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Scientists Reclassify Pluto as ‘Dwarf Planet’

August 24, 2006 at 6:45 PM EDT

RAY SUAREZ: Nine planets no more. An international meeting in the Czech capital, Prague, has redefined what had been the furthest heavenly body orbiting the sun. Alan Boss is an astrophysicist and planet formation theorist at the Carnegie Institution in Washington. I spoke to him earlier today about how we wound up with one fewer planet today than yesterday.

RAY SUAREZ: Alan Boss, welcome.

ALAN BOSS, Astrophysicist: Thank you.

RAY SUAREZ: Why isn’t Pluto recognized as a planet any longer?

ALAN BOSS: The reason is that Pluto is not the only major body in its area of the solar system. It’s actually one of a whole swarm of objects which have been the Kuiper Belt objects. And, in fact, recently Michael Brown of Caltech discovered another object in the same swarm, which has been called Xena as a temporary name.

And so we now realize that not only is Pluto not alone, there’s even another object out there which is even bigger than Pluto. And so, in some sense, it’s really just a member of a whole collection of objects rather than a splendid single isolation object like one of the major planets.

RAY SUAREZ: But objects that don’t cross a certain threshold of definition to be considered planets?

ALAN BOSS: Pluto is massive enough to have its figure pulled into a round shape because of gravity, and that has been proposed as a very basic criterion that any planet should hold. And that is part of the definition that was adopted today in Prague by the astronomers at the International Astronomical Union General Assembly.

So, yes, being large enough to be massive enough to have a round shape is a part of the planet definition. And so the eight major planets share all that, Mercury through Neptune.

But in addition, the IAU group today decided that there should be another criterion, as well, namely that you should be the biggest bad boy in your part of the neighborhood. And so certainly Earth, Venus, Jupiter, and Saturn, Uranus, Neptune, even Mercury and Mars meet that criterion, but Jupiter — sorry, but Pluto and Xena, the new object, do not.

They are, as I said at the beginning, just a member of a whole swarm of objects. It’s rather like the situation in the asteroid belt where Ceres is the largest asteroid. When Ceres was discovered back in 1800 or so, it was thought to be the missing planet between Mars and Jupiter. But then, a few years later, they discovered another asteroid called Vesta, and then another one called Pales, and then they realized, “Well, maybe we shouldn’t call Ceres a planet after all.” So they started calling those objects the asteroids or the minor planets, and we now know there are tens of thousands of those objects.

Astronomical debate

RAY SUAREZ: So now that Pluto has been reclassified, what do we call it now?

ALAN BOSS: Pluto has now been given an official name by the IAU. It is now a dwarf planet. If you want to say Pluto is a planet, you have to say, "But it is not a regular planet or a major planet or a regular planet, it's a dwarf planet." It's part of a new class of objects which are the objects which are massive enough to have a round shape, but are not massive enough that they dominate their entire neighborhood.

RAY SUAREZ: Well, you mentioned the IAU. Tell us a little bit more about how this process works. A group of scientists essentially made the decision for the rest of humanity that Pluto's not a planet.

ALAN BOSS: That's right. The IAU has been worrying about this, actually for several years. There was another group that was chartered a little over two years or so ago, and I was actually a part of that group. It was roughly a group of 19 scientists who debated this issue for over a year.

People have strongly held opinions about this, because there are good reasons to declare that Pluto is a planet, as well as good reasons to declare it as not a planet. It's not something which is absolutely black and white. It's a very grey decision.

And so this group of 19 astronomers argued for a year, essentially all through e-mail. I think by the end I had generated a stack of e-mail about two inches thick. And no one really changed their mind during that process.

There was one-third roughly that thought that Pluto should be a planet. As long as you're round, you're a planet. Roughly one-third thought you really had to be dynamically dominant, be the biggest guy in your area of the solar system. And one-third were sort of, you know, willing to settle on a compromise where we say Pluto is an historical planet and the big guys are the major planets.

But, you know, there was no consensus among those three possibilities of the end members (ph). And so the IAU decided to start all over again and create a new group. Just more like a half a year or so ago, they created a smaller group, not of 19 astronomers, but six, which then grew to become seven. And those seven met again in private and among themselves came up with something they thought would be a consensus definition, which was then presented to all of us for the first time last Wednesday in Prague.

RAY SUAREZ: Well, as I understand it, 2,500 astronomers and astrophysicists attended this conference, but only a very small number actually cast a vote on whether Pluto's in or out.

ALAN BOSS: That's true. I watched the proceedings today by streaming video. And I would estimate, based on the number of people in the room -- and I was in that room last week -- that there are roughly 400, maybe 450 or so people. I think that's consistent with the number of votes that were cast.

If you add up the pros and cons of the various votes, it's roughly something like 400, 420 people were there out of the 2,500. But that's normal for scientific conferences. It's a two-week-long meeting. Two weeks is a long period of time. It was natural for scientists to come and see some sessions but not stay for the bitter end.

Eternal review

RAY SUAREZ: Is it over, though? Once this decision is made by the IAU, can it be revisited?

ALAN BOSS: Certainly, I think these can be revisited. For this particular three-year period -- it will be three more years before the IAU has another General Assembly -- I think this decision will pretty much stick.

But I think it's possible, if someone really wanted to press the issue, to try and restart the process and argue otherwise. I understand, for example, that some of the astronomers who did not care for the decision that was reached today are beginning a petition drive to try and re-address this question. But whether or not that will gain traction, no one knows at this point.

RAY SUAREZ: Well, 76 years since Pluto was defined as a planet.

ALAN BOSS: Indeed.

RAY SUAREZ: Why is it important for science to revisit things like this, come up with new definitions for old ideas?

ALAN BOSS: It's really a very positive situation, I think, that we're having this debate, because it does let the general public know that science keeps marching on. We cannot take a stone tablet and engrave on it what a planet is and expect that definition still to hold a thousand years from now, much less 10 years from now.

We've learned so much about what is occupying the outer regions of our solar system that it just simply made sense, given what we know now and especially this Michael Brown discovery of the Xena object. It just made sense.

Now was the time to address this issue, and get it out, and get a definition which is scientifically defensible, and make it clear that, at least from the scientific perspective, that Pluto should not be called a plain, old planet. It's a dwarf planet. It doesn't really address the cultural aspects, but it does, I think, address the scientific aspects quite well.

RAY SUAREZ: Well, now we're down to eight, but at the same time there are all these wonderful new tools for seeing further and further out into space. Could we be adding to the eight and coming up with ninth and tenth planets eventually?

ALAN BOSS: I think it would be rather hard for us to find another object on the edge of the solar system, which is the only region that has not been explored yet, which meets the criterion of being the biggest guy in the neighborhood. We'd have to find something which is considerably larger than either Pluto or Xena by probably a factor of 10 or something in mass.

And if such an object existed, it probably would have been seen already. It's not to say that it's impossible, but I think it's unlikely. But we will find many more objects which are similar, perhaps in mass, to Pluto and Xena. And so there will be many more dwarf planets.

I understand Michael Brown has maybe a dozen or so sitting in the back of his computer waiting to be announced. And he probably -- he and other astronomers will find even more of these objects. So the number of objects in the dwarf planet category is bound to increase by a large number.

RAY SUAREZ: Well, Alan Boss, I guess I can be one of the last people to say it, "My very eager mother just sent us nine pizzas," no more pizzas any more.

Now we don't have to use that to remember the nine planets. Thanks for being with us.

ALAN BOSS: Thank you, Ray.