and their satellites cannot be well observed on account of their
great distances from the earth. The indistinctness of surface
markings makes it impossible to determine the period of rotation of
these two outer planets on their axes. It is believed that their
rotation is very rapid, however, as is the case with the other
planets Jupiter and Saturn.
All the planets in the solar system fall naturally into two groups.
Jupiter, Saturn, Uranus, and Neptune, the members of the outer group,
have on the average, diameters ten times as great and, therefore,
volumes one thousand times as great as Mercury, Venus, Earth and
Mars, the members of the inner or terrestrial group.
[Illustration: A. VENUS. B. MARS. C. JUPITER. D. SATURN.
Taken by Prof. E. E. Barnard with the 40-inch telescope of the Yerkes
Observatory, with exception of Saturn, which was taken by Prof.
Barnard on Mt. Wilson.]
NOTE: The reader must bear in mind that the telescopic views of
the four planets have not been reduced to the same scale and so
are not to be compared in size.
The terrestrial planets are the pigmies of the solar system, the
outer planets are the giants. The densities of the planets Mercury,
Venus, Earth and Mars are several times greater than the density
of water. They are all extremely heavy bodies for their size, and
probably have rigid interiors with surface crusts.
The existence of life on Mercury is made impossible by the absence
of an atmosphere. Venus and Mars both have atmospheres and it is
possible that both of these planets may support life. Mars has
probably been the most discussed of all the planets, though Venus is
the Earth's twin planet in size, mass, density and surface gravity,
just as Uranus and Neptune are the twins of the outer group. It
is now believed that water and vegetation exist on Mars. The reddish
color of this planet is supposed to be due to its extensive desert
tracts. The nature of certain peculiar markings on this planet, known
as canals, still continues to be a matter of dispute. It is generally
believed since air, water and vegetation exist on Mars, that some
form of animal life also exists there.
The length of the day on Mars is known very accurately, for the
rareness of its atmosphere permits us to see readily many of its
surface markings. The length of the day is about twenty-four and
one-half hours, and the seasonal changes on Mars strongly resemble
our own, though the seasons on Mars are twice as long as they are
on our own planet since the Martian year is twice as long as the
terrestrial year.
The question of life on Venus depends largely upon the length of the
planet's rotation period. This is still uncertain since no definite
surface markings can be found on the planet by which the period of
its rotation can be determined. So dense is the atmosphere of Venus
that its surface is, apparently, always hidden from view beneath a
canopy of clouds. It is the more general belief that Venus, as well
as Mercury, rotates on its axis in the same time that it takes to
make a revolution around the sun. In this case the same side of the
planet is always turned toward the sun and, as a result, the surface
is divided into two hemispheres--one of perpetual day, the other of
perpetual night.
This peculiar form of rotation in which the period of rotation and
revolution are equal is by no means unknown in the solar system. Our
own moon always keeps the same face turned toward the earth and there
are reasons for believing that some of the satellites of Jupiter and
Saturn rotate in the same manner.
Life on any one of the outer planets is impossible. The density of
these planets averages about the same as the density of the sun,
which is a little higher than the density of water. The density of
Saturn is even less than water. In other words, Saturn would float
in water and it is the lightest of all the planets. It is assumed
from these facts that the four outer planets are largely in a gaseous
condition. They all possess dense atmospheres and, in spite of their
huge size, rotate on their axis with great rapidity. The two whose
rotation periods are known, Jupiter and Saturn, turn on their axis in
about ten hours. On account of this rapid rotation and their gaseous
condition both Jupiter and Saturn are noticeably flattened at the
poles.
The terrestrial planets are separated from the outer group by a wide
gap. Within this space are to be found the asteroid or planetoid
group. There are known to be over nine hundred and fifty of these
minor bodies whose diameters range from five hundred miles for the
largest to three or four miles for the smallest. There are only four
asteroids whose diameters exceed one hundred miles and the majority
have diameters of less than twenty miles. The total mass of the
asteroids is much less than that of the smallest of the planets. It
was believed at one time that these small bodies were fragments of a
shattered planet, but this view is no longer held. The asteroids as
well as the comets and meteors probably represent the material of the
primitive solar nebula that was not swept up when the larger planets
were formed.
With few exceptions the asteroids are only to be seen in large
telescopes and then only as star-like points of light. Most of them
are simply huge rocks and all are necessarily devoid of life since
such small bodies have not sufficient gravitational force to hold an
atmosphere.
