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
Fernández. 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 Pečnik (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 Fernández, 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 Pečnik 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 Pečnik.
That's why
Pečnik prefers another physical property: ability of the body to keep its
atmosphere against the vacuum of the interplanetary space. In previous work,
Pečnik 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 Pečnik'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).
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