Evening star., 1951-12-03
Moon Maybe Planet,
Astronomer Concludes
Eurth's Satellite Believed to Be
10th, Smallest in Sun Family
By Thomas R. Henry

1Vths SiIusicSt w JUL usHsty
By Thomos R. Henry
The moon and the earth are twins.
Only by accident does the former hap
pen to be satellite of the other for it
is planet in its own right-the tenth,
Ightest and smallest of the suns family.
which seems to have been entrusted to
big sister.
This is one of the conclusions of DR..
Gerard P. Kuiper, University of Chicago
astronomer from study of the probable
origins of the 80 known satellites in the
solar system. The moon, he finds. is
unique. 1t is child of the sun, whereas
the others are only grandchildren.
In the first place, DR.. Kuiper points
out, it is one eighty-f1rst of the mass
of the earth. This is altogether too big
to fit into the regular pattern. The
biggest of the other satellites is only one
thousandth as heavy as the star around
which it revolves.
Two Groups of Moons.
Also. DR.. Kuiper says, the structure
of the moon. so far as it can be deter
mined, does not fit into any hypothesis
earth. The absence of an iron core is
especially important. A familiar theory
has been that the moon was torn out
Of the earth (probably leaving the pres
ent basin of the Pacif1c) by some terrific
gravitational disturbance long before the
appearance of life on this planet.
The other moons" in the solar sys-
tenn are grouped by DR.. Kuiper into two
groups-regular and irregular satellites.
The regular group includes the two
moons Of Mars. the five inner moons of
Jupiter. the seven inner moons of Saturn
and the five moons of Uranus.
They all are arranged in a definite
pattern, DR.. Kuiper believes. and it is
precisely the pattern of the planets
around the sun. Their spacings are in
geometrical progression with the largest
the farthest away.
This fits with the present widely ac-
cepted theory of the formation of stars
and planets. The sun, as well as all the
other stars, it is thought, was derived
from an enormous mass of whirling gas,
a "protostar." Some OF this gas did not
enter into the composition of the sun
itself but continued as smaller whirling
balls. the protoplanets. Small cast-offs
from the protoplanets resulted in the
satellites.
Under this relatively recent theory
1t can be conjectured that the majority
Of the hundred billion stars in the
Milky Way galaxy have planets Of their
own and that these planets have their
own moons. 1t never will be possible to
verify this, because the nearest star is
far too distant for a planet to be dis
eliminated by any possible device.
The results of these protostar and
protoplanet setups. it is explained, are
such that common pattern is formed.
essentially that of the solar system.
"Orphans of Eternity"
But. DR.. Kuiper says in his report to
the National Academy only 19 of the
moons" in the solar system fit into the
expected geometrical pattern. Ten, in
addition to the earth's moon, are left
out. forming group of their own with
no common pattern Of behavior. These
include the six outer moons of Jupiter.
Some of them move in the wrong direc-
ton. None of them seem to obey
strictly the geometrical pattern and the
pattern of movement which would be
expected from the operations of the
law of gravity.
Some of these. DR.. Kuiper declares.
may be explained as satellites resulting
from protoplanets which happened to
be too small or too distant in the first
place to fall within the gravitational
pattern of the parent planet. They
were true orphans of eternity, left
parentless in the vast emptiness of
solar system space. They would con-
tinue, to move around the sun in orbits
very similar. although not necessarily
identical, with those of the parent
planets. Sooner or later lost satel-
lite may get in the way of a parent
planet and be given a titanic shove
backwards, resulting in the retrograde
motion Of some of the lost moons"
It is quite possible that some of the
larger asteroids. the host of bodies which
range from small chunks of rock to
masses almost half that of the moon
and which are particularly abundant
between the orbits of Mars and Jupiter,
may be cast-off satellites Of Jupiter,
DR.. Kuiper says. They usually are con-
sidered as fragments of a major planet
which exploded" a billion or more
years ago. Observations now are in
progress which may decide the validity
OF this theory, since the orphans would
be expected to show slightly different
behavior from true asteroids.