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When the International Astronomical Union (IAU) meets in August this
year, it will have to resolve the vexed question of Pluto.
Should the IAU stick to the decision it made 18
months ago but had put on hold due to vigorous protests by teachers,
schoolchildren, and planetary scientists or should it reverse
itself?
The IAU’s big idea was to clear the solar
system of bodies that did not belong by changing the definition of
the term “planet.” Only those objects in our solar system may be
called planets. Furthermore, they must be gravitationally dominant
in that they have “cleared the neighborhood around [their]
orbit.” Clear, however, is unclear. Would Jupiter, with countless
asteroids twirling in its orbital path, qualify? If demoting Pluto
was controversial, would the reclassification of Jupiter be less so?
The IAU’s intent was to recognize only those
bodies that are more and more massive with increasing distance from
the Sun. Pluto was originally accepted as a planet because it was
wrongly thought to be five times more massive than Earth. If Mars
were even half an astronomical unit (AU) farther from the Sun, it
too would fail to qualify. (An AU is equal to 1.495 x 1911 meters.)
The teachers’ and schoolchildren’s
attachment to Pluto was somewhat sentimental. The rejection by the
planetary scientists of the IAU definition was however driven by the
rapid increase of knowledge about objects in outer space. Their
interest was mainly in the geophysical characteristics of these
bodies and not their dynamics.
Ahead of the August meeting, the Planetary
Science Institute (PSI) in Tucson, Arizona laid out its position in
a paper by Mark Sykes, (Science, March 28, 2008).
The PSI defined a planet as “a round object
(in hydrostatic equilibrium) orbiting a star.”
In physics, hydrostatic equilibrium is complete
balance between the force of gravity and the pressure force. This
definition increases the number of planets even in our solar system.
The PSI count is 12: the present nine plus Ceres, Charon and Eris.
The IAU’s classification if it agrees could become a subset of
PSI’s list.
One reason for PSI favoring a broader definition
is the increasingly detailed information gathered by spacecraft
about “volcanism, tectonics, atmospheric circulation and
chemistry, erosion, fluvial processes and the potential for life”
on objects in the visible universe.
As these observations accumulate and as theories
are proposed and tested, our understanding of how Earth works is
refined and modified. For planetary scientists, Earth is the
“ultimate planet” against which what has been observed elsewhere
is assessed and measured.
The common characteristic of all these worlds is
they are all round. Why? Massive objects in space are crushed by
gravity to become round. This is also the shape that’s in
hydrostatic equilibrium.
The mechanisms of planetary formation account
for the differences among them. Heavier materials sink to the
center; lighter materials rise to the surface; the volatile
materials like oceans and atmospheric gases that are produced by
these processes are retained if there’s sufficient gravity; the
mantle gives rise to tectonic activity and volcanism. Much has still
to be discovered about the sequence of these processes.
But when we see an object in our solar system or
orbiting another star and is massive enough to be round, then we can
expect to understand it in conjunction with what we know about
terrestrial processes.
The geophysical definition of a planet that the
PSI proposes makes understanding and teaching simpler for both
scientists and lay persons like ourselves.
The issue that the IAU should keep in view in
August is that space exploration will continue to accelerate. More
and more spacecraft and more powerful telescopes are being launched
for very specific missions. There will be more discoveries that need
to be explained in physical rather than dynamical terms.
But whether or not the astronomers accept the
larger perspective of the planetary scientists, it will not really
matter to science.
opinion@manilatimes.net
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