Television

Neil deGrasse Tyson On Why Pluto Is Transgressive And Nobody Hugs The Moon

Astrophysicist Neil deGrasse Tyson poses with a costumed Pluto the dog.

hide captionThe Pluto Files, premiering tonight on PBS, isn't about this Pluto. Well ... maybe a little.

Gene Duncan/Walt Disney World

Tonight on PBS (check your local listings, as they say) is the premiere of NOVA's The Pluto Files, the Neil deGrasse Tyson special about the massive controversy that erupted when Pluto was placed with other icy little chunks rather than with the set of planets it had been placed with before. It's lots of fun, great for kids, very entertaining, and ... well, it features Tyson, who, as we learned a few days ago, is pleased to be in your face.

This is the actual Pluto part of the conversation we had in January. Tomorrow, I will run the brief epilogue — a few comments he made about pop culture and how it fits into some of his bigger theories about teaching and learning. But for now: Pluto.

Linda Holmes: All right. Pluto. When you originally made the decision to put Pluto not with planets, but with ... chunks of ice?

Neil deGrasse Tyson: You got it.

LH: What goes into that other than — I saw the demonstration [in the special] relating to the size. Other than size, can you talk a little bit about Pluto's non-planet nature?

NdT: Yeah. Size, it turns out, is a weak argument, because Jupiter is "more bigger" compared with Earth than Earth is compared with Pluto. So if we were Jupiterians — or Jovians — we would have more argument to kick Earth out of the pantheon of planets than Earth has to kick Pluto out. If size is the criterion that you invoke. So that's not the criterion that dominated our decision. It was simply the fact that Pluto, over the years, has revealed itself to be peculiar in many ways. Yes, it's small — smaller than all the other planets. There are six moons in the solar system bigger than Pluto. And like I said, that's a lighter concern.

But let's get more serious. Pluto's orbit is so elongated that it crosses the orbit of another planet. Now that's ... you've got no business doing that if you want to call yourself a planet. Come on, now! There's something especially transgressive about that. Not only that — its orbit is tipped significantly out of the plane of the solar system. Seventeen degrees, if you're measuring.

LH: And are the rest of them relatively in line?

NdT: Yes, Mercury is like eight degrees out, that's the next one, but all the rest are just very close to what we call the plane of the solar system. Plus Pluto is quite icy — no other planet has that much ice. So its properties, if you put checks in the "oddball" category, there's a lot of checks in the "oddball" list. But we didn't know what else to do with it. You can't have a category of one, scientifically. It just doesn't work. So you're counting planets; you just include Pluto, because what else are you going to do with it?

LH: So you feel that Pluto's original planetary status was sort of a default setting.

NdT: Perfectly stated. No place else to put it, so of course, it was a planet. And in fact, at the time it was discovered, people were looking for Planet X. Planet X! Why [did they] think there's another planet out there? Because the movement of Neptune was not following the trajectory you would expect from Newton's laws of gravity.

LH: So they were thinking something else was affecting ...

NdT: Right, so either you throw away Newton's laws of gravity — but we weren't prepared to do that just yet, because Newton is my man — Newton is a significant contributor to our understanding of the universe. You'd need pretty serious data to start throwing Newton out and coming up with a new idea.

What happens after you decide not to dismiss Newton, after the jump.

So the first thought is: maybe there's an unseen planet whose gravity has not yet been accounted for in the movements of bodies in the outer solar system. So you start looking.

By the way, we're not dreaming this up — Neptune was discovered in just this way. The sequence of planets — Jupiter, Saturn, Uranus, Neptune — when Uranus was discovered, it wasn't following Newton's laws, people said maybe there's something else out there. Bada bing, you've got to do the math, when you do the math, you discover Neptune.

So we said — this is getting good to us. Neptune's got some funny business in its orbit, let's look for another planet. How massive would that planet have to be to perturb Neptune the way we see it? It's got to be something as massive as Neptune. More massive. So the day Pluto was discovered, it was assigned a mass commensurate with what we thought Planet X should have.

LH: Because of the effect they were assuming it was having.

NdT: Exactly.

LH: So its mass was based on behavior that they assumed it was exhibiting.

NdT: Precisely.

LH: And not because of measurable anything about the mass.

NdT: Exactly. Measurements would come later. And as we start measuring the thing, we find out, no, it's not quite as big as we thought, not quite as massive ... Pluto's size didn't settle out until the 1970s. It's far away and dim, so.

LH: How do you measure the size of a planet, incidentally?

NdT: In the early days, you know what you do? You say, "I think it's this big." And as we move around the sun and as the planets move around the sun themselves, every now and then, the planet will cross in front of a distant star.

LH: Of which you already know ...

NdT: Well, you know the exact place where that star is in the sky. So you say to yourself, "If it's this big, it will block out the light of that star. So what happens? Because you can't see the surface of Pluto. It's too far away; it's just a dot of light. So it goes by; the star doesn't change a lick in its brightness. So Pluto has to be smaller than what you thought it was. So every next star that you thought would get blocked by Pluto moving in front of it was not blocked. And that told people: we're dealing with something much smaller than anyone had ever bargained for.

Then in 1978, Pluto's moon, Charon, was discovered.

LH: Charon the Moon.

NdT: Charon the Moon. The name for the ferry boat driver that carried your unfortunate soul across the river Acheron into Hades.

LH: Well ... not MY unfortunate soul. The generic unfortunate soul.

