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Learn more at copilot.microsoft365.com. You're

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listening to Shortwave from

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NPR. Hey, Shortwavers.

0:50

So as part of our Shortwave Space Camp

0:52

series we're dropping every Tuesday, we recently asked

0:55

you to tell us what's your favorite planet,

0:58

and you called in from all over. This

1:01

is John Erickson, a retired planetarium operator

1:03

from Richmond, California. My name is Summer,

1:05

and I live in Wheat Ridge, Colorado.

1:07

My name is Marina, and I live

1:10

in New Little Georgia. You named a

1:12

lot of good ones. Saturn

1:15

is a favorite planet of mine.

1:17

It's got bright rings, awesome moons,

1:19

and atmospheric waves that make a

1:21

hexagon around the North Pole. My

1:24

favorite planet is Earth because we

1:26

live here, and everyone I love

1:28

is here. It is the only

1:30

planet with dogs, and I have

1:32

two dogs. My favorite planet is

1:34

Uranus because it's been sideways. But

1:36

the one you didn't name was

1:39

Pluto. That's because Pluto

1:41

isn't a planet. It's now a

1:43

dwarf planet. In 2006,

1:45

a meeting in Prague changed its fate

1:48

forever. At the International Astronomical

1:50

Union, 424 members representing over 1,000 scientists passed

1:55

a resolution to decide what the word

1:57

planet would actually mean in our solar

1:59

system. And what kind of scientist would

2:01

you call yourself? Old.

2:05

That's Virginia Trimble. She's an astronomer and

2:07

also studies the history of science. And

2:10

she was there in Prague when the vote

2:12

happened. I was there as an ordinary

2:14

IAU member. I also skippered

2:16

the team of tellers who counted the

2:19

votes. Everyone who was entitled to vote

2:21

had a yellow card. And

2:23

you voted by holding your yellow card up. And

2:26

so the tellers came down, the IELTS counting, the

2:28

number of yellow cards were being held up in

2:30

the row and doing basic arithmetic to

2:34

get the total numbers. Leading up to the

2:36

meeting, multiple objects that were around the same

2:38

size as Pluto had been found. So

2:40

the IAU decided to reopen the question of what makes

2:43

a planet a planet. They

2:45

decided that in order to be a planet in

2:47

our solar system, an object needed to meet three

2:49

criteria. One, it had to

2:52

orbit the sun. Two, it had

2:54

to be big enough to assume hydrostatic equilibrium, which

2:56

is a fancy way to say it needed to

2:58

be round. And three, it

3:00

had to, quote, clear the neighborhood

3:02

around itself, basically to have a

3:05

strong enough gravitational pull that there wasn't

3:07

anything left immediately around it, like no

3:09

asteroids or other small bits of rocker

3:11

ice. And that third

3:13

requirement, that's what disqualified Pluto.

3:17

At the time, it felt that something

3:19

had to be done about Pluto. And with

3:21

that, I mean that Pluto was always the

3:24

odd ball among the planet. That's

3:26

Vladimir Lyra. He's a computational astrophysicist. And he

3:29

says even though scientists made up the decision

3:31

back in 2006, it's still kind of

3:34

controversial now, depending on who you

3:36

ask. Planetary sciences used

3:39

to be part of astronomy. Now,

3:42

there is some mixing, of course, but by

3:44

and large, we're talking about two different communities.

3:48

So the fate of a planet was

3:50

being decided by people who don't

3:52

study planets, mostly. And that is,

3:55

that was, and is, is still

3:57

one of the criticisms about

3:59

the vote. That

4:01

astronomers were voting

4:04

on the definition

4:06

of planet. And

4:09

who is that the planets are planetary

4:11

scientists. So today on

4:13

the show, the case for Pluto. What

4:16

it can tell us about how planets

4:18

were created and why are some planetary

4:21

scientists still Pluto defenders almost 20 years

4:23

later. I'm Regina Barber and

4:25

you're listening to shortwave from NPR.

4:28

Support for NPR and the following

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Society. Dr. Alpa Patel leads a

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voices of Black women study is that it

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Listen to Shark Week the podcast wherever

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you get your podcasts. Okay

5:38

Vlad, let's get into these discoveries that ultimately

5:41

led to the vote in 2006 to demote

5:44

Pluto. Like first of all there was Ceres

5:46

in the early 1800s and it

5:48

was born out of this search for a planet

5:51

in between Mars and Jupiter which scientists

5:53

kind of focused on because like the four

5:55

planets closest to the Sun are somewhat

5:57

evenly spaced out until you get to Mars and then

5:59

And there's this big gap between Mars

6:02

and Jupiter, where there's this asteroid

6:04

belt, where there's no planet. And

6:06

this perplexed scientist. So the

6:08

idea that there should be something there was

6:11

already in the mind of

6:14

astronomers. They found Ceres. It was inherited

6:16

as a planet. But

6:18

then they kept looking and kept finding more stuff. They

6:21

found more and more objects, nothing as big

6:24

as Ceres. But Ceres was sharing

6:26

its orbit with many other objects.

6:29

Right. And initially they were calling a

6:31

bunch of things planets. It seemed at

6:33

the time there wasn't really a super

6:35

clear distinction between planets and asteroids. Right.

6:38

So that led then to astronomers

6:41

to rethink what these

6:43

minor planets were. And then they were

6:46

called the asteroids. So I guess

6:48

there's precedent for Pluto being demoted.

