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ZL2VAL > SETI 31.10.03 18:53l 140 Lines 6614 Bytes #999 (0) @ WW
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Subj: All that planetary jazz
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Sent: 031031/0801Z @:ZL2AB.#46.NZL.OC #:29141 [New Plymouth] FBB7.00g
From: ZL2VAL@ZL2AB.#46.NZL.OC
To : SETI@WW
All That Planetary Jazz
By Ignacio Mosqueira
Principal Investigator, SETI Institute
And Paul Estrada
NRC Fellow, NASA Ames Research Center
posted: 06:08 am ET
30 October 2003
Among the most exciting and challenging questions facing 21st century
astronomy are these: Is our solar system typical of the planetary
systems around stars? In what sense? And how does this affect the
chances for life elsewhere?
Certainly, such questions will keep astronomers busy for many years to
come. Yet, our efforts have already met with some success. Planet
hunters have discovered over a hundred planets orbiting nearby stars,
and the hunt has only just begun. Unfortunately, most of these planetary
systems seem to us rather bizarre. Though recently a few planets have
been found that are more Jupiter-like (in terms of their sizes and
orbital properties), the search method itself artificially inflates the
number of large planets found near their star, and fails to detect
planets like Earth altogether.
Planets as Moons of the Sun
So does this mean that we are stuck with only one familiar planetary
system for now? In two companion papers to be published in Icarus, we
suggest otherwise: in a real sense planets can be thought of as moons of
the Sun, and the regular satellites of the giant planets (Jupiter,
Saturn, Uranus and Neptune) formed in a similar way to that of the
planets themselves.
The genesis of this idea goes as far back as 1610, when Galileo
discovered the four moons of Jupiter and urged other astronomers to
study them. In those days, planetary orbits were generally perceived as
infallible celestial machinery, with the Earth at the center. But
Jupiter offered clear evidence that not everything revolved around the
Earth. Still, the orbits of the Galilean satellites, like the orbits of
the planets, were remarkably well behaved (nearly circular, lying on the
equatorial plane of the planet and periodic). Indeed, it was for this
reason that the satellites were at first regarded as small-sized planets.
Order and Chaos: Jazz
Since Galileo's days many more satellites have been discovered, some
regular and some irregular (these terms refer to their orbits), and the
music of the spheres has given way to something more akin to a planetary
jazz, with order and structure existing side-by-side with chaos and
happenstance. When satellites with orbital characteristics quite unlike
that of the planets were found, such as the 1898 discovery of retrograde
Phoebe by William Pickering (a retrograde satellite orbits in the
opposite sense to a planet's rotation, much like driving on the wrong
side of the road), the paradigm of satellite formation begun to shift to
a more random, catastrophic process. In fact, some of the planets
themselves were known to be subject to chaotic behavior given
sufficiently long time-spans. Thus, a new view of worlds in collision
seized the imagination of the lay public and of astronomers alike,
culminating with the spectacular collision of comet Shoemaker-Levy with
Jupiter in 1994.
But just because catastrophic events are known to take place does not
mean that order cannot lay hidden amid the chaos; only that one needs to
exercise caution when ascribing particular properties to specific
causes. While it is probably true that Earth's Moon, Pluto's moon
Charon, and perhaps the moons of Mars formed in the aftermath of the
collision of two large solid objects, our study suggests that the
regular satellites of the giant planets should be seen as an ordered,
even predictable family of celestial objects.
We argue that there is a sequence that relates the orbital properties
and other characteristics of the observed satellites to the mass of the
giant planet and of its long-gone, associated protosatellite disk. Also,
within each satellite system there is a tendency for larger satellites
to be found farther away from the planet.
Gas Disks and Satellites
It is likely that such regular behavior is at least in part due to the
presence of gas during the formation of the regular satellites. Since
giant planets also formed in a gas disk, their satellites must contend
with some of the same issues encountered in planet formation. In
particular, the interaction of planets and satellites with the gas disk
results in the inward migration of their orbits. This evolution may take
place in a time shorter than the time it takes for the gas to dissipate.
This means that a satellite's orbit could keep shrinking until finally
the satellite is lost by accretion onto the giant planet (just as the
giant planet itself might accrete onto the central star).
So how was this loss prevented? We argue that sufficiently large
satellites will form a gap in the gas disk (a similar process has been
suggested for the giant planets). Once such a gap forms, satellite
migration may stall. Moreover, by efficiently dispersing gas in the disk
the large, gap-opening satellites may have allowed smaller, neighboring
satellites to survive.
Tuning In
Where does this leave us? Although planetary jazz is difficult to grasp
at first, it does not mean that it is arbitrary or without discernable
structure. We are lucky to have a chance to listen to a few more pieces
before we venture to guess what its rules might be and what form such
music may take. There is hope that our basic notions regarding its
general outline may be correct, and that knowing which dials to tweak we
may in time tune into the music while learning to suppress the noise and
random hiss of the recording.
If so, it may indeed be possible to develop reliable models to constrain
the diversity of extrasolar planetary systems and of their satellites,
and begin to meaningfully address the question of whether such systems
may harbor life.
Of course, much more work will be required to verify our hypothesis. An
upcoming visit of the Cassini spacecraft to Saturn's satellite system,
especially to its enigmatic, cloudy moon Titan, may afford an
opportunity to test it. This moon has a thicker atmosphere than that of
Earth; it is only slightly smaller than Mars; and it may have produced
the kind of organic chemistry that on Earth gave rise to life. The
sirens of Titan beckon. An island world awaits.
=========================
73 de Alan
AX25:ZL2VAL@ZL2AB.#46.NZL.OC
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Message timed: 22:49 on 2003-Oct-31 (NZ local)
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"It's no exaggeration to say that the undecided could go one way or
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