for National Geographic News
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.
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.
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