[meteorite-list] Astronomers Sharply Divided on New Planet Definition

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Aug 17 17:15:27 2006
Message-ID: <200608172112.OAA19291_at_zagami.jpl.nasa.gov>

http://www.space.com/scienceastronomy/060817_planet_support.html

Astronomers Sharply Divided on New Planet Definition
By Robert Roy Britt
space.com
17 August 2006

A 12-person committee representing the world's largest group of
planetary scientists today threw its support behind a new
planet-defintion proposal that would increase the tally of planets in
our solar system to 12.

More dissent emerged, too, from several prominent planet experts.

The definition proposed yesterday at a meeting of the International
Astronomical Union (IAU) in Prague, preserves Pluto's planet status and
essentially classifies as planets all round objects that orbit the Sun
and do not orbit another planet. The tally of planets is expected to
eventually soar into the hundreds if the resolution is passed by a
vote next week.

The Division for Planetary Sciences (DPS), a group within the American
Astronomical Society, has the opposite view. The 12-member DPS
Committee, elected by the membership, "strongly supports the IAU
resolution," according to a statement released today.

"The new definition is clear and compact, it is firmly based on the
physical properties of celestial objects themselves, and it is
applicable to planets found around other stars. It opens the possibility
for many new Pluto -like planets to be discovered in our solar
system," the DPS statement reads.

An informal SPACE.com survey of astronomers who study planets in and out
of our solar system found six in favor of the resolution and seven
against. A separate private straw poll being conducted by the National
Academies of Sciences has so far yielded an overwhelming "No" response,
a source told SPACE.com.

'Terrible definition'

Clearly no consensus has emerged, however.

"I think it's a terrible definition," said David Charbonneau, a
researcher at the Harvard-Smithsonian Center for Astrophysics who
searches for and studies planets around other stars. Charbonneau joins
two other astronomers close to the issue who sharply criticized the plan
[see yesterday's story
<http://www.space.com/scienceastronomy/060816_planet_definition.html>].

-------------------------------------------------

More Views

In email interviews, several experts in planetary science share their views:

"The definition itself is not that important. There are lots of
interesting bodies out there for us to study. We need to have a
definition, though, because it makes it easier for people to understand
what we mean."
-Amy Simon-Miller, NASA scientists and member of the DPS Committee that
endorsed the IAU resolution

"I think most astronomers agree that there are eight planets, and (like
myself) are not particularly passionate about either Pluto's status or
the outcome of the 'debate.' It's clear, however, that removing Pluto
from the list rouses strong emotions within the public (who ultimately
pay the bills). So I would just retain the eight planets plus Pluto."
-Gregory Laughlin, University of California, Santa Cruz extrasolar
planet researcher

"It [the definition] makes a lot of sense. There has to be a physically
meaningful definition for a planet since we are finding lots of KBOs and
planets around other stars. If you had an arbitrary cutoff at say Pluto
or even Mercury, how would you justify it when looking for other bodies
in the solar system or in other stellar systems?"
-Larry Lebofsky, senior research scientist at the Lunar and Planetary
Laboratory

"My prejudice is to restrict the definition of planet and put Pluto and
its large Kuiper Belt cousins in a different class...with a name to be
determined (planetoid seems to serve well)."
-Jonathan Lunine, professor of planetary science and of physics at the
Lunar and Planetary Lab

"The whole debate, with Pluto as the pivot point, seems a bit silly to
me, to make such a big deal of it. If planets are round, then there are
a whole lot more than 12 of them."
-Laurance Doyle, SETI Institute extrasolar planet researcher

"The astronomers who oppose the resolution on pure or ostensibly pure
science grounds find the criterion that makes Charon a planet - the
center of mass is outside the body of the more massive partner - most
objectionable. I also think that this criterion is new to them and they
might fine it less objectionable after it gets to be a familiar rule."
-Stephen Maran, retired NASA astronomer and author of " Astronomy for
Dummies"

-------------------------------------------------------------------

Charbonneau said the definition was motivated by a desire to determine
whether Pluto and another object, 2003 UB313, are planets. But the IAU
now says there are a dozen other objects that might be planets but need
further study.

