Do Scientists think there's an undiscovered Planet X?

Summary

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Who Smarted? – Do Scientists Think There’s an Undiscovered Planet X?

Podcast: Who Smarted?
Episode Date: December 8, 2025
Main Theme: The search for an undiscovered ninth planet ("Planet X") in our solar system and the science, history, and imagination wrapped around the mystery.
Host: "Trusty Narrator" (A)
Featuring: Personified Pluto (B), mysterious Planet X (C)

Episode Overview

This episode takes listeners on a hilarious, kid-friendly journey beyond Neptune into the outer edges of the solar system, all in search of the elusive "Planet X"—a world that astronomers suspect might exist but have never seen. With a blend of playful banter between the Narrator and Pluto, fun analogies (like M&Ms for distances), and interactive questions, the episode explores the science behind the search, the evidence for its existence, and the reasons it’s so hard to find (plus, a twist: could it even be a black hole?).

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Key Discussion Points and Insights

1. Setting the Scene: Pluto’s Place

• Pluto’s Feelings: The episode opens with Pluto feeling left out after being demoted from planet status.
  • Quote:
    • "You'll always be the ninth planet in my heart." – A (02:22)
• Planetary Classification: The Narrator explains that while Pluto is a dwarf planet, astronomers are searching for a true ninth planet.

2. Distance in the Solar System

• M&M Demonstration: To help kids understand astronomical distances, the podcast uses M&Ms as a fun analogy:
  • 1 M&M = distance from Earth to Sun (1 Astronomical Unit, or AU)
  • To Pluto: 39 M&Ms (39 AU)
  • To Planet X: Could be as far as 600–1,200 AU!
    • Quote:
      • "We're gonna need many bags. Astronomers believe this ninth planet could be 600 to 1200 AU from the sun..." – A (06:32)

3. Why Do Scientists Think Planet X Exists?

• Orbit Oddities: The strange orbits of objects in the Kuiper Belt (like Sedna) suggest that something massive—possibly a new planet—is tugging on them.
  • Quote:
    • "Sedna has an unusual orbit...it’s elongated, stretched out, as if it was yanked by the gravity of a distant object." – A (07:36)
• Evidence for Planet X: At least six other objects share this odd, tilted trajectory, making random chance unlikely.
• Caltech Astronomers' Theory: They suggest an undiscovered planet about 10 times Earth's mass is shaping these orbits.

4. Meet "Planet X" – The Mystery Continues

• What Does 'X' Mean?: In "Planet X," the X stands for something unknown, not 'extra special' or 'X marks the spot.'
• Possible Attributes:
  • Size: Maybe like Uranus or Neptune
  • Mass: About 10x Earth's mass
  • Orbit: Takes 10,000–20,000 years to go around the Sun!
    • Quote:
      • "The answer according to the Caltech astronomers is somewhere between 10,000 and 20,000 years." – A (11:12)

5. If It Exists, Why Haven’t We Found It?

• Visibility Challenge: So distant and faint it can only be seen by the most powerful telescopes.
• "Planet X" is just a placeholder name:
  • Historically, any proposed planet beyond Neptune has been called "Planet X"—even Pluto was once "Planet X."
    • Quote:
      • "You were once Planet X." – A to Pluto (14:17)

6. What If Planet X Isn’t a Planet?

• Alternative Theory: Black Hole?
  • Some scientists speculate that the gravitational influence might come from a tiny black hole, not a planet.
  • If so, "Planet X" could be the size of an orange—with 10x Earth's mass!
    • Quote:
      • "How small would this Planet X be if it's a black hole?...The answer...is C, the size of an orange." – A (15:26)
• Impossible to Find? An object so small and dark would be almost invisible even to strong telescopes.

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Notable Quotes & Memorable Moments

• Pluto’s Comic Relief:

  • "So now a mysterious unknown object that might not even be a planet gets to be called Planet X. How is that fair?" – Pluto (13:34)
  • "We're gonna need a bigger bag of M&Ms." – Pluto (06:27)

• Imagination and Encouragement:

  • "Who knows? Maybe a smarty pants listening right now will be the one to find the mysterious Planet X." – A (15:54)

• Mind-Blowing Fact:

  • "That means whenever planet X is found, it will be in a location it hasn’t been in since well before ancient times." – A (11:12)

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Important Timestamps

• [01:24] – Episode theme introduction: searching for a new planet beyond Pluto.
• [05:01] – M&M space distance analogy begins.
• [06:27] – Distances to Pluto & Planet X in AUs.
• [07:36] – Sedna’s orbit and clues about Planet X.
• [08:21] – Caltech astronomers' findings on Planet X's possible mass & location.
• [10:05] – Twin planets Uranus and Neptune; Planet X’s possible size.
• [11:12] – Orbit time for Planet X: 10,000–20,000 years!
• [13:34] – Pluto expresses frustration at "Planet X" naming.
• [15:06] – Black hole theory for Planet X.
• [15:54] – Episode wrap and encouragement for curious listeners.

