http://www.space.com/scienceastronomy/060824_planet_definition.html
Capping years of intense debate, astronomers resolved today to
demote Pluto in a wholesale redefinition of planethood that is
being billed as a victory of scientific reasoning over historic and
cultural influences. But already the decision is being hotly
debated.
Officially, Pluto is no longer a planet.
"Pluto is dead," said Caltech researcher Mike Brown, who spoke with
reporters via a teleconference while monitoring the vote. The
decision also means a Pluto-sized object that Brown discovered will
not be called a planet.
"Pluto is not a planet," Brown said. "There are finally,
officially, eight planets in the solar system."
The vote involved just 424 astronomers who remained for the last
day of a meeting of the International Astronomical Union (IAU) in
Prague.
"I'm embarassed for astornomy," said Alan Stern, leader of NASA's
New Horizon's mission to Pluto and a scientist at the Southwest
Research Institute. "Less than 5 percent of the world's astronomers
voted."
"This definition stinks, for technical reasons," Stern told
SPACE.com. He expects the astronomy community to overturn the
decision. Other astronomers criticized the definition as
ambiguous.
The resolution
The decision establishes three main categories of objects in our
solar system.
Planets: The eight worlds from Mercury to Neptune.
Dwarf Planets: Pluto and any other round object that "has not
cleared the neighborhood around its orbit, and is not a
satellite."
Small Solar System Bodies: All other objects orbiting the
Sun.
Pluto and its moon Charon, which would both have been planets under
the initial definition proposed Aug. 16, now get demoted because
they are part of a sea of other objects that occupy the same region
of space. Earth and the other eight large planets have, on the
other hand, cleared broad swaths of space of any other large
objects.
"Pluto is a dwarf planet by the ... definition and is recognized as
the prototype of a new category of trans-Neptunian objects," states
the approved resolution.
Dwarf planets are not planets under the definition, however.
"There will be hundreds of dwarf planets," Brown predicted. He has
already found dozens that fit the category.
Contentious logic
The vote came after eight days of contentious debate that involved
four separate proposals at the group's meeting in Prague.
The initial proposal, hammered out by a group of seven astronomers,
historians and authors, attempted to preserve Pluto as a planet but
was widely criticized for diluting the meaning of the word. It
would also have made planets out of the asteroid Ceres and Pluto's
moon Charon. But not now.
"Ceres is a dwarf planet. it's the only dwarf planet in the
asteroid belt," Brown said. "Charon is a satellite."
The category of "dwarf planet" is expected to include dozens of
round objects already discovered beyond Neptune. Ultimately,
hundreds will probably be found, astronomers say.
The word "planet" originally described wanderers of the sky that
moved against the relatively fixed background of star. Pluto,
discovered in 1930, was at first thought to be larger than it is.
It has an eccentric orbit that crosses the path of Neptune and also
takes it well above and below the main plane of the solar
system.
Recent discoveries of other round, icy object in Pluto's realm have
led most astronomers to agree that the diminutive world should
never have been termed a planet.
'A farce'
Stern, in charge of the robotic probe on its way to Pluto, said the
language of the resolution is flawed. It requires that a planet
"has cleared the neighborhood around its orbit." But Earth, Mars,
Jupiter and Neptune all have asteroids as neighbors.
"It's patently clear that Earth's zone is not cleared," Stern told
SPACE.com. "Jupiter has 50,000 trojan asteroids," which orbit in
lockstep with the planet.
Stern called it "absurd" that only 424 astronomers were allowed to
vote, out of some 10,000 professional astronomers around the
globe.
"It won't stand," he said. "It's a farce."
Stern said astronomers are already circulating a petition that
would try to overturn the IAU decision.
Owen Gingerich, historian and astronomer emeritus at Harvard who
led the committee that proposed the initial definition, called the
new definition "confusing and unfortunate" and said he was "not at
all pleased" with the language about clearing the
neighborhood.
Gingerich also did not like the term "dwarf" planet.
"I thought that it made a curious linguistic contradiction,"
Gingerich said in a telephone interview from Boston (where he could
not vote). "A dwarf planet is not a planet. I thought that was very
awkward."
Gingerich added: "In the future one would hope the IAU could do
electronic balloting."
Years of debate
Astronomers have argued since the late 1990s, however, on whether
to demote Pluto. Public support for Pluto has weighed heavily on
the debate. Today's vote comes after a two-year effort by the IAU
to develop a definition. An initial committee of astronomers failed
for a year to do so, leading to the formation of the second
committee whose proposed definition was then redefined for today's
vote.
Astronomers at the IAU meeting debated the proposals right up to
the moment of the vote.
Caltech's Mike Brown loses out in one sense. The Pluto-sized object
his team found, called 2003 UB313, will now be termed a dwarf
planet.
"As of today I have no longer discovered a planet," he said. But
Brown called the result scientifically a good decision.
"The public is not going to be excited by the fact that Pluto has
been kicked out," Brown said. "But it's the right thing to
do."
Textbooks will of course have to be rewritten.
"For astronomers this doesn't matter one bit. We'll go out and do
exactly what we did," Brown said. "For teaching this is a very
interesting moment. I think you can describe science much better
now" by explaining why Pluto was once thought to be a planet and
why it isn't now. "I'm actually very excited."
http://www.space.com/scienceastronomy/060824_planet_definition.html