NdT: Had you died, you would've met Charon, okay? And you'd have to give him a tip, too.

LH: Gotcha.

NdT: Otherwise, he'll leave your behind right where it was. That's how that works. I'm told.

LH: So, Pluto's moon Charon.

NdT: So we discover a moon and that allows you to do the gravitational dynamics and really nail the mass for Pluto, and so Pluto is something like ... I forgot the number, we can look it up, but it's about one-fifth the mass of Earth's moon. I mean, it's puny. And so then ... what of Planet X?

LH: What of Planet X?

NdT: I know! We left that back in Chapter Three. So it turns out Planet X, we would later learn, never existed. Because the data we had for Neptune was faulty. There was one observatory making measurements of Neptune that everybody assumed was accurate. If you removed that observatory and used everybody else's data? Neptune falls right on track.

LH: So among other things, the behavior on which the size of Pluto was originally estimated never happened.

NdT: Never happened.

LH: What a scam.

NdT: What a total ... we're all just misled. Well, this happens on the frontier of science. The public doesn't always see the fits and starts. They typically only read about the discoveries.

LH: So the way the special tells the story, when you place Pluto like this, you do not anticipate necessarily that it will be the gigantic brouhaha that it eventually was.

NdT: Because we didn't kick it out of the solar system! We didn't say, "There are now only eight planets." We were not confrontational about it. All we said, in fact — we went to the other eight and said, "How would we group them?" So we took the small rocky ones and put those together. Hey, you've got four big ones that are giant and gaseous — group them.

LH: So there was no single group of eight from which Pluto was being excluded.

NdT: Thank you! Don't get me started.

LH: What you really did was break the [existing nine planets] into three groups ...

NdT: She's getting me started. She's getting me.

LH: These four, these four, and this one.

NdT: And icy ones! That's right! We did not recount the planets in the solar system. We did not recount the planets at all. What we did with Pluto, we did with everybody else. We were not singling out Pluto in this scientific and pedagogical exercise of organizing the contents of the solar system.

LH: And then ... The New York Times.

NdT: Well, it took them a year.

LH: To notice where Pluto was. See? And if nobody notices where Pluto is for an entire year ...

NdT: Then how important would it have been? That's what we thought! Until they put it on page one. And even then, the reporter wasn't the one who discovered it. A reporter, off-duty, overheard somebody else trying to find Pluto. He looked, he couldn't find Pluto, he called the science reporter, he couldn't find Pluto, there it was, page one. The reporting day of Bush's inauguration, it shares page one with that story. "'New President Inaugurated For The United States Of America!' 'Pluto Not A Planet: Only In New York.'"

LH: [laughing]

NdT: Getting me started!

LH: So was there consideration of undoing this?

NdT: Nnnno!

LH: Or did you have an internal debate?

NdT: Nnnno! [he picks up my recorder and speaks as closely into it as possible] Nnnnnnnnnno! Because we thought about it. I wasn't autocratically deciding this; I had a committee. We would not have put Pluto out there were it not for the fact that other icy objects were being discovered in the outer solar system.

LH: So Pluto is not actually a category of one, it's a category with lots of other things.

NdT: Hence, it's got brethren.

LH: Brothers of Pluto.

NdT: It's got family. It's got a place to call home. These other objects are icy like Pluto. Small like Pluto. Cockamamie orbits like Pluto. They cross the orbit of Neptune like Pluto. Come on, now. Walks like a duck, talks like — it's a duck!

LH: So leaving it with the other planets would be like leaving it away from its family. Tearing it away from its family.

NdT: I think it's happier there. It's one of the kings of the comets out there.

LH: So what do you think is the basis for the sentimental attachment to the idea of Pluto as a planet? Why do people care?

NdT: I didn't know. Is it because it's the underdog? There are a lot of things where it's the underdog. I don't believe that, because Mercury's pretty small, and you don't hear anybody waxing poetic about Mercury. So I'm pretty sure it's Disney. I blame it all on Disney.

LH: Because of the dog.

Neil deGrasse Tyson helps Pluto look through a telescope.

hide captionThis cuddly dog (at right) is blamed by this astrophysicist (at left) for sentimental attachment to Pluto.

Gene Duncan/Walt Disney World

NdT: Yeah, and a point that I sometimes forget to make, but I will make here and now, is that Disney — the dog Pluto, Mickey's dog, was first sketched the same year Pluto was discovered. They have the same tenure in the hearts and minds of Americans. The same tenure. How can you shake that? That's an unshakeable correspondence.

LH: So basically, it's the Disney planet.

NdT: And how many cosmic objects do you look at in the night sky, would you learn about, that bring you warm, fuzzy, cuddly feelings? None, other than Pluto.

LH: Just Pluto. Aside from the moon, perhaps.

NdT: No!

LH: Well, with its reputation for being made of cheese and so forth.

NdT: Yeah, but that's not warm-fuzzy-cuddly. There's nothing cuddly about cheese.

LH: That is ... true.

NdT: Plus, kids don't like cheese! Unless it's Velveeta. American kids certainly don't. I'm talking about warm, fuzzy things that trigger emotions within you. Not that people don't have thoughts about what was on the moon. Yeah, made of cheese! Sure! That's an entertaining thought. Not a cuddly thought. Not a thought that distracts you during the day. Not a feel-good thought. That thought doesn't lick you in the face.

LH: Like Pluto.

NdT: The way a dimwitted bloodhound Mickey's-dog does.

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