There goes the solar system.
Once an elite society of nine lordly bodies of rock, ice and gas, our solar system would grow to at least 12 and perhaps as many as 53 members under a new definition of a planet proposed Tuesday by the International Astronomical Union.
The core of the definition? Planets are round. And they orbit a star.
The proposal was hammered out after two years of intense astronomical debate among leading experts of the IAU, which is the sole authority on Earth for naming celestial objects.
“We now have a new way to put the solar system together,” said Richard Binzel, a member of the IAU executive committee that drafted the definition. “We think this definition is reasonable.”
The proposal will be voted on next week by the group’s general assembly, which is meeting in Prague, Czech Republic.
Binzel, an astronomer at Massachusetts Institute of Technology, said he was “optimistic” the definition would be approved.
The new list of round planets would consist of one recently discovered object beyond the orbit of Pluto named UB313 – nicknamed Xena – as well as two bodies that previously were rejected for planetary status: Pluto’s moon Charon and Ceres, the largest member of the asteroid belt between Mars and Jupiter.
Charon and Pluto would become the solar system’s first double planet, meaning they twirl around each other but neither dominates.
They would become part of a new subclass of planets called “plutons,” defined by the fact that their orbits around the sun take at least 200 years.
There could be dozens more plutons added after the objects are more thoroughly reviewed by the IAU. There are currently 12 awaiting evaluation.
Ceres also would get a new designation as the sole member of a subclass called “dwarf planets.”
Gibor Basri, chairman of astronomy at the University of California, Berkeley, praised the IAU for coming up with a reasonable definition that could help quell the stubborn arguments over what makes a planet, a debate provoked by critics who questioned tiny Pluto’s status as a planet.
“I feel that they have made the most rational and scientific choices,” he said. “It does mean some adjustment for the public.”
The definition has riled some astronomers as well.
Perhaps surprisingly, one of the strongest critics of the new solar-system lineup is the man who discovered one of the proposed new planets, astronomer Michael Brown of the California Institute of Technology. He called the decision an “odd solution.”
“In my book, the word ‘planet’ was special. I liked it back when planets meant something other than: It’s round,” said Brown, who has discovered 15 objects that appear to now qualify as planets, making him history’s most prolific planet hunter.
Brown said he had counted 53 objects that appeared to meet the proposed definition of a planet.
It’s an open question how the new definition will be received by the general public, which grew up with mobiles of the nine-planet solar system in their bedrooms, and learning mnemonic devices to memorize the planets, such as My-Very-Excellent-Mother- Just-Served-Us-Nine-Pizzas.