6:51

As I said before, Pluto was

6:53

always the oddball, right? It was seen that

6:56

Pluto just didn't fit among the

6:58

eight planets. And that was partly

7:00

because of the dwarf planet Aries. Like it's in

7:03

the Kuiper belt with Pluto towards the end of

7:05

the solar system, right? Yes. When

7:07

Aries was found, there was

7:09

the catalyst to

7:12

declassify Pluto. Because all of a sudden

7:14

you found an object that

7:17

was more massive than Pluto.

7:19

Right. So either you also call that

7:21

object a planet or

7:23

you have to rethink the definition

7:25

of planet. Okay. So let's

7:27

talk about Pluto for a little bit. It was discovered by an

7:29

American in 1830, not even a hundred years ago. And

7:33

after that, I.A.U. vote, it's not even a

7:35

planet anymore. So why do you think Pluto

7:38

is so fascinating? If you think Pluto is

7:40

fascinating. Oh, I think Pluto

7:42

is absolutely fascinating for

7:44

sure. Pluto

7:47

holds many clues to understand how

7:49

planets in general form. So

7:53

as a scientist who

7:56

who studies how planets form, for

7:59

me, Pluto. Pluto is

8:01

a brick that helps me understand

8:04

the building. Yeah, no, that

8:06

totally makes sense. So let's just step back

8:08

for a second though and like think about

8:10

and talk about how do planets and dwarf

8:12

planets like Pluto form in the first place?

8:15

Right, so the way that planets form

8:18

is you start

8:20

from a cloud

8:22

of gas and dust. And

8:25

modern astronomy has found these

8:28

disks of gas orbiting young stars and

8:30

what we see is that once you

8:32

have dust in a disk of gas,

8:35

coagulation will take place. So you build larger

8:37

grains out of the dust that is in

8:39

this disk. A bit

8:41

like if you don't clean your room

8:43

often enough, you're gonna get dust bunnies,

8:46

right? Now imagine that you don't

8:48

clean your room for 10 million years. That's

8:50

how big those dust bunnies will get, right?

8:53

So with that- You're gonna take the whole room.

8:55

Yeah, so if that you form the first grains,

8:57

right? The first dust grains. And

9:00

then there are mechanisms happening in this

9:02

disk that help concentrate this dust grains

9:05

so that you catch an

9:07

object of the size of asteroids. We

9:10

call this what is planetesimos, which is a

9:13

portmanteau of a planet plus infinithesimo.

9:15

So a very small part of

9:17

a planet. So asteroids

9:19

and comets, they are the so-called

9:21

planetesimos or as I like to

9:24

call them the building blocks of

9:26

planets. Right? And then once you

9:29

build it that size, gravity comes into play

9:31

and then they can keep growing to the

9:33

size of the Earth. That's so cool. Or

9:35

bigger. And at some point it got so

9:37

massive that they attract gas from the disk

9:40

and then form a gas planet like Jupiter. Okay,

9:42

which brings us back to our friend Ceres,

9:45

right? The dwarf planet in the asteroid belt

9:47

between Jupiter and Mars, it's got all these

9:49

asteroids around it. Like how come they haven't

9:51

combined together and turned into a bigger planet? So

9:54

the asteroid belt did not become a big

9:56

planet because of the presence of Jupiter. Really?

9:59

Okay. part

14:00

of the definition, the clearing of

14:02

the orbit. And planetary scientists have

14:05

been advocating for a purely geophysical

14:07

definition. In this case Pluto is

14:10

a planet. The moon

14:12

is a planet,

14:14

right? And one of the

14:16

arguments that has been given by that is that,

14:19

oh, if we do that, then there's going to

14:21

be too many planets. How are

14:23

kids supposed to remember the name of all

14:25

of the planets if you have so many?

14:28

Like, can you tell me

14:31

the lineup of the

14:33

U.S. women's soccer team? No.

14:35

I don't know all names there. Yeah, I don't

14:38

either. But they're still pro soccer players.

14:40

Yeah. And so this

14:42

argument, I think, doesn't

14:44

hold much water. So classification

14:47

wise, I am completely comfortable calling

14:49

Pluto a planet. The

14:51

moon a planet. Europa, one

14:53

of Jupiter's moons, is a planet. Europa is a

14:55

planet, yes. If you put

14:57

Europa in an independent orbit around

14:59

the sun, you would call it

15:01

a planet. 100

15:03

percent. I would. Okay, well, thank

15:06

you so much, Vlad, for talking

15:08

to me today and making me

15:10

really excited about Pluto. My pleasure. Thank

15:12

you so much. Again, we'll

15:14

be back tomorrow with our regular

15:16

shortwave and Back Tuesday with our

15:18

next installment of the Space Camp

15:20

series. And I have

15:22

the sneak preview from one of our experts. Hey,

15:26

shortwave-ers, it's Dr. Serafina Nance, your

15:28

supernova guide back on Earth. I

15:31

hear you're getting to leave our solar system and

15:33

are on your way to visit stars other than

15:35

the sun. One of my

15:37

favorites is Betelgeuse, and I'm so jealous that

15:40

you might get to see it explode from

15:42

space. In case you didn't

15:44

know, Betelgeuse is a red supergiant fated

15:46

to explode any day now. Take

15:49

a selfie with Betelgeuse for me. Thanks and

15:51

have a blast. This

15:55

episode was produced by Hannah Chin, edited

15:57

by our showrunner Rebecca Ramirez, and fact-checked

15:59

by me.