Astrophysicist: Pluto's Planet Demotion Justified

In August 2006 the International Astronomical Union (IAU) demoted Pluto's status as a planet. Public protest flared over the landslide vote by astronomers, and so-called "Pluto Huggers" have been pushing ever since to reinvoke Pluto's planetary status.

Neil DeGrasse Tyson, an astrophysicist at the American Museum of Natural History in New York City, openly led the movement to rethink how stuff in the Solar System is classified. At an event deemed the "Great Planet Debate" at Johns Hopkins University, Tyson is to face off over the issue with Mark Sykes, director of the Planetary Science Institute.

The hope? To settle the planet controversy once and for all.

Dave Mosher: What the heck is going on with our Solar System?

Neil DeGrasse Tyson: The Solar System could care less what we think about it, first of all, let's get that straight. Second, the history of science shows that anytime you have a new wave of data that descends on your previous understanding of how things work, either the data fit right in and you go on about your business. When they don't, it forces a reassessment of how you used to think.

You ask, "what's going on in the Solar System?" What's going on in the Solar System is we're discovering objects in orbit around the sun beyond Neptune; Pluto is not alone out there. We're discovering ice on Mars long suspected of being there, but now we've ... got it in the laboratory that landed on Mars. ... We've found countless thousands of asteroids with orbits that cross the orbit of Earth, so now Earth is in a shooting gallery -- something we never knew could have been the case and now is...

Jupiter's moon's Europa: all evidence shows that it's got liquid water thriving beneath the frozen surface. ... It's been liquid for a billion years.

DM: And you've got Enceladus too...

NT: Yes -- you've got Enceladus with geysers, and you've got Titan with a possibly prebiotic, nascent Earth. You've got the Kuiper Belt of comets in the outer Solar System. So the Solar System is a much more interesting place.

I think we should celebrate that fact rather than somehow be concerned or complain about it.

DM: So you're attending the "Great Planet Debate" this week in Laurel, Maryland. What are the stakes? What does everyone hope happens by the end of this conference?

NT: People view me as some kind of ... gladiator combatant -- and someone had the audacity had to title it the "great" planet debate. I'm not responsible for that.

I think people expect that all of the dirty laundry will be hung out. You know, both Mark Sykes and I are very informed about what's out there, so the audience ... will get -- I think -- a very good overview of the relevant and important facts about the Solar System, old and new. And it'll be my duty and his duty to convince the audience of where the scientific community should take its next steps.

My sense is that the concept of "planet" is simply outdated. I and my institution ... have been caricatured, really, as kicking Pluto out of the Solar System. And that's not really what we did. All we did was organize the contents of the Solar System in a way that, to us, made scientific and pedagogical sense. And that meant no longer counting or enumerating planets.

It's not an interesting scientific exercise given what else you can talk about regarding these objects. There was a day when you didn't know much about the planets -- just that they were points of light and the sky and moved in funny ways.

DM: Part of the celestial map.

NT: Yeah, so there they were and they move against the background stars. And so, sure let's count them, let's number, them, let's name them, let's memorize them. What the heck else is there to talk about -- that's all they are. Oh, then we've got telescopes! Oh, they've got moons, they've got rings -- and the moons aren't just dots of light, we took our space probes to determine this -- the moons are more interesting than the planets themselves. The Solar System becomes exponentially more interesting than had ever been previously been imagined.

Look at early text books, the size that they give for Pluto: it's huge! Then over the decades as we got better and more data, the size of Pluto got smaller and smaller and smaller, and it finally settled out in the 1970s to something even smaller than our moon. So once you realize this, the concept of "planet" -- whatever was your definition to begin with -- really ought to be revisited.

If the purpose of a classification scheme is to group objects of like properties, then when you have the likes of Pluto -- smaller than our moon, icy -- having the same classification as Jupiter, which is more massive than all of the planets combined and it's gaseous, then what insight do you glean by the term ["planet"] at all, if that term includes both such objects?

So all we did was group objects by like properties: we took the terrestrial planets -- Mercury, Venus, Earth and Mars -- and called them a family. They're all small, rocky, near the Sun. The Asteroid Belt: craggy chunks of rock, that's a family. The gas giants -- Jupiter, Saturn, Uranus and Neptune. And then we have the Kuiper Belt of icy bodies, including Pluto.

We grouped Pluto with this newly discovered class of objects. ... If you look at any of these groups, these families, objects within each family have more in common with each other than any one of them have with anything else in the Solar System.

DM: Including planets?

NT: No. I'm saying remove the word "planet" altogether and just look at them as families. You have the terrestrial family, the Asteroid Belt, the gas giants, the Kuiper Belt, the Oort Cloud. Good... we're out of the starting blocks. We've got the beginnings of something, where a classification scheme is now useful. Not only that, we're discovering planets in orbit around other stars.

So what an opportunity it is to broaden the classification scheme -- don't come up with a classification scheme that only works for our own Solar System. What kind of hubris is that? Let's back up, look at the now 300 planets that we now know about and wonder whether there's some deeper way to group objects that orbit a host star than an enumeration of spherical objects from closest to farthest.

DM: And get away from Earth-centric terminology?

NT: Yeah... the problem when Copernicus came along: Is Earth a planet? Well, after Copernicus, the whole concept of planet had to be thrown out the window, but it wasn't -- we kept it. So we kept this outdated term and sort of grandfathered in Pluto... But people have known since the '70s, when the size of Pluto finally settled, that Pluto was kind of a weird beast. And now you look at other Kuiper Belt objects that are as weird as Pluto...

DM: Some people say "who cares, it's just a term!" What's your response?