ACLogoDate: April 18, 2009


Title: Why Isn’t Pluto a Planet?

Podcaster: The Astronomy Cast team of Fraser Cain and Dr. Pamela Gay

Organization: Astronomy Cast
Description: This is our second contribution to 365 Days of Astronomy. This time, we’re going to give you a short explainer on why Pluto is no longer a planet. If you’re friends have cornered and asking you technical questions on why Pluto isn’t a planet, you can point them here and let me and Pamela set them straight.

Bio: Fraser Cain is the publisher of Universe Today and Dr. Pamela Gay is a professor at Southern Illinois University-Edwardsville. They team up to do Astronomy Cast, a weekly facts-based journey through the cosmos.

Today’s Sponsor: This episode of “365 Days of Astronomy” is sponsored by AAS.


Fraser Cain: This is your 365 Days of Astronomy for April 18, 2009. Hi, everyone. My name is Fraser Cain. I’m the publisher of Universe Today and with me is Dr. Pamela Gay, Professor at Southern Illinois University, Edwardsville. As you might know, Pamela and I do Astronomy Cast, the weekly facts-based journey through the cosmos – This is our second contribution to 365 Days of Astronomy and this time we’re going to give you a short explainer on why Pluto is no longer a planet. So, if your friends have you cornered and they’re asking you technical questions on what’s the story with Pluto, you can point them here and let me and Pamela set them straight.

Okay, Pamela, so just give the official answer – how many planets in the Solar System?

Pamela Gay: Eight.

Fraser Cain: Eight planets. So, just run out, grab your encyclopedias, grab your dictionaries, grab your kid’s books, scratch out nine, put in eight. Wherever it says the word planet behind Pluto, replace it with dwarf planet. Pluto is no longer a planet. Get over it.

Pamela Gay: It makes it all easier to remember. You only have eight things to remember now.

Fraser Cain: Yeah. So, why are there only eight planets? Why did Pluto lose its planethood?

Pamela Gay: Well, it was kind of this dilemma of either we were going to have several dozen planets or we were going to have eight planets and so they realized, well, and they being the International Astronomical Union, realized they should probably come up with a physics/astrophysics-based definition of what is a planet that went beyond the historical decision of Pluto as a planet and tackled things in terms of, well, what makes Mercury, Venus, Earth, Mars, Jupiter what they are, Saturn, Neptune, Uranus, and what is it that makes Pluto and Makemake and Sedna just something a little bit different? So, they came up with a fairly straightforward set of three different parts of the definition.

Fraser Cain: Right, and this was triggered by the discovery of Eris, right, which is an object bigger than Pluto orbiting further out than Pluto. So, if Pluto is a planet, then Eris should be a planet.

Pamela Gay: And this actually comes down to a very simple silly argument of they were trying to figure out should the planet-naming committee or should the small-body-naming committee get the right to name Eris? So, it all comes down to who does the committee work?

Fraser Cain: And whether it’s defined as a planet or not, right?

Pamela Gay: And so this is where they had to come up with the definition, which required even MORE committee work, and so the definition that came out of the committee starts with it has to orbit around the sun, so Titan has an atmosphere, has lots of planet-like characteristics but Titan is not a planet. Titan orbits Saturn instead of orbiting the sun, so kick it out of all potential of ever being a planet.

Fraser Cain: Right. So, three rules: Rule No. 1 – it must orbit the sun. Well, Pluto still fits the bill, right? It still orbits the sun.

Pamela Gay: Still doing pretty good.

Fraser Cain: Okay.

Pamela Gay: Now, the next one we have is the object has to be round. It technically has to assume a hydrostatic equilibrium shape that is a sphere. So, if you look at an asteroid, most asteroids are kind of potato-looking and this is because they have so little mass that the mass doesn’t pull itself together with so much gravitational force that it forces itself into a sphere. When you start getting the planets like Earth, you actually can’t build mountains much bigger than Everest because gravity prevents them from getting any taller. It just sort of crushes them down, makes them crumble, causes avalanches.

You can actually have bigger mountains on Mars, which is a smaller planet, than you can have on Earth and when you get to the really small stuff, you get potatoes. But Pluto is actually big enough that it is still a spheroidal shape.

Fraser Cain: Yeah, I mean, we haven’t seen Pluto up close yet. We will in a few more years with New Horizons but, yeah, I mean, all the pictures we have of Pluto so far, it’s a sphere. So, that’s two rules… check.