The revolution of the planets around the sun and of the satellites
of the planets around the primary planets are performed according to
known laws of motion that make it possible to foretell the positions
of these bodies years in advance. Asteroids and comets also obey
these same laws, and after three observations of the positions of
one of these bodies have been obtained its future movements can be
predicted. All the planets and their satellites are nearly perfect
spheres. They all, with few exceptions, rotate on their axes and
revolve around the sun, or, in the case of moons, around their
primaries, in the same direction, from west to east. Only the two
outermost satellites of Jupiter, the outermost satellites of Saturn
and the satellites of Uranus and Neptune retrograde or travel in
their orbits from east to west, which is opposite to the direction of
motion of all the other planets and satellites.
The paths of all the planets around the sun are ellipses that are
nearly circular, and they all lie nearly in the same plane. The
asteroids have orbits that are more flattened or elliptical and these
orbits are in some instances highly inclined to the planetary orbits.
The comets have orbits that are usually very elongated ellipses or
parabolas. Some of the comets may be only chance visitors to our
solar system, though astronomers generally believe that they are all
permanent members. Paths of comets pass around the sun at all angles
and some comets move in their orbits from west to east, while others
move in the opposite direction or retrograde. The behavior of the
asteroids and comets is not at all in accord with the theory that
was, until recently, universally advanced to explain the origin of
the solar system.
Some astronomers have made attempts to modify the nebular hypothesis
that has held sway for so many years, in order to make it fit in with
more recent discoveries, but others feel that a new theory is now
required to explain the origin of the solar system. Several theories
have been advanced but no new theory has yet definitely replaced the
famous nebular hypothesis of the noted French astronomer La Place.
XVIII
THE ORIGIN OF THE EARTH
It is not possible to consider the question of the origin of the
earth apart from the question of the origin of the solar system.
That all the planets, as well as the asteroids, originated from a
common parent-mass has never been seriously questioned. All of these
bodies revolve about the sun, and rotate upon their axes in the same
direction--from west to east. Moreover, all of the planetary orbits
lie very nearly in the same plane and are nearly circular in form.
The orbits of the asteroids are more elliptical and more highly
inclined to one another than are the orbits of the planets, but on
the average they are neither very elliptical nor very highly inclined
to the planetary orbits.
The sun rotates upon its axis in the same direction in which the
planets rotate and perform their revolutions, and the orbits of
the planets are inclined at small angles to the plane of the sun's
equator.
These facts are all significant and cannot be overlooked in
formulating a theory to explain the origin of the planetary system in
general and of the earth in particular. Presumably the planets and
asteroids formed at one time a part of a central body which rotated
on its axis in the direction in which they now revolve about the sun.
When and by the operation of what force, external or internal, they
were separated from this central body is the question.
In 1796 La Place advanced his celebrated _nebular hypothesis_ to
explain the origin of the solar system. It was received with favor
both by scientists and laymen, and in a short time was almost
universally accepted as closely approximating to the truth.
According to the nebular hypothesis the solar nebula from which the
planetary system was formed, originally extended at least as far as
the orbit of Neptune and rotated slowly in the direction in which
the planets now revolve. As it lost heat by radiation and contracted
under the gravitation of its parts its rate of rotation increased.
When the centrifugal (center-fleeing) force at the equator equalled
the gravitational force directed toward the center, a ring would
be left behind by the contracting nebula. Such a ring would not be
absolutely uniform and would break at some point and gather into a
planetary mass under the gravitation of its parts. This planetary
mass would abandon rings in turn and these would break up to form
satellites. Successive rings were supposed to have been abandoned at
intervals by the solar nebula at the present distances of the planets
from the sun in the manner described above until the original solar
nebula had contracted to its present size.
The rings of Saturn were supposed to be the single example remaining
of this process of forming planets and satellites from a _contracting
nebulous mass_.
The La Placian hypothesis attempted to explain why all the planets
and their satellites revolve in the same direction in which the sun
turns on its axis, in nearly circular orbits and nearly in the same
plane. At the time it was advanced it appeared to be in accord with
all the facts then known regarding the solar system.
The planetoids with their interlacing and in some instances highly
inclined and elliptical orbits were then undiscovered. It would have
been impossible for them to have been formed by the abandonment of
successive rings from a central, rotating mass.
The constitution of Saturn's rings was unknown at this time; also
the fact that the moonlets of the inner ring revolve about Saturn in
_half_ the time required for the planet to turn on its axis--another
impossibility under the nebular hypothesis, for, according to the
assumptions of the nebular hypothesis it would be impossible for a
satellite to revolve about a central body in a shorter time than that
body turns on its axis.
The satellites of Mars were not discovered until many years later,
as well as the retrograding satellites of Jupiter and Saturn,
all presenting difficulties in the way of accepting the nebular
hypothesis without radical changes. Attempts, mostly unsuccessful,
have been made from time to time to make these exceptional cases fit
in with the requirements of the nebular hypothesis.
