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

tion, as the central part of the cloud become ionized by
heating, the galactic magnetic field was trapped by the
conducting gas and compressed along with the matter.
Thus, we visualize a disk with a slowly rotating cold rim
about to condense into the gas-giant planets, and
progressively more and more rapidly rotating inner parts
until we come to the hot ionized central region. Our
problem is to slow down the rapid central rotation so
that this part may shrink to become a spherical star.
Several mechanisms have been proposed. Simple
viscosity is one. Because of the velocity gradient with
radius, collisions between particles will have the overall
effect of slowing down the inner ones and speeding up
the outer ones. This mechanism does not appear
adequate. If one invokes large-scale turbulences, the
effect can be increased but other difficulties arise. We
now know that any ionized gas in the inner part of the
disk would be trapped by, and forced to move along,
rather than across, the flux lines of the central rotating
magnetic field. This constitutes a powerful angular
momentum transfer mechanism. Matter is spun out of
the central region picking up angular momentum in the
process and carrying it away from the Sun. It now
appears that the Sun itself could have ejected much of
this material.
Certain young stars such as T-Tauri, which (from
their positions on the HR diagram) appear to be entering
their main sequence phase, are observed to be ejecting
large amounts of matter. Violent flares occur that can be
detected even on present radio telesco_s. The Sun is
now believed to have ejected a substantial fraction of its
initial mass in this fashion. This emission would easily
have been enough to slow down its rotation and to
dissipate most of the gases in the disk out to the orbit of
Mars or beyond. We note that the Sun still emits matter
at a greatly reduced rate, in the form of the solar wind.
In the outer regions of the disk most of the
primordial gases of the disk probably went into forming
the dense atmospheres of the Jovian planets, with their
high abundance of hydrogen and helium. In the inner
part of the disk it is not clear whether the terrestrial
planets formed early atmospheres that were then largely
dissipated by the Sun in a "T-Tauri" stage, or whether
most of the nebular gases had already been dissipated
before these planets formed.
The formation of the disk and its evolution into
planets is obviously a very complicated problem in
statistical mechanics involving the interplay of just about
every known property of matter and radiation. Very
likely the exact sequence of events will not be known
until the whole problem can be modeled on a large
computer and the evolution followed step-by-step. When
we consider that only one seven-hundredth of the mass
of the solar system is outside the Sun the real mystery
appears to be, not why a few planets were formed, but
rather why a great deal more debris was not left behind.
How, in fact, could the cleanup process have been so
efficient?
The return to the nebular hypothesis is a key factor
in the developing scientific interest in extraterrestrial
life. For, with this mechanism of planetary system
formation, single stars without planets should be the
exception. (We cannot say this about binary stars, but
we see no reason to rule out terrestrial planets even for
them, if the separation of the stars is some 10 AU or
more.) Thus in the last two decades many astronomers
have become convinced that there are some billion times
as many potential sites for life in the Galaxy as were
thought to exist earlier in this century. This opinion is
not unanimous. Kumar (ref. 10) argues that the condensation
process may have several outcomes and that
planetary systems, while far commoner than catastrophic
theories would predict, may not be ubiquitous.
Some Evidence For Other Planetary Systems
Planets around other stars have never been observed
directly with telescopes. The huge brightness difference
between the planet and the star, together with the close
separation of the images, make such observation all but
impossible. Nevertheless we do have a little evidence for
other planets or planetlike objects.
Figure 2-8 shows the observed positions of Barnard's
star, the nearest star after the t_-Centauri system. Van de
Kamp reported in 1963 that the observed wobbling
could be partly accounted for by a dark companion of
roughly Jupiter's size in a highly elliptical orbit. He later
reported an alternate solution based on two masses in
circular orbits. In 1971, Graham Suffolk and David
O
--- - l-- --T " I l I I I
t,_. _ • I O.Ol
__ ./. _ ....
• •
O " RIGHT ASCENSION o
DECLINATION
I/..A. O0 . . • oo o
0 _ - --t----T_o_---
-I_ -
• - o-- o
t94o _95o t96o 197o
Figure 2-8. Observed motion of Barnard's star. (Right
ascension and declination are the coordinates used by
astronomers to measure motion against the star
background.] The error bar in the upper right hand
corner indicates the average error associated with any
point.
14