Pamela Gay: And so the next one here is the real kicker. The third one says it has to have cleared the neighborhood around its orbit and this is a kind of tricky one because we know that there is near-Earth objects. We know that there’s asteroids that, in fact, pass between the moon and the Earth on their orbit around the sun. So, it’s clear that the Earth didn’t completely clear its orbit but if you look around, there aren’t a bunch of objects that have orbits or almost identical to the planet Earth’s orbit. We’re pretty much the only thing in our orbit.

Now, Pluto is not quite so lucky. When you go out and you start looking at the band of space that passes inside and outside of Neptune’s orbit, you find a whole flock of what we call trans-Neptunian objects… objects that cross Neptune’s orbit. They’re small, they’re icy, they’re in some cases spheroidal, and in some cases they’re Pluto-sized and all these different objects basically form a band not all that different from the asteroid band.

Fraser Cain: Right, and we can give a bit of a comparison here. So, Earth has about a million times as much mass as everything else in its orbit combined while Pluto has less than one percent of all the mass in its orbit, so it’s the biggest out there, you know, in that orbit but it’s still just a fraction of the entire mass of all the objects that are out there. That’s the difference.

Pamela Gay: And so this is where we decide, okay, we’re going to kick out Pluto and we’re also not going to end up incorporating some of the other objects because the asteroid Sirius is big enough that it’s actually a spheroid too. So, had they decided to come up with a scientific definition that would have made Pluto a planet, we also would have ended up with Sirius being a planet. We would have ended up with Eris and Makemake being planets.

So, all these other objects would have grown the list causing small children everywhere to suddenly make up the longest sentences anyone has ever heard to try and remember the names of all these different objects.

Fraser Cain: Right, my very excellent mother would just go on and on and on.

Pamela Gay: Exactly.

Fraser Cain: Right, so they cut it at eight but now that doesn’t rule out the chances of there being other planets in the future, right?

Pamela Gay: No, and in fact this definition is going to have to get rewritten because that whole is in orbit around the sun part kind of means that your favorite extra solar planet is not, according to the International Astronomical Union, actually a planet.

Fraser Cain: Right, around its parent star.

Pamela Gay: Exactly. So, we’re going to have to rework this definition and when it gets reworked there is always the possibility that maybe Pluto will get a second chance. Now, honestly, I think that Pluto probably shouldn’t be classified the same way we classify Mercury. It is part of the belt the same way the asteroids are part of a belt, but there’s also the possibility we could still find a couple of really large objects out there that we haven’t even imagined yet… but probably not. 

Fraser Cain: Right. You could have another object very large, very far out that HAS cleared out the entire orbit but it would have to be something special.

Pamela Gay: Right, and right now we have no evidence that such a thing exists but you can never close all the doors on what the Universe is going to throw at you.

Fraser Cain: Now, astronomers did throw Pluto a bit of a bone giving it a sort of king of a new kind of object.

Pamela Gay: Yes. So, according to – they created this new class of object called the Plutoid on June 11, and their formal definition of a Plutoid reads that Plutoids are celestial bodies and orbit around the sun at a semi major access greater than that of Neptune that have sufficient mass for their self-gravity to overcome rigid body forces so that they assume a hydrostatic equilibrium near a spherical shape and that have not cleared the neighborhood around their orbit. Satellites of Plutoids are NOT Plutoids themselves. So, in English, what this says –

Fraser Cain: It’s Pluto.

Pamela Gay: It’s Pluto and it’s not Charon. In English what this means is anything that has the long part of its orbit, the furthest distance that it gets from the sun that’s bigger than Neptune’s for this distance from the sun and that’s spherical in shape is a Plutoid. So, we have this new class of object.

Fraser Cain: So, Eris and Pluto and Makemake and a few others out there.

Pamela Gay: Yeah, it’s a new set of objects to keep track of and, yeah, the IAU threw Pluto a bone.

Fraser Cain: Now what do you think are the chances of this ever being reversed?

Pamela Gay: I think it’s pretty low but I think it’s going to force us to rethink the physical definitions of planethood and that’s kind of cool. Anytime you force people to think hard, define things carefully, and be scientific instead of emotional, that’s when we make progress. 

Fraser Cain: All right. Well thanks, Pamela, and if you want to hear more from me and Pamela, you can go to our website at and we have probably close to 200 episodes, including a whole episode just on Pluto, a whole tour of the solar system, and many more episodes. That’s All right, well thanks, Pamela.

Pamela Gay: It’s been my pleasure, Fraser, and hope to see all of you over at

End of podcast:

365 Days of Astronomy
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