Under new definition, Pluto would become a planet again

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.”

For years, astronomers, planetary scientists and other space researchers have fought about what to call the small, icy world at the edge of our solar system. Is it a planet, as scientists believed for nearly seven decades? Or must a planet be something bigger, something more dominant, as was decided by vote at the International Astronomical Union in 2006?

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 ignored his own advice last week when he attended the annual Lunar and Planetary Science Conference in Houston. He attempted 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.

When the IAU voted in 2006, scientists decided 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 was reclassified as a “dwarf planet” — a body that resembles a planet but fails to “clear its neighborhood,” in the IAU’s parlance.

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, but not so massive that it starts to undergo nuclear fusion and become a star.