The theory that the sun's heat was maintained by the contraction of
the original solar nebula, which would cause its temperature to rise,
appeared to give considerable support to the theory of La Place, but
the mathematicians got to work and showed that the amount of heat
that would be furnished by the contraction of the sun from beyond
the orbit of Neptune to its present dimensions would be sufficient
to supply heat to the earth at the present rate for only twenty-five
million years, a period far too brief, the geologists and biologists
said, to cover all the vast cyclical changes that are known to have
taken place upon the surface of this planet since its surface crust
was formed. Evidently gravitational contraction is by no means the
only or even the chief source of the sun's heat.
It was also shown indisputably, that it would have been impossible
for successive rings to have been abandoned at certain definite
intervals by a contracting nebula, and granted a ring could have
been formed it would have been impossible for it to condense into a
planet, since forces residing in the sun would offset the gravitation
of its parts.
When La Place advanced his famous theory it was, to use his own
words, "with that distrust which everything ought to inspire that is
not a result of observation or of calculation."
Were La Place living today he would be, we believe, the first to
abandon a theory that is now known to be in accord neither with
observation nor calculation.
Deprived of a theory that has served to explain the outstanding
features of the solar system more or less adequately for one hundred
and twenty-five years, astronomers are seeking in the light of recent
observations and discoveries to formulate a satisfactory theory of
the origin of the solar system.
In the planetesimal theory of Chamberlin and Moulton and the recently
suggested theory of the well-known English mathematician, Jeans, _a
second sun passing close to our own sun is assumed to have been the
cause of the origin of the planetary system_.
The effect of the close approach of such a sun would be the
ejection of a stream of matter from our sun, as we term it, in the
direction of the passing body and also in a diametrically opposite
direction. This ejection would be continuous as long as the stars
remained near one another, the height attained by the ejected stream
decreasing as the passing star receded. The result would be the
formation of a _spiral nebula_ in which the motion of the ejected
particles--planetesimals--would be across the spiral arms, toward
and away from the passing star. After the sun had receded so far as
to have no further effect upon these ejected particles they would
revolve about the sun in more or less elliptical orbits which would
gradually be reduced to nearly circular forms by repeated collisions
between planetesimals. Larger nuclei would be formed and these would
gradually sweep up smaller fragments and become the planets of the
present system. Smaller nuclei in the vicinity of larger ones would
become their satellites and in the course of many millions of years
all of the larger fragments would be swept up by the planetary nuclei
and their satellites--leaving only the asteroids, comets and meteors
as survivors of the original spiral system.
It must be borne in mind that a spiral nebula formed by the close
approach of two suns would resemble in form only the great spiral
nebulæ that are known to exist by hundreds of thousands in the
heavens. These are far too extensive to form anything so small as a
single solar system, but might condense into systems composed of many
suns--either galaxies or star clusters.
Jean's suggested theory of the origin of the planetary system differs
in its details from the above, though a passing sun is assumed to
be the disturbing force that causes the ejection of a stream of
matter which condenses to form the planets and their satellites. The
origin of the inner planets is left greatly in doubt by this theory,
however, and the system which interests us chiefly--the earth-moon
system--is the one about which it is most difficult to arrive at any
definite conclusion. Our own sun, it is assumed, was dark and cold,
of low density and with a diameter about equal to that of Neptune's
orbit at the time of the catastrophe which is placed at some
300,000,000 years ago. In Jean's words, "... The time for arriving
at conclusions in cosmogony has not yet come--and it must be left to
future investigators armed with more mathematical and observational
knowledge than we at present possess to pronounce a final decision."
However, since La Place advanced his celebrated nebular hypothesis,
great advances in astronomy have been made, and man is in a better
position to theorize on this fascinating problem today than he was
one hundred and twenty-five years ago.
All such theories must necessarily be regarded as working hypotheses
only, to be discarded or modified as our knowledge and understanding
of the laws of the universe increase. No theory can ever be regarded
as final or perfect.
The discovery of radio-activity furnishes us with new material
for new theories. The sun and the planets may be and probably are
far older than we ever dreamed could be possible. It is no longer
necessary or reasonable to assume that a greatly extended solar
nebula once existed and supplied the planets with heat through
gravitational contraction or to place a time limit upon the period
required for the formation of the planets and their satellites that
is not in accord with the requirements of other sciences.
We know today that there exist within the sun powerful repulsive
forces, which even under present conditions occasionally eject
gaseous matter to heights of five hundred thousand miles or more with
a velocity of over two hundred miles per second. Small changes in the
velocity of ejection produce great differences in the height of the
ejected columns.
With an initial velocity of three hundred and eighty miles per
second, matter would be thrown from the solar surface to a height
of fifty million miles. Were the velocity of ejection three hundred
and eighty-three miles per second the height of the column would be
five hundred million miles, while a further increase in the initial
velocity would send matter away from the sun, never to return.
