THE LITTLE PLANETS
Along toward the end of the eighteenth century and the beginning of the
nineteenth, astronomers were leading a quiet unexcited life. Sir William
Herschel had been knighted by King George for his discovery of the outer
planet Uranus, and practically everything seemed to be known and
discovered in the solar system with a single exception. Between Mars and
Jupiter there existed an obvious gap in the planetary brotherhood.
Could it be possible that some time in the remote cosmic past a planet
had actually existed there, and that some celestial cataclysm had blown
it to fragments? If so, would they still be traveling round the sun as
individual small planets? And might it not be possible to discover some
of them among the faint stars that make up the belt of the zodiac in
which all the other planets travel?
So interesting was this question that the first international
association of astronomers banded themselves together to carry on a
systematic search round the entire zodiacal heavens in the faint hope of
detecting possible fragments of the original planet of mere hypothesis.
The astronomers of that day placed much reliance on what is known as
Bode's law--not a law at all, but a mere arithmetical succession of
numbers which represented very well the relative distances of all the
planets from the sun. And the distance of the newly found Uranus fitted
in so well with this law that the utter absence of a planet in the gap
between Mars and Jupiter became very strongly marked.
Quite by accident a discovery of one of the guessed-at small planetary
bodies was made, on January 1, 1801, in Palermo, Sicily, by Piazzi, who
was regularly occupied in making an extensive catalogue of the stars.
His observations soon showed that the new object he had seen could not
be a fixed star, because it moved from night to night among the stars.
He concluded that it was a planet, and named it Ceres (1), for the
tutelary goddess of Sicily.
Other astronomers kept up the search, and another companion planet,
Pallas (2) was found in the following year. Juno (3) was found in 1804,
and Vesta (4), the largest and brightest of all the minor planets, in
1807. Vesta is sometimes bright enough when nearest the earth to be seen
with the naked eye; but it was the last of the brighter ones, and no
more discoveries of the kind were made till the fifth was found in 1845.
Since then discoveries have been made in great abundance, more and more
with every year till the number of little planets at present known is
very near 1,000.
The early asteroid hunters found the search rather tedious, and the
labor increased as it became necessary to examine the increasing
thousands of fainter and fainter stars that must be observed in order to
detect the undiscovered planets, which naturally grow fainter and
fainter as the chase is prolonged. First a chart of the ecliptic sky had
to be prepared containing all the stars that the telescope employed in
the search would show. Some of the most detailed charts of the sky in
existence were prepared in connection with this work, particularly by
the late Dr. Peters of Hamilton College. Once such charts are complete,
they are compared with the sky, night after night when the moon is
absent. Thousands upon thousands of tedious hours are spent in this
comparison, with no result whatever except that chart and sky are found
to correspond exactly.
But now and then the planet hunter is rewarded by finding a new object
in the sky that does not appear on his chart. Almost certainly this is a
small planet, and only a few night's observation will be necessary to
enable the discoverer to find out approximately the orbit it is
traveling in, and whether it is out-and-out a new planet or only one
that had been previously recognized, and then lost track of.
Nearly all the minor planets so far found have had names assigned to
them principally legendary and mythological, and a nearly complete
catalogue of them, containing the elements of their orbits (that is, all
the mathematical data that tell us about their distance from the sun and
the circumstances of their motion around him) is published each year in
the "Annuaire du Bureau des Longitudes" at Paris. But these little
planets require a great deal of care and attention, for some astronomers
must accurately observe them every few years, and other astronomers must
conduct intricate mathematical computations based on these observations;
otherwise they get lost and have to be discovered all over again.
Professor Watson, of the University of Michigan and later of the
University of Wisconsin, endowed the 22 asteroids of his own discovery,
leaving to the National Academy of Sciences a fund for prosecuting this
work perpetually, and Leuschner is now ably conducting it.
