[meteorite-list] When is a Planet not a Planet?

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Sep 22 20:41:49 2005
Message-ID: <200509230040.j8N0eZf15509_at_zagami.jpl.nasa.gov>

http://www.newscientistspace.com/article/dn8039-when-is-a-planet-not-a-planet.html

When is a planet not a planet?
Maggie McKee
New Scientist
22 September 2005

Astronomers are arguing bitterly over how to define a planet, with some
proposing that the term be abandoned completely in favour of more
specific labels based on where objects are located. Two competing
proposals are expected to be put forward to a formal task group on
Friday, but astronomers say the debate could drag on indefinitely.

The International Astronomical Union (IAU), which is responsible for
settling such issues, assembled a special working group to come up with
a new definition about 18 months ago, when a large new body called Sedna
was found in the outer solar system.

"The hope was that we would come to some agreement before anything else
dramatic happened," says the working group's chairman, Iwan Williams of
Queen Mary, University of London, UK. "But then 2003 UB313 turned up."

This large object was discovered in July 2005 in a ring of rocky bodies
beyond Neptune called the Kuiper Belt. It was dubbed the "tenth planet"
by its discoverers because it is larger than Pluto, reigniting the
debate over what constitutes a planet.
          
Going in circles

But the 19-member working group has been unable to arrive at a
consensus. The discussion has "just been going in circles", says group
member Alan Stern, an astronomer at the Southwest Research Institute in
Boulder, Colorado, US.

One of the two proposals to be submitted on Friday is based simply on an
object's size - a definition that Stern favours. That would increase the
number of planets in the solar system, with Pluto being just one of
several known Kuiper Belt objects of about the same size that would
qualify as a planet.

But some of the team say choosing a size cutoff for the definition is
arbitrary. "There is no scientific value in maintaining that there are
nine planets, including Pluto as one out of many similar small bodies,"
says group member Brian Marsden, head of the IAU's Minor Planet Center
in Cambridge, Massachusetts, US.

The second proposal would decrease the number of planets in the solar
system by demoting Pluto. This definition says a planet is the dominant
body in its immediate neighbourhood - a title based on its size relative
to its neighbours and the dynamics of its orbit. Objects in the asteroid
belt beyond Mars, for example, would not be planets because there are so
many of them in the same region, while Pluto would not count because it
crosses the orbit of its more massive neighbour Neptune.

Fudge factor

But a third proposal has been discussed that does away entirely with the
term "planet" - an option Williams calls a "fudge factor". It uses
qualifying adjectives to define subclasses of planets based on
characteristics like location, composition or culture. In this scheme,
Earth might be a "terrestrial planet" and Pluto a "historic planet".

Marsden supports this idea, explaining it "allows us to educate a public
that thinks Pluto is somehow very special." But Stern is vehemently
opposed to it. "Our charter is to define a planet, not subgroups," he
told New Scientist. "Either we do our job or we don't."

"If the working group actually ratifies a statement that says there is
no such thing as a planet, the IAU will be the laughing stock of the
world," he says. "Everyone will ask why egghead PhDs can't tell when an
object is a planet if regular people can."

The working group may vote on the proposals within the next two weeks.
But Williams says: "We may just decide to start from scratch again
rather than going on the two proposals."
Received on Thu 22 Sep 2005 08:40:35 PM PDT