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Summary Table

| Timestamp | Segment/Topic | Highlights | |-----------|------------------------------------|--------------------------------------------------------------------| | 01:24 | Introduction to search for Planet X | Pluto joins the adventure; clue setup | | 05:01 | M&M Analogy | Visualizing AU and solar system distances | | 06:27 | Distance Estimates | How far out Planet X could be (600-1200 AU!) | | 07:36 | Sedna and Orbits | Clues from Kuiper Belt; orbits suggest influence of a big object | | 08:21 | Scientist Theories | Planet X might be 10x Earth’s mass | | 10:05 | Twin Gas Giants | Uranus/Neptune comparison | | 11:12 | Orbit Length | Planet X's year = 10,000–20,000 Earth years | | 13:34 | History of Planet X as a Concept | Pluto’s role; the meaning of “Planet X” | | 15:06 | Alternative: Black Hole | Could it be a tiny black hole? | | 15:54 | Ending/Encouragement | Listeners motivated to keep wondering and exploring |

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Tone and Style

• Engaging, playful, and humorous, using banter, puns, and silly voices to make complex science accessible and memorable.
• Interactive—asks listeners questions throughout.
• Encouraging curiosity—invites kids to dream about making discoveries themselves.

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Episode Takeaways

• Scientists believe there may be a large, distant planet—Planet X—based on how far-off objects move in space.
• If it exists, it could be ten times heavier than Earth, orbiting absurdly far out, and might take 20,000 years to circle the Sun.
• It hasn’t been seen yet because it is SO far away and faint.
• There’s even a possibility it’s not a planet at all—but a tiny, massive black hole!
• Any unknown world past Neptune gets called “Planet X” until it gets found and officially named.
• Listeners are encouraged to keep being curious—maybe THEY will discover Planet X one day!

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Transcript

[A]

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And now it's time for who's Smarted? Psst. Hey there, smarty pants. Welcome to another adventure in the far reaches of the solar system. Right. Hello. I'm coming to you from the USS Smartypants, boldly going beyond the farthest known planet circling the sun in search of me. No, Pluto, not you. We're looking for something nobody has ever seen before.

Ever since Neptune was first discovered in 1846, astronomers have been searching for another world beyond the ninth planet of the solar system.

[B]

But that's me. I'm the ninth planet.

[A]

Well, you know, Pluto, we did do a whole who Smarted? Episode on this. You're now known as a dwarf planet. And you aren't included with the major solar system planets.

[B]

But.

[A]

But, But. Yes.

[B]

I thought maybe you changed your mind.

[A]

Sorry, Pluto. Besides, I'm not the one calling the shots here. You'll always be the ninth planet in my heart. Aw. But most astronomers say Planet nine has yet to be found. If it exists at all.

[B]

But I do exist. The New Horizons probe stopped by for a visit. There are lots of pictures of me.

[A]

I know. Once again, you're not a planet. You're a dwarf planet.

[B]

And dwarf planets don't matter much of course they do.

[A]

In fact, it's the behavior of a dwarf planet that has astronomers on a new search to. To find a world beyond Neptune.

And that's what we're exploring today, smartypants. Oh.

[B]

So I'm helping astronomers find another planet. That makes me feel a lot better.

[A]

Hmph.

[B]

They thought they could just push little old Pluto aside. Well, it looks like this dwarf planet's about to get some major solar crit. How do you like them apples?

[A]

Um, I don't know how to say this without upsetting you, Pluto, but it wasn't you I was talking about. When I said a dwarf planet was helping, I meant someone else. Actually, it's a bunch of objects in the far end of the solar system known as the Kuiper Belt. And all of them are acting kind of funny. But these objects.

Are not you.

[B]

Oh.

[A]

But maybe you can help me find it today.

[B]

Really? You mean we get to go on an adventure?

[A]

Sure, but we'll have to travel far.

[B]

How far?

[A]

That's a question for the smarty pants. Where is the mysterious ninth planet? Does it really exist? How big might it be? And why is it so hard to find? It's time for another whiff of science and history. On who smarted.

Who's smarted?

