[meteorite-list] Planetary Definition Showdown!

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue Aug 22 20:07:42 2006
Message-ID: <200608222332.QAA04849_at_zagami.jpl.nasa.gov>

http://www.space.com/adastra/adastra_planet_def_060822.html

Planetary Definition Showdown! As Debate Rages, One Astronomer Says
'It's All About the Atmosphere'

By George Whitesides
National Space Society
22 August 2006

The biggest astronomical debate of the young millennium culminates
this Thursday at International Astronomical Union's (IAU) meeting in
Prague, where national representatives will give thumbs up or down to
IAU's latest planet definition proposal.

Last Wednesday, the IAU Executive Council (EC) offered a controversial
planet definition which would confirm Pluto's planetary status, and
instantly promote Pluto's moon Charon, the asteroid Ceres, and the
newly-discovered UB313 to planets as well. This proposal was voted
down by a wide margin two days later in an internal vote open
exclusively to the planetary community. It lost 60% of the votes to an
alternative definition proposed by Gonzalo Tancredi and Julio
Fernandez. But the real vote comes this Thursday.

Although the EC's original proposal seems simple and based on physical
concepts (a planet is massive enough to be rounded by its gravity,
orbits a star, and is neither a star nor a satellite of a planet), the
definition stirred controversy among the planetary community. The
challenge: under the new definition, there could soon be dozens of new
"planets" in our solar system. That struck many astronomers as the wrong
result.

At the same time, two important papers dealing with the planethood
definitions appeared on the online preprint site arXiv.org. In the
first paper, Dr. Steven Soter (Dept. of Astrophysics, American Museum of
Natural History, NY) detailed the concept of the gravitational dominance
in the orbital zone of the planetary body, while Croatian scientist Dr.
Bojan Pecnik (Dept. of Physics, Univ. of Split, Croatia) argued that the
necessary planethood criterion should be ability to keep an atmosphere.

The most popular alternative proposal, by Tancredi and Fernandez,
requires the planet to be by far the largest body in the local
population, and massive enough to be round. A body rounded by its own
gravity but accompanied by others of similar size would not qualify and
be called something else. This would kill the planetary aspirations of
Pluto, along with other trans-neptunian objects (TNOs), and all asteroids.

Reasoning similar to the alternative proposal was applied by Steven
Soter in a work submitted to the Astronomical Journal. Soter argues that
"a planet is an end product of disk accretion around a primary star or
substar. I quantify this definition by the degree to which a body
dominates the other masses that share its orbital zone."

Croatian scientist Dr. Bojan Pecnik agrees, saying "Gravitational
dominance in one's orbital zone around a star or stellar remnant should
be required to be a planet, but a planet should also posses an intrinsic
physical property, irrespective of the dynamics of its environment".

Currently, the most popular property is roundness caused by object's own
gravity. The problem with that definition is that celestial bodies can
be made round through processes other than those considered by the EC
definition. For example, violent kinetic event can shatter a
potato-shaped asteroid into the spherical rubble pile (which is what
most likely happened to Dactyl, a 1 kilometer moon of the asteroid Ida),
or an iron meteorite can experience primordial melting and solidify into
a sphere.

"Discriminating roundness caused by gravity from roundness due to
cosmogony might prove difficult for objects on the border between major
and minor planets, especially for the bodies far-out in the Kuiper Belt,
or even more so for exoplanets", continues Pecnik.

That's why Pecnik prefers another physical property: ability of the body
to keep its atmosphere against the vacuum of the interplanetary space.
 In previous work, Pecnik developed a new concept needed to quantify his
criterion. This was a critical step, because earlier attempts to
associate an atmosphere with planethood failed - the criterion was not
quantifiable, nor it was possible to discriminate dilute atmospheres
from vacuum.

According to Pecnik's work submitted to Journal of Planetary and Space
Science's special issue on exoplanets, "only the ability to hold the
atmosphere is required, not the atmosphere itself." Therefore, Mercury
would likely, although marginally, keep its planetary status.

With so many things happening at the same time, all bets are off what
will happen on Thursday in Prague. Only one thing is certain: the
solar system will never be seen the same way.

George Whitesides is executive director of the National Space Society
(NSS).
Received on Tue 22 Aug 2006 07:32:47 PM PDT