[Since the article's first appearance, Pluto was
stripped of its status as a planet and redefined as
a "dwarf planet," leaving just eight classical
planets in the solar system. European Weekly]
Sharp divisions in
opinion among some of the world's top astronomers
about how to define a planet could scuttle more than
a week's worth of debate and proposals on how to
settle an issue that has divided the field for
Puny Pluto's status
as a planet, a frequent matter of debate, looked no
closer to resolution Aug. 22 as astronomers of the
International Astronomical Union's (IAU's) triennial
convention packed into a large hall of Prague's
Congress Center and failed to endorse a draft
resolution defining what a planet is.
The committee wants
to define a planet as a celestial body that has
enough self-gravity to pull itself into a round
shape and orbits a star rather than another object.
Moreover, the committee recommends categorizing
Pluto and three other new planets as "dwarf
The full assembly of
the IAU, more than 2,500 astronomers, will vote on
the matter Aug. 25. If approved, three new planets —
Charon, a moon of Pluto; Ceres, a large asteroid;
and a newly discovered body known as Xena — will
join our solar system.
But during more than
an hour of heated debate, it became clear that
warring disciplines within astronomy, mostly between
gravitational physicists and dynamic physicists, are
making it hard to produce a simple resolution.
"This is not a
scientific issue of what is correct," said IAU
President Ron Eckers. "There is no correct answer to
this issue. This is about a compromise that will
Still, a growing
number of astronomers called on the IAU to put off
the issue for another three years, when the general
assembly reconvenes in Rio de Janeiro.
screamed into a microphone that the committee's
failure to take into account quantum dynamics in its
definition was "an insult to the entire astronomical
has focused on Prague's Congress Center in Vyšehrad
for more than a week, and the planet issue has cast
a shadow over what scientists say are a lot more
important concerns for the field of astronomy.
everybody's talking about," one astronomer said.
Of course, many are
surprised that in a field as complex as astronomy, a
collection of some of its most brilliant
practitioners are still debating what a planet is.
"It never mattered
before," said Virginia Trimble, of the University of
But then came Xena —
or, officially, 2003 UB313 — which California
Institute of Technology Astronomer Mike Brown
discovered in 2003 well beyond Pluto. Careful
measurements revealed that it was indeed bigger than
Pluto, a conclusion confirmed earlier this year by a
team at the University of Bonn in Germany.
"It was the first
object that was larger than Pluto," said Frank
Bertoldi, the team's leader. "The issue was acute.
It had to be dealt with."
After two years of
failing to define a planet, the IAU convened a new
committee in Paris in June. During the course of a
few sleepless nights, in the words of one member,
the group reached consensus.
Owen Gingerich, a
Harvard professor who chaired the IAU's recent
"definition committee" said Pluto was the "elephant
in the parlor" throughout the process.
Pluto's status has
been debated since the planet's discovery in 1930,
and, despite what millions learn in school,
astronomers long ago concluded that it's not really
a planet: Its orbit is too different, its size —
one-quarter of 1 percent as massive as Earth — is
To make matters
worse, it has remained largely a mystery, hard to
observe at some 40 times further from the sun than
Earth (which is itself 93 million miles, or 150
million kilometers, from the sun).
Still, Pluto has a
strong lobby supporting it.
"People like Pluto in
the same way they tend to favor the underdog," said
Richard Binzel, an astronomer at the Massachusetts
Institute of Technology. "There is an emotional
attachment to Pluto that is uncommon in the world of
Keeping it simple
astronomers is settling not so much on Pluto, but on
how to find a definition that is not too simple, but
not too complex either.
"People want an easy
definition, but I don't think a lot of this stuff is
easy to explain," said David Morrison, who writes
In the end, the
recommendation on the table defines a planet in both
intrinsic and extrinsic ways: Shape, size and orbit
are broad enough to understand, but leave enough
room to get specific.
Still, at press time,
e-mails were pouring into the IAU with alternate
suggestions on how to best fine-tune the definition
of "planet." At the Aug. 22 debate, several new
ideas were floated, and it was unclear what kind of
airing they would get.
One overall concern
was that defining a planet broadly flings open the
door to literally millions of potential other
"The proposal I
actually favor is to say that everybody on this
planet has grown up with the idea that there are
nine planets. ... Sorry, all you other guys, you
just don't make it," said Paul Weissman of NASA's
Jet Propulsion Laboratory.