Probing Question: Why did Pluto get demoted?

pluto and earth
NASA

Pluto's volume is about 0.66 percent that of Earth's.

Word came down in stages this summer. First, the rumor that our long-exclusive planetary club was ready to expand—not only renewing Pluto's status, but adding three new members in the bargain. The world's astronomers seemed to be sending a message of inclusion: There's always room for one more.

Scratch that, they told us two weeks later. Never mind Charon and Ceres and Eris. And while you're at it, forget Pluto too. Our solar system, from henceforth, would include only eight full-fledged planets, not the nine we had all memorized in elementary school.

What happened? Why the dramatic reversal? Why did poor Pluto have to get the ax? Last month, as part of the 2006 Friedman Lecture Series in Astronomy, a panel of Penn State experts convened to take up these questions.

Truth is, said Darren Williams, associate professor of astronomy and astrophysics at Penn State Erie, Pluto was always a misfit—too small, too distant, and too eccentric in its orbit to be anything but an exception. Discovered almost by accident in 1930, the little anomaly was basically grandfathered in. Which was okay until the early 1990s, when advances in digital imagery allowed astronomers to see the first planetoid objects in the Kuiper Belt, a region of solar system out beyond Neptune's orbit.

Since 1992, over 800 of these objects have been noticed in the vicinity, each new discovery casting doubt on Pluto's privileged status. The last straw came in 2005, with the discovery of Eris, a Kuiper Belt object that is actually larger than old Number Nine.

Clearly, something had to be done. So, in August 2006, in Prague, the General Assembly of the International Astronomy Union (IAU) reconsidered Pluto's fate.

The process was anything but smooth, according to Mercedes Richards, professor of astonomy and astrophysics, who attended the meeting. A planet definition committee had been secretly deliberating since June. On the Assembly's first day, as astronomers gathered from 63 countries, the committee's report was leaked to the media. As a result, "There was a major uproar" among the rank-and-file members, Richards remembered. "As the kids would say, we had a hissy fit."

The committee's initial proposal was simple: A planet was any object orbiting a star that was large enough for its own gravity to make it "nearly round" in shape. (Nobody's perfect.) Astronomy graduate student Avi Mandell reviewed the advantages of this approach: "It makes it easy to assess a given object, just by looking at it," he said. "And it allows us to keep Pluto."

The disadvantages: Though the planets immediately added by this definition would be only three, there were 12 more likely candidates waiting in the wings. In fact, over 50 known objects might fit this definition. It would have opened the floodgates.

For the next week and a half, the issue was hotly debated, until finally a third criterion was agreed on. A planet, in addition to orbiting the Sun and being large enough to be round, has to have "cleared the neighborhood around its orbit," in the IAU's wording. As Mandell explained, "planets suck up most of the surrounding mass during their formation, leaving lots of little objects behind. This remaining stuff is then scattered" by the force of the planet's gravity, "and the orbit slowly gets cleared of debris."

"Since Pluto is found with hundreds of other objects in the Kuiper Belt, it clearly hadn't cleared its neighborhood," said Richards. So when the new definition passed overwhelmingly on the assembly's final day, it was a done deal. Pluto was a journeyman pitcher sent down to the minors: a dwarf planet.

The decision was controversial, to be sure. Even sad, Mandell allowed. But "It provides a natural dividing line," he said. "On one side are the planets, and on the other are the asteroids, dwarf planets, and Kuiper Belt objects. There's nothing in between."

Sentiment aside, Richards added, "There's no going back for Pluto. We're discovering more and more of these objects out in the Kuiper Belt every day."

To her, the change in Pluto's status—and the discussion that surrounds it—is cause for celebration. "It shows that astronomy is alive, and our solar system is alive, and the future is wide open."

"What is a Planet? Why not Pluto?," an event in the 2006 Friedman Lecture Series in Astronomy at Penn State, took place at University Park on November 7. Panel members from the astronomy and astrophysics department in the Eberly College of Science included assistant professor Kevin Luhman, Ph.D.; doctoral student Avi Mandell, B.A.; lecturer and outreach fellow Chris Palma, Ph.D.; professor Mercedes Richards, Ph.D., mtr@astro.psu.edu; and from Penn State Erie, associate professor Darren Williams, Ph.D., dmw145@psu.edu.

Last Updated December 11, 2006