August 24, 2006; updated August 25A
stronomers meeting in Prague have decided by a wide margin that Pluto no longer merits consideration as a full-fledged planet. The historic decision follows a week of sometimes contentious debate over how to define a planet.
|Xena discoverer Mike Brown; Alan Stern, lead scientist on the New Horizons mission to Pluto; and Patricia Tombaugh, widow of Pluto's discoverer, react to the decision in Astronomy's podcast.|
Jocelyn Bell Burnell, an expert on neutron stars at the University of Oxford in England, moderated the proceedings with the help of some playful visual aids. A blue toy balloon stood in for Neptune, while a plush toy of Disney's cartoon dog Pluto played the ninth rock from the Sun. Applause followed this morning's critical vote at the International Astronomical Union's (IAU) General Assembly meeting.
"I was relieved," says Caltech's Michael Brown. Brown's January 2005 discovery of Xena (officially, 2003 UB313
), which is larger than Pluto, forced the IAU to address this long-simmering issue. "Scientifically, there is no question this is the right way to go," he says. "Eight is enough."
"I am delighted that rationality has prevailed," says Richard Conn Henry of Baltimore's Johns Hopkins University.
However, not everyone is happy with the decision. The American Astronomical Society 's 1,300-member Division of Planetary Sciences — the world's largest group of planetary scientists — recommended acceptance of the original proposal announced last week. If passed, this would have retained Pluto's status and declared three other bodies — Xena, the largest asteroid, and Pluto's largest moon — planets as well.
"I just think the IAU has embarrassed itself," says Alan Stern, principal investigator for NASA's New Horizons mission to Pluto. "If you read the definition that they have adopted in that room today, it is scientifically indefensible."
"I think there's going to be a protest," says Mark Sykes at the Planetary Science Institute in Tucson. "A minority of the astronomical community passed this."