Who's smart? Is it you? Is it me? Is it science? Or history? Listen up, everyone. We make smarting lots of fun. On who's Smarted.

Hey, smartypants, I'm so glad you've beamed aboard. We're now traveling billions of kilometers to find the mysterious ninth planet of the solar system. Along for the ride is our good friend, the formerly known as planet Pluto.

[B]

Hi, trusty. Are you sure it's okay that I come with you?

[A]

Well, we're probably breaking numerous laws of physics by having a dwarf planet travel with us as small as you might be, but since this is a podcast, let's just go with it.

[B]

Great.

So just how far are we going?

[A]

Good question. Let's think of it in terms of M and M candies. Smarty pants. Imagine one M&M is equal to the distance from the sun to the Earth, with the sun at one edge and the Earth on the opposite side. In scientific terms, this is called 1 Astronomical Unit, or AU.

Now, how many MMS would you have to place side by side in a line to represent the distance from the sun to Pluto? 20, 39 or 57. Again, each mm represents the distance from the Earth to the Sun.

[B]

Did you bring any M&Ms.

[A]

For me? Uh, no. These are not for snacking. They're for demonstrating planet distances.

[B]

Oh, I see. You'd rather use your delicious chocolate candy on a scientific demo than share some with your old pal, sad little lonely planet.

[A]

Dwarf planet.

[B]

Right, Dwarf planet.

[A]

Okay, you can have some MMs. Thanks.

Back to our demo, Smarty Pants. If 1 M&M is equal to 1 AU, the distance from the sun to the earth, you'd need 39 of them in a straight line to illustrate the distance from the sun to Pluto. That's 39 AU. Next question. How far away do astronomers think this mysterious ninth planet might be? 50 AU, 100 AU, or 200 AU?

[B]

200 AU. We're gonna need a bigger bag of M&Ms.

[A]

Actually, we're gonna need many bags. Astronomers believe this ninth planet could be 600 to 1200 AU from the sun beyond the icy Kuiper Belt where Pluto lives. Its great distance from the sun makes it very hard to find.

[B]

So why do astronomers think this planet even exists?

[A]

Oh, because of a dwarf planet called Sedna.

[B]

Hold on, hold on.

Gotcha there, Captain. Narrator In a dwarf planets episode, you said there are only five dwarf planets, and Sedna ain't one of them.

[A]

Smartypants. What do you think? True or false? There are only five official dwarf planets. The answer is true. Aha. But, but, but those are the dwarf planets officially named by the International Astronomical Union. You might remember that astronomers have discovered more than 700 objects that could be dwarf planets. And one of those is nicknamed Sedna. Oh.

[B]

I'm fine. So what's so great about Sedna anyway?

[A]

Oh, Sedna has an unusual orbit that takes it far, far away from the Sun. Instead of being circular like other Kuiper Belt objects, it's elongated, stretched out, as if it was yanked by the gravity of a distant object.

[B]

The ninth planet.

[A]

Maybe objects do get knocked around in space. And at first, Sedna's orbit seemed like a random occurrence. But it's not alone. Astronomers have found at least six other Kuiper Belt objects with similar orbits, tilted the same way, being pulled in the same direction as sedna.

The chances of this happening randomly are incredibly small. After studying these objects, two astronomers from the California Institute of Technology.

Concluded that a ninth planet might be affecting their orbits. They say this planet could be located about 600 astronomical units from the sun, which, according to my made up calculations, is right where we are right now.

[C]

Hello.

[A]

Bwah. Are.

[B]

Are you the ninth planet of the solar system?

[C]

I might be. The name's X. Planet X. X.

[B]

But isn't X the roman numeral for 10?

[C]

Why yes, it is.

[B]

Wait, could that mean I'm still number nine and you're number ten?

[A]

Am I a planet again? No, Pluto. Sorry, I don't think that's the case.

[B]

Okay, I guess lil old me will just have to settle for more M&Ms.

[A]

Smarty Pants, why would the mysterious ninth planet be called planet X? Does X stand for extra special? X marks the spot or something unknown?

Well, you can X out the first two answers because X means something unknown.

[C]

And there's a lot unknown about me. I like to remain mysterious.

[B]

Is that why you're wearing sunglasses, a ski mask and a giant coat?

[C]

Maybe. It's also really cold out here.

[A]

Either way, the Caltech astronomers have been unlocking the mystery.

[C]

Really? What do they know?

[A]

They believe planet X has 10 times the mass of Earth and is about the size of one of the two so called twin gas giants.

[B]

Sorry, M and Ms. Give me gas too.

