[meteorite-list] Definition of 'Planet' Expected in September

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Sat Jun 10 12:39:54 2006
Message-ID: <200606092031.NAA19952_at_zagami.jpl.nasa.gov>

http://www.space.com/scienceastronomy/060608_planet_definition.html

Definition of 'Planet' Expected in September
By Robert Roy Britt
space.com
08 June 2006

Historians and educators have joined astronomers in an effort to break a
deadlock on contentious discussions over a definition for the word planet.

A decision is expected in September, but history suggests rewriting the
textbooks could be more challenging than finding tiny new worlds at the
edge of the solar system.

The International Astronomical Union (IAU) is expected to propose wording
to delineate planets from other small, round objects at its 12-day General
Assembly meeting in Prague this August. The proposal will be based on
recommendations from a newly formed committee that includes experts
outside the realm of astronomy tasked to break a deadlock in earlier
committee discussions.

Depending on the outcome of a separate controversial procedural
issue - whether IAU members should be allowed to vote on such
things - astronomers might then have the chance to weigh in on the
definition later in the same meeting, SPACE.com has learned.

If approved, the definition would then be announced in September.

Rewrite the textbooks...

Some might think it ironic that the world's governing body for astronomy
does not have a definition for planet.

The problem stretches back to the late 1990s, when astronomers began
discovering Pluto-like objects in the distant reaches of our solar
system.

All the newfound worlds - there are several known now - were until
recently smaller than Pluto, but they are round and orbit the Sun,
two characteristics that had for centuries been sufficient for the
implicit definition of planet. The hitch: These small objects are
typically on wild, elongated orbits that stretch well above and below
the main plane of the solar system where eight of the traditional
planets travel (Pluto has a wild orbit, too, which is one reason many
astronomers do not consider it a planet anymore).

So what to call them? Astronomers have been arguing about it in earnest
since 1999.

The controversy came to a head with the July 2005 announcement of 2003
UB313, an object roughly the size of Pluto that orbits the Sun beyond
Neptune. The object's discoverer, Mike Brown of Caltech, has argued it
should be called a planet.

But other astronomers say that if planethood is bestowed upon 2003
UB313, then several similar way-out bodies should gain the same status,
and the number of planets in our solar system could ultimately climb
into the thousands as search technology improves.

Still waiting...

The IAU had deferred judgment until it could come up with a definition.

That process was debated in an IAU committee for more than a year.
But the dozen or so astronomers on the committee could not agree whether
to define planet strictly by mass, or to consider orbital
characteristics as well as how and where a planet formed, among other
things. Last fall they argued over possibly putting adjectives in front
of planet, such as gas giant, terrestrial, asteroidal and perhaps even
traditional or historic to grandfather Pluto into the family of "regular"
worlds. Those talks broke down, however.

Recently, the issue was handed off to a new committee that includes
historians and educators, said Alan Boss, a planet-formation theorist at
the Carnegie Institution of Washington who was on the first committee.

"They wanted a different perspective from that of planetary scientists,"
said Edward Bowell, an astronomer at Lowell Observatory who is also vice
president of the IAU's Division III-Planetary Systems Sciences group.

Uncertain future

Neither Bowell nor Boss knows what exactly might happen next, however.
Nor does Brian Marsden, leader of the Minor Planet Center where newfound
objects are catalogued. Marsden was also on the first definition committee.

"The new committee is supposed to recommend what 'should be done' about
Pluto, 2003 UB313 and other 'largish' small bodies, but it is not clear
that what they decide will depend on mass," Marsden told SPACE.com.

Marsden said it's also unclear how the IAU will reach an ultimate
resolution.

"The IAU made the interesting policy decision in 2003 to disenfranchise
its members, and they were therefore not allowed to vote on 'scientific
matters' (such as what a planet is) at the last plenary General Assembly
session at the Sydney meeting [in 2003]," Marsden said. "There are
rumors that there may be an administrative decision to re-enfranchise us
at the first of the upcoming plenary sessions this August in Prague - so
that suggested vote might be possible at the second."

That is indeed the plan, IAU General Secretary Oddbjorn Engvold
explained yesterday. The advisory Committee is scheduled to meet June
30, Engvold said by email.

"Their proposal and advice will be forwarded to the IAU Executive
Committee, who will present the matter for decision at the IAU General
Assembly in Prague," Engvold said. "Assuming that the proposed change in
voting rules will be accepted at the first session of the General
Assembly, all IAU members will be allowed to vote on all scientific
issues at this General Assembly."

IAU officials appear to have some confidence this will all work out. A
statement on the IAU web site states: "The IAU will publish beginning of
September 2006 the definition of a 'Planet.'"

Broad definition possible

Astronomers, meanwhile, are eager to know if the definition will include
just mass, which would likely mean 2003 UB313 and eventually hundreds of
other worlds will be added to the list of original nine planets, or if it
will exclude those worlds by defining planets as being also in somewhat
circular orbits or some other qualifier.

Might the definition go beyond mass, to include orbit characteristics
and formation scenarios?

"Yes," IAU President Ronald Ekers told SPACE.com. "The scope of the
definition may include all these aspects."
Received on Fri 09 Jun 2006 04:31:56 PM PDT