The Great (and Sometimes Serious) Debate About Pluto
The artist's concept gives a view of the Pluto system from the surface of Nix or Hydra, two of its moons discovered in 2005. Nix and Hydra are two to three times farther from Pluto than its large moon, Charon (to the right of Pluto), which was discovered in 1978.
Credit: NASA, ESA and G. Bacon (STScI).

LAUREL, MD — The entrance to the debate over Pluto's planet status said it all: With techno music blaring in the background, the two debaters and a moderator walked into the auditorium, cameras flashing and the audience clapping.

One debater, Neil deGrasse Tyson, did the boxing entrance à la Rocky. That's how hot the matchup is between Pluto as a planet and Pluto as a plutoid.

Tyson, director of the American Museum of Natural History's Hayden Planetarium in New York, supports the demotion of Pluto. In the other corner, Mark Sykes, director of the Planetary Science Institute in Tuscon, Ariz., does not agree with the recent ruling that essentially booted Pluto from the planet lineup.

The debate over whether Pluto should be considered a planet is part of "The Great Planet Debate: Science as Process" conference here at the Johns Hopkins University Applied Physics Laboratory (APL) that runs through Saturday.

Before the idea-throwing began, debate moderator Ira Flatow of National Public Radio's Science Friday threw out his own rule, "No throwing of perishable items or missiles of any kind at the stage," Flatow said with a smirk.

In fact, the debate was filled with lots of applause, laughter and some snide remarks, but mostly it was a friendly tussle. In fact, neither Tyson nor Sykes clearly defined their specific positions on Pluto and the definition of a planet

How many planets?

In general, Tyson said astronomers need to come up with an entirely new lexicon to group planets and planet-like objects together. He also said Pluto is not like the other eight major planets in the solar system and that it instead fits into the Kuiper Belt, a vast region of objects beyond the orbit of Neptune.

"And I am certain Pluto is happier there," Tyson said.

Sykes said that if a non-stellar object is massive enough to be round and it orbits a star, it ought to be a planet. Under this definition, the solar system would have 13 planets, although more might be found in the future beyond the orbit of Pluto. In addition to Pluto and the other eight major planets, these would also include Ceres, Pluto's moon Charon, Eris, and recently discovered Makemake.

In response to Sykes wanting to call all of these objects "planets," Tyson responded, "You need that word. I'm saying define it however you want and then recognize how useless it is and then find another term to group objects of like properties that are useful to planetary scientists," Tyson said.

Tyson would rather not count planets and instead group objects together that have similar properties, even if that means having handfuls and handfuls of planets.

Pluto tale

The debate marks another chapter in the Pluto saga, which began when Pluto was discovered in 1930, as this object was an oddball compared with its solar system buddies in its eccentric orbit, small size and low mass (it is less massive than Earth's moon).

Some argued, then, that Pluto didn't fit in with the rest of the solar system planets. The plot thickened in 2004 with the discovery of Sedna, an object about three-fourths Pluto's size and about three times as far from the sun. If Pluto fits the planet build then so would Sedna.

Caltech's Mike Brown added another twist to the story in 2005 when he announced the discovery of 2003 UB313, a hopeful 10th planet in our solar system. The object was round, orbited the sun, and the kicker — it turned out to be larger than our then ninth planet, Pluto. In 2006, UB313 was officially named Eris.

"The Pluto controversy boiled up when Eris came up, because you couldn't leave things the way they were," said Jack Lissauer of NASA Ames Research Center in California. "You really had to contort things to say Pluto was a planet and Eris wasn't. Things really came to a head."

Since then, the International Astronomical Union (IAU) has labeled Pluto a "dwarf planet" and then later, a "plutoid." Many planet scientists were disgruntled over the 2006 IAU decision, which they said involved a vote of just 424 astronomers out of some 10,000 professional astronomers and many other planetary scientists around the globe.

"Having a group of graybeards getting together and issuing a formal definition is not a good idea," David Morrison of NASA Ames, told SPACE.com, referring to the IAU's 2006 vote.

Delusion or debate?

Hal Weaver of JHU's Applied Physics Laboratory, called this week's debate "a real scientific conference to lay out all the issues and discuss them."

But Lissauer pointed out that even this conference has its flaws. "This meeting isn't representative of planetary scientists either. There is a very, very skewed distribution," Lissauer said during a panel discussion.

No consensus was reached Thursday.

At the end of the debate, Pluto, as far as many astronomers are concerned, remains in some sort of limbo.

And closing the debate, Sykes said, "I get the feeling Neil [Tyson] is coming over to the right side of the fence."

Tyson's response: "The delusion continues."