[A]

Right, smarty pants? Do you remember what two gas planets are called? Twins. Because of their similar size and color. Is it A, Jupiter and Saturn, B, Uranus and Neptune, or C, Earth and Venus? The answer is B, Uranus and Neptune.

[C]

Maybe I'm their long lost triplet.

[A]

You're certainly a long distance from them.

[B]

Yeah. How long does it take you to orbit the Sun? From way out here? It takes me about 248 Earth years longer than any of the so called major planets.

[C]

After 248 Earth years, my voyage around the sun is just beginning.

[A]

Smarty pants, how many years would it take for planet X to orbit the sun? Just shout out your answers. Five hundred? Nope. A thousand? Not even close. A million? Okay, that's too long. The answer according to the Caltech astronomers is somewhere between 10,000 and 20,000 years. That means whenever planet X is found, it will be in a location it hasn't been in since well before ancient times.

[C]

That is if you ever find me.

[A]

Oh. With top scientists using math and science to calculate your location and with the Earth's most powerful telescope searching, I think if there's another planet out there, we'll eventually find it.

[C]

But what if I'm not a planet?

[B]

Not a planet.

[A]

Whoa, smarty pants. If planet X isn't a planet, then what could it be? We'll explore the answer right after this quick break.

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Now back to who's smarty.

So smarty pants, if you're as far out in space as we are, you're gonna be pretty hard to find from Earth. Planet X would be very, very, very faint, with only the most powerful telescopes on the planet able to see it. And with space being as massive as it is, finding a planet that looks like a faint, tiny speck requires a lot of scientific knowledge, mathematics and a bit of luck.

[C]

That is, if I'm a planet at all.

[A]

Oh, that's right. Some say Planet X might not be a planet.

[B]

Wait, how can Planet X not be a planet? Think of it, lil old me, whom you've all seen and love is kicked out of the planet club, while some mysterious unknown object that might not even be a planet gets to be called Planet X. How is that fair?

[A]

Oh, Pluto. Planet X isn't a permanent name. For decades, astronomers have referred to any unknown planet beyond Neptune as Planet X. You were once Planet X. I was? Yup. But so far, all the discoveries thought to be Planet X turn out to not be planets at all. And sadly, that includes you too, Pluto.

Oh, it's not that bad. And if this Planet X isn't a planet, it won't be called Planet X anymore.

[C]

You'll have to find me to find out.

[B]

Okay, but if you're not a planet.

[A]

What might you be, smarty pants? Any guesses? It can't be a star because we'd be able to easily see a bright star. So what other dark object in space can have the mass of a planet and A strong gravitational pull. Well, very astute. Smartypants might remember we did a who Smarted episode on just such an object. Did somebody say a black hole? That's right. And as you may remember, a black hole has extremely condensed matter. So one with just 10 times the Earth's mass, as Planet X supposedly is, would be very, very small.

[B]

Smaller than me?

[A]

Oh, much smaller. Smarty pants, how small would this Planet X be if it's a black hole, A, the size of the moon, B the size of Australia, or C, the size of an orange? The answer, believe it or not, is C the size of an orange.

[B]

Wait, how can we find a Planet X in this ginormous universe if it's a dark black hole the size of an orange?

[A]

Oh, it'd be nearly impossible. So if we want to find it, we better hope it's a planet.

[B]

Well, while we're here, let's just ask a Planet X. Are you a planet or a black hole? Planet X. Uh, where'd it go?

[A]

It looks like the mysterious Planet X has vanished.

[B]

Aw, so now what?

[A]

Well, the astronomers at Caltech, hoping to find Planet X, have made their research known so that others might join their search. Who knows? Maybe a smarty pants listening right now will be the one to find the mysterious Planet X.

[B]

Oh, and smarty pants, if you get the chance, can you please make me a planet again?

Thanks.

[A]

An out of this World shout out to Toby in Montreal, Canada. Your mom said she gets a nice break while you listen to three or four episodes of who's Smarted in a row. And I hear you love telling your parents what you learned and that the Scorpions episode is your favorite. You're my kind of smarty pants, buddy. Thanks. This episode, Planet X, was written by Dave Blackhole Beaudry and voiced by Teya Galactic Garland, Chris Ortcloud Okawa and Jerry Colbert. Technical direction and sound design by Josh Hubbell Hahn, who Smarted is recorded and mixed at the Relic Room Studios. Our associate producer is Max Cosmos Kamasky. The theme song is by Brian Singularity Suarez, with lyrics written and performed by Adam Dark Matter Davis, who Smarted was created and produced by Adam Tex Davis and Jerry Colbert. This has been an Atomic Entertainment production.