLOOM: You can also subscribe to our Smarty Pass.

MOMO: Super fun, ad-free episodes and bonus stuff just for you.

MOLY BLOOM: All right, Momo, are you ready to hear that mystery sound again?

MOMO: Yes.

MOLY BLOOM: All right, here it is.


What are your new thoughts?

MOMO: I'm starting to think like a laundry machine or like someone dropping like if there's a big thing of water and someone dropping like fruits into it to wash them or something.

MOLY BLOOM: Both excellent guesses. Are you ready for the answer?

MOMO: Yes, please.

MOLY BLOOM: Here it is.

SYDNEY: Hi, my name is Sydney, and I'm from Encino, California. That was the sound of my pet turtle splashing in his tank to say hi.

MOLY BLOOM: Whoa. Turtle.

MOMO: Not even close.

MOLY BLOOM: Well, if the turtle was pooping, then maybe.

MOMO: I don't think the poops would be big enough.

MOLY BLOOM: Probably not big enough to make noise, you're right. He was just saying hi. That's very sweet. It's a happy sound. You had excellent guesses, though. I enjoyed your guesses very much.


Now it's time for the Brains Honor Roll. These are the incredible kids who keep the show going with their questions, ideas, mystery sounds, drawings, and high fives.


We'll be back next week with more answers to your questions.

MOMO: Thanks for listening.

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