The gathered scientists reacted far more favorably to a new planet-definition presented Friday by Julio Fernández of the University of the Republic in Montevideo, Uruguay. The proposal, signed by 17 astronomers from nine nations, would demote Pluto and give us a solar system with eight planets.
"I much prefer the competing second proposal to the original," says Boss, who is headed to Prague. Nearly 75 percent of the scientists polled during Friday's debate felt the same way.
While both definitions argue that a planet must be nearly round in shape, Fernández's version adds that it must also be "by far the largest object in its local population" and "not produce energy by any nuclear fusion mechanism." This last point solidifies the distinction between gas-giant planets and brown dwarf stars, which can undergo a flicker of fusion while young.
"Pluto, Ceres, and other large trans-neptunian objects … should be not considered as planets, since they never were the dominant bodies in their accretion zones," reads the new plan.
That would be fine with Paul Weissman. "The correct definition of a planet that I would offer is that it is a body orbiting a star that is massive enough to clear its dynamical zone of debris (asteroids and comets) over the history of the solar system," he commented. "This definition is intrinsic to each planet and to its unique position in the solar system."
Other scientists agree that dynamics must play a role in any definition of a planet. On Wednesday, Steven Soter of New York's American Museum of Natural History submitted a paper
to The Astronomical Journal
that argues a planet can be defined by the degree to which it dominates other masses that share its orbital zone. His analysis shows that eight planets — Mercury through Neptune — affect nearby objects 100,000 times more than Ceres, Xena, or Pluto and Charon.