Pluto fans are attempting to reignite a contentious astronomy debate: What is a planet?
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Is Pluto a planet?

It’s not a question scientists ask in polite company.

“It’s like religion and politics,” said Kirby Runyon, a planetary scientist at Johns Hopkins University. “People get worked up over it. I’ve gotten worked up over it.”
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The issue can bring conversations to a screeching halt, or turn them into shouting matches. “Sometimes,” Runyon said, “it’s just easier not to bring it up.”

But Runyon will ignore his own advice this week when he attends the annual Lunar and Planetary Science Conference in Houston. In a giant exhibit hall crowded with his colleagues, he’s attempting to reignite the debate about Pluto’s status with an audacious new definition for planet — one that includes not just Pluto, but several of its neighbors, objects in the asteroid belt, and a number of moons. By his count, 102 new planets could be added to our solar system under the new criteria.
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When the IAU voted in 2006, scientists came to the conclusion that gravitational dominance is what distinguishes the eight planets from the solar system’s other spheres. From giant Jupiter to tiny Mercury, each is massive enough to make them the bullies of their orbits, absorbing, ejecting or otherwise controlling the motion of every other object that gets too close. According to the definition, planets must also orbit the sun.

Pluto, which shares its zone of the solar system with a host of other objects, was reclassified as a “dwarf planet” — a body that resembles a planet but fails to “clear its neighborhood,” in the IAU’s parlance.
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But to Runyon, that distinction is less important than what dozens of solar system worlds have in common: geology.

“I’m interested in an object’s intrinsic properties,” he said. “What it is on its surface and in its interior? Whether an object is in orbit around another planet or the sun doesn’t really matter for me.”

Runyon calls his a “geophysical” definition. A planet, he says, is anything massive enough that gravity pulls it into a sphere (a characteristic called “hydrostatic equilibrium”), but not so massive that it starts to undergo nuclear fusion and become a star.
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If you talk to enough scientists on either side of this debate, you’ll notice that their arguments start to echo each other. They use the same terms to criticize the definitions they don’t like: “not useful,” “too emotional,” “confusing.” Both groups want the same thing: for the public to understand and embrace the science of the solar system. But each is convinced that only their definition can achieve that goal. And each accuses the other of confusing people by prolonging the debate.

Source: A new definition would add 102 planets to our solar system — including Pluto

Give us Pluto back!