Project Cyclops, A Design... - Department of Earth and Planetary ...

nothingspecial abouthesolarneighborhood. Oursisas
goodasanyotherin thedisk,andconversely. Furthermore,thediskisthetypicalenvironment
of theGalaxy.
FORMATIONANDEVOLUTION
OFPLANETARYSYSTEMS
Oneof themoststrikingfeaturesof thesolarsystem
istheextremeorderliness of themotionsof theplanets
andsatellites. Theorbitsof theplanetsareall nearly
circular and, except for far-out Pluto and, to a lesser
extent, near-in Mercury, lie very nearly in a common
plane (see Appendix A). This plane makes only a small
angle with the plane of the Sun's equator. All the planets
revolve in the direction of the Sun's rotation. Within the
solar system are two well-developed "subsolar systems":
the satellite systems of Jupiter and Saturn. The major
satellites of these two planets also exhibit the same
regularity of motion with respect to their primaries as
the planets have with respect to the Sun.
These facts strongly suggest that the collapsing gas
cloud, which was to become the Sun, contracted to an
initial disk shape with a concentrated center just as was
the case for the Galaxy as a whole. In the galactic cloud
about a trillion local condensations occurred to form
stars. In the Sun's disk, because of the vastly smaller
scale, only a few local condensations occurred to form
planets, and the larger of these repeated the process to
form satellites. Thus, although we have only one
planetary system to study, we see certain aspects of its
formation process repeated in miniature within it, and
other aspects repeated on a much larger scale for the
Galaxy
as a whole.
Nebular vs. Catastrophic Theories
of Planetary
Formation
The earliest theories of planetary formation were the
nebular hypotheses of Kant and LaPlace. These pictured
a large cloud condensing into a disk much as we have
indicated. These theories were brought into question in
the mid-1800s by the observation that the Sun, which
contains the overwhelming majority of the total solar
system mass, has only a small fraction of the angular
momentum (see Appendix A). Since no way was known
for the Sun to get rid of angular momentum, this
disparity constituted a real objection to any condensation,
or nebular theory.
Thus the question was inverted: If the Sun, however
it formed, had so little angular momentum, how did the
planets acquire so much? To explain this excess, various
catastrophic' theories of planetary formation were devised.
There were two major versions. In one version, an
original binary companion of the Sun destroys itself(or
is destroyed by collision with a third body) leaving
debris, which condenses into the planets. In another
version, a wandering star brushes past the Sun at such
close range that tidal forces draw out filaments of
matter, which separate into globules and cool to form
the
planets.
The catastrophic theories, popular in the early part of
this century, failed for a variety of reasons. Self-destruction
of a star produces a nova or supernova in which
much of the matter is cast into space in excess of escape
velocity and the core remains behind as a white dwarf,
or a neutron star. The violence of a collision with a third
body would scatter matter in all directions and would
hardly produce coplanar and circular planetary orbits.
The same objection applies to the globules condensed
from tidal filaments. These could hardly all have had
exactly the right initial velocities to produce the
observed nearly circular orbits of the planets. The coup
de grace was Spitzer's demonstration, in the late 1930s,
that hot gases extracted from the Sun in a tidal filament
would explode rather than condense into planets.
The implication of all catastrophic theories is that
planetary systems must be extremely rare. Stars are so
widely separated, and their orbital motions around the
galactic center are, for the majority, so nearly concentric,
that close encounters might be expected to occur
only a few tens of times in the entire history of the
Galaxy. Thus in the early twentieth century astronomers
would have rejected the prevalence of extraterrestrial life
solely on the basis of the scarcity of planetary systems.
The last 30 years have seen a return to the nebular
hypothesis, with the newly discovered principles of
magnetohydrodynamics playing a key role in solving the
angular momentum problem. We should be clear that the
problem is not, as the catastrophic theories supposed, to
explain any angular momentum excess of the planets,
but rather to explain the angular momentum deficiency
of the Sun. The gas globule that fragmented out of the
galactic gas cloud to become the Sun was on the order of
one light-year across. If it merely shared the general
rotation rate of the Galaxy its period of rotation would
be about 240 million years. After contracting to a diameter
of l0 -3 light-years, which is roughly the diameter of
the solar system, its rotation rate would be 10 6 times as
fast, or one revolution in 240 years. This is about the
orbital period of Pluto and so is an appropriate rotation
rate for the planets.
At this stage the cloud would be greatly flattened by
its rotation and would consist of a disk with a denser,
rapidly rotating, hot central region. Irregular density
distributions in the disk produced nucleation centers for
the formation of planets. In the course of its contrac-
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