"Pluto Huggers" Fight to Renew Planet Status

Anne Minard
for National Geographic News
August 15, 2008
Two years after Pluto was ousted from the planet lineup, some supporters of the now dwarf planet are fighting to restore its title. But others say it's time to move on.

The current 8-planet system versus the 13-planet system—which would include Pluto—was the subject of a boisterous debate Thursday at the "Great Planet Debate: Science as a Process" conference at the Johns Hopkins Applied Physics Laboratory near Columbia, Maryland.

The International Astronomical Union (IAU)'s new definition of a planet rejects some rightful planetary members, said Mark Sykes, director of the Tucson-based nonprofit Planetary Science Institute, one of the scientists who took to the stage Thursday.

"If you want to find a term that's going to touch on everybody, you do it broadly," he said.

But Neil deGrasse Tyson, an astrophysicist with the American Museum of Natural History and host of the PBS TV program NOVA Science Now, argues that the very word planet should be replaced with a wider term that captures the diversity of celestial objects that orbit stars.

"I'm saying define it however you want," he said, "then recognize how useless it is."

From the get-go, the debate was fast-paced, with few ground rules, frequent interruptions, and surges of laughter from the audience.

At one point, Tyson got so excited about how "Pluto huggers" have grasped at straws, he accidentally hit the moderator.

Goodbye, Planets?

The debate stems from a decision by the IAU in 2006 to revisit the definition of a planet, which hadn't been changed since it was coined in ancient Greece, according to Tyson.

The organization voted that a planet must have three characteristics: orbit the sun; be big enough that its gravity makes it round, or nearly so; and have cleared its path of debris by flinging the debris with its gravity or absorbing it.

The IAU made a new category for Pluto, its moon Charon, and the giant asteroid Ceres. Those were called dwarf planets—the only difference being that they had not cleared their orbits of debris.

Public Outcry

Besides a vocal public outcry about Pluto's demotion, scientists also lamented the decision based on both procedure and scientific integrity.

"A vote saying this is the definition we shall impose on everyone, that kind of gives a bad impression about science," Sykes said in a telephone interview before Thursday's debate.

"I think the IAU did harm to the public understanding of science."

Sykes says that if a nonstellar object is massive enough to be round and orbits a star, it ought to be a planet.

Tyson has less of a problem with the mechanics of the definition—it's the very word planet, which he said has "lost all scientific value."

He noted that a planet can be rocky or gaseous, among a number of other traits.

The word planet "had utility when there wasn't much to talk about when you talked about these wandering bodies. We are in desperate need for a lexicon to accommodate this new knowledge."

New System

Sykes and his camp would like to see a 13-planet system, which includes not only Pluto but its outer solar system companions Eris, Charon, and the newly named Makemake, formerly known as 2005 FY9 or Easterbunny.

Ceres, the largest known object in the asteroid belt between Mars and Jupiter, would be admitted as the solar system's smallest planet.

Tyson calls for an open-minded system of terms that will be applicable not only to our own solar system, but to the hundreds of so-called planets orbiting other stars.

Three terms: "terrestrials," "asteroids," and "jovials" for gas giants, would be a good start.

He added that he sees no cause for concern for schoolteachers, who must explain more complex categories of astronomical bodies in the classroom.

(Related: "Pluto's Demotion: What Will We Tell the Children?" [August 30, 2006].)

"The last time I saw a third-grader, they'd memorized far more dinosaurs than planets," he said. "Don't worry about that."

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