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 (IAU) 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 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.