[Illustration: JUPITER, LARGEST OF THE PLANETS. The irregular belts
change their mutual relation and shapes because they do not
represent land, but are part of the atmosphere. (_Photo, Yerkes
Observatory._)]
[Illustration: THE PLANET NEPTUNE AND ITS SATELLITE. The photograph
required an exposure of the plate for one hour. (_Photo, Yerkes
Observatory._)]
[Illustration: SATURN, AS SEEN THROUGH THE 40-INCH REFRACTOR, at
the time when only the edge of the rings is visible, showing
condensations. (_Photo, Yerkes Observatory._)]
[Illustration: SATURN, PHOTOGRAPHED THROUGH THE 40-INCH REFRACTOR.
The rings appear opened to the fullest extent they can be seen
from the earth. The picture was made July 7, 1898. (_Photo,
Yerkes Observatory._)]
While the number of the asteroids is gratifyingly large, their
individual size is so small and their total mass so slight that, even if
there are a hundred thousand of them (as is wholly possible), they would
not be comparable in magnitude with any one of the great planets. Vesta,
the largest, is perhaps 400 miles in diameter, and if composed of
substances similar to those which make up the earth, its mass may be
perhaps one twenty-thousandth of the earth's mass. If we calculate the
surface gravity on such a body, we find it about one-thirtieth of what
it is here; so that a rifle ball, if fired on Vesta with a muzzle
velocity of only 2,000 feet a second, might overmaster the gravity of
the little planet entirely and be projected in space never to return.
If, as is likely, some of the smallest asteroids are not more than ten
miles in diameter, their gravity must be so feeble a force that it might
be overcome by a stone thrown from the hand. There is no reliable
evidence that any of the asteroids are surrounded by atmospheric gases
of any sort. Probably they are for the most part spherical in form,
although there is very reliable evidence that a few of the asteroids,
being variable in the amount of sunlight that they reflect, are
irregular in form, mere angular masses perhaps.
The network of orbits of the asteroids is inconceivable complicated.
Nevertheless, there is a wide variation in their average distance from
the sun, and their periods of traveling round him vary in a similar
manner, the shortest being only about three years. While the longest is
nearly nine years in duration, the average of all their periods is a
little over four years. The gap in the zone of asteroids, at a distance
from the sun equal to about five-eighths that of Jupiter, is due to the
excessive disturbing action of Jupiter, whose periodic time is just
twice as long as that of a theoretical planet at this distance.
The average inclination of their orbits to the plane of the ecliptic is
not far from 8 degrees. But the orbit of Pallas, for example, is
inclined 35 degrees, and the eccentricities of the asteroid orbits are
equally erratic and excessive. Both eccentricity and inclination of
orbit at times suggest a possible relation to cometary orbits, but
nothing has ever been definitely made out connecting asteroids and
comets in a related origin.
No comprehensive theory of the origin of the asteroid group has yet been
propounded that has met with universal acceptance. According to the
nebular hypothesis the original gaseous material, which should have been
so concentrated as to form a planet of ordinary type, has in the case of
the asteroids collected into a multitude of small masses instead of
simply one. That there is a sound physical reason for this can hardly be
denied. According to the Laplacian hypothesis, the nearness of the huge
planetary mass of Jupiter just beyond their orbits produced violent
perturbations which caused the original ring of gaseous material to
collect into fragmentary masses instead of one considerable planet. The
theory of a century ago that an original great planet was shattered by
internal explosive forces is no longer regarded as tenable.
To astronomers engaged upon investigation of distances in the solar
system, the asteroid group has proved very useful. The late Sir David
Gill employed a number of them in a geometrical research for finding the
sun's distance, and more recently the discovery of Eros (433) has made
it possible to apply a similar method for a like purpose when it
approaches nearest to the earth in 1924 and 1931. Then the distance of
Eros will be less than half that of Mars or even Venus at their nearest.
When the total number of asteroids discovered has reached 1,000, with
accurate determination of all their orbits, we shall have sufficient
material for a statistical investigation of the group which ought to
elucidate the question of its origin, and bear on other problems of the
cosmogony yet unsolved. Present methods of discovery of the asteroids by
photography replace entirely the old method by visual observation alone,
with the result that discoveries are made with relatively great ease and
rapidity.