"It is ironic that we are left with more, not fewer objects for which we
are uncertain of their 'planetary' status," Charbonneau told SPACE.com.
"Perhaps astronomy will undergo a schism, with sects of astronomers
proclaiming different numbers of planets."

"As representatives of an international community of planetary
scientists, we urge that the resolution be approved," said the DPS
statement, signed by chairman Richard French of Wellesley College.

In an email interview, French said he supports the defintion but
realizes its shortcomings.

"My own personal definition would have been different from the final IAU
resolution, but scientists have been stalemated for years by defending
their own pet definitions," French said. "I understand the appeal of a
simple declaration that Pluto is no longer a planet and that the solar
system has only eight, but I also think there is value in the present
definition that has applicability to planets around other stars as well."

The DPS has about 1,300 members, at least one-quarter of which are
outside the United States. The statement does not represent the views of
all members, said DPS Press Officer Sanjay Limaye. "There has been some
feedback saying, 'I don't like it,'" he said.

'Worst' decision

The definition would make a planet of the asteroid Ceres and also
reclassify Pluto's moon Charon as a planet, on the logic that the
center of gravity around which Charon and Pluto orbit is not inside
Pluto but rather in the space between them. (Earth's Moon orbits our
planet around a center of gravity that is inside Earth.)

Pluto and Charon would be called a double planet, and they'd also be
termed "plutons" to distinguish them from the eight "classical" planets.
Ceres would be termed a dwarf planet.

The definition entirely misses the key element of a solar system object,
namely its role in the formation of the solar system," Charbonneau said.
"There are eight fully formed planets. The other objects - Ceres, Pluto,
Charon, [2003 UB313], and hundreds of thousands of others, are the
fascinating byproducts of the formation of these eight planets."

David Jewitt, an astronomer at the University of Hawaii who searches for
objects in the outer solar system, told SPACE.com that the proposal is
"the worst kind of compromised committee report."

Jewitt has long avoided the whole debate over whether Pluto is a planet
"because I think it is essentially bogus and scientifically it is a
non-issue." He waded in reluctantly this week.

"Scientifically, whether Pluto is also a planet is a non-issue," Jewitt
writes on his web site. "No scientific definition of planet-hood exists
or is needed. Is that a boat or a ship? It doesn't matter if you are
using it to float across the ocean. Scientists are interested in
learning about the origin of the solar system, and setting up arbitrary
definitions of planet-hood is of no help here."

Geoff Marcy, who has led the discovery of more planets around other
stars than anyone, called the definition arbitrary.

"Pluto, its moon, and large asteroids cannot suddenly be deemed
planets," Marcy said in an email interview. "How would we explain to
students that one large asteroid is a planet but the next biggest one
isn't?"

Astronomers made a mistake when they deemed Pluto a planet in the
1930's, Marcy and many other astronmoers say. "Scientists should show
that they can admit mistakes and rectify them," he said.

'Just might work'

However, one mild endorsement came today from Brian Marsden, who heads
the Minor Planet Center where asteroids, comets and other newfound solar
system objects are catalogued.

Marsden was on an IAU committee of planetary scientists that tried for a
year but failed to come up with a definition for the word "planet,"
which was never needed until recent discoveries of Pluto-sized worlds
out beyond Neptune. The newly proposed defintion was crafted by a second
IAU committee of seven astronomers and historians.

Marsden is a firm believer that there are eight planets, but the new
proposal has him sounding more flexible than in the past.

In an email message from Prague, Marsden said the new definition is
"intended to satisfy the eight-planet traditionalists (such as myself)
and the 'plutocrats.'" He added that he's "not against" the idea of
using roundness as a determining factor.

The IAU proposal will be voted on by IAU members Aug. 24.

"It all just might work," Marsden said.
Received on Thu 17 Aug 2006 05:12:44 PM PDT