Instead of suns and solar systems evolved from nebulæ we are now
more familiar with the idea of nebulæ evolved from stars through some
terrific cataclysm as in the case of novas or temporary stars.
It is now known that there exist in certain parts of space a number
of sharply defined stars surrounded by extensive nebulous envelopes.
Are these possibly suns that are going through the process of forming
their planetary systems?
It is now known that pressure of light and electrical repulsion are
forces to be reckoned with in the evolution of stars and nebulæ as
well as gravitational contraction. It has long been felt that the
peculiar formations existing among the vast irregular gaseous nebulæ
could not be explained as gravitational effects alone.
_Light-pressure and electrical repulsion_, as well as _gravitation_
are at work within the solar system and the sun is the seat of
powerful disturbances which produce periodic outbursts of exceptional
activity and which may have produced in the distant past more
startling effects than any with which we are familiar at present.
The earth and moon form a system that is in a way unique. No
satellite in the solar system is so large in proportion to its
primary as is our own moon. Seen from the distance of Venus or Mars,
the two bodies would apparently form a _double star_. The diameter of
the moon is one-fourth that of the earth. Satellite III of Jupiter
far exceeds our own moon in actual size but its diameter is only
about four-hundredths of the diameter of the planet about which
it revolves. The diameter of Titan, the largest satellite of the
Saturnian system, bears the same ratio to the diameter of Saturn.
Moreover, all the nearer satellites of Jupiter and Saturn lie nearly
in the equatorial planes of these planets, but the orbit of the moon
is inclined at a high angle to the plane of the earth's equator.
It is not difficult to believe that the satellites of Jupiter or
Saturn were at some time thrown off from the equatorial belts of
their primaries, just as the planets themselves may have been ejected
from the equatorial belt of the sun, but we cannot so readily believe
that our own satellite was formed from the earth in a similar manner.
The moon's orbit lies nearly in the plane of the sun's equator,
however, and it is conceivable that both earth and moon were
simultaneously ejected from the equatorial zone of the sun, the two
nuclei being so close together that the smaller one remained under
the gravitational control of the larger.
The difficulties in the way of believing that the moon once formed
a part of the earth are very great. It can be shown mathematically
that if the two bodies at one time formed a single mass it would have
been impossible for the moon to break away from the earth, unless the
force that caused the separation were sufficient to hurl the moon to
a greater distance than two and a half times the earth's radius. The
mathematician, Roche, found out by computation that a satellite could
not remain intact within this distance of the planet, but would be
broken up into small fragments under the effects of the tides raised
by the larger body. If, then, the moon had originally been ejected
from the earth to a less distance than two and one-half radii of the
earth (2.44 to be exact) it would have been disintegrated into small
particles, or moonlets, under the tidal strains exerted upon it by
the earth and would have been gradually distributed about the earth
in the form of a meteoric ring which, in the course of ages, would be
absorbed by the earth, just as Saturn is now gradually absorbing its
rings.
The planets differ greatly in density. The more distant and larger
planets--Jupiter, Saturn, Uranus and Neptune--have densities
equal to or less than that of the sun. The densities of the inner
planets--Mercury, Venus, Earth and Mars--are, relatively, extremely
high, the density of the Earth's core being about that of meteoric
iron. The densities of Mercury and Venus are slightly less than that
of the earth and the densities of Mars and the moon about equal to
that of the earth's crust.
If a stream of matter were ejected from the sun under the influence
of some external force, such as that exerted by a passing star, the
outlying parts of the stream would consist of the lighter elements
and the lower parts of the heavier elements, since the lighter solar
elements lie at or near the surface of the sun and the heavier
elements at greater depths. At the time of ejection the lighter
elements would be thrown to great distances and would go to form the
less dense outer planets; the heavier elements would go to form the
inner planets of high density.
It is conceivable that ejection of solar material might have taken
place under the influence of certain forces at work within the sun
itself, such as electrical repulsion or pressure of light which might
become powerful enough under certain conditions to overcome the
effect of gravitation.
Next to nothing is known about the physical state of matter at great
solar depths, where abnormal conditions of temperature and pressure
must exist, and where great physical changes and disturbances may
have taken place in the past. Even today solar activity goes through
a cycle of change during the sun-spot period, and many millions of
years ago the sun-spot cycle of solar activity may have been far
different from what it is today and a far more powerful factor in
producing changes in the solar system.
Outbursts of novas indicate that agencies making for peace and
order are not the only ones at work among the stars. The cause of
such outbursts has never been satisfactorily explained. The theory
that they are caused by the close approach of two suns or by the
encounter of a star with a dark nebula does not account for all of
the circumstances of such outbursts. The nebulous matter seen about
a nova after the outburst is now generally believed to have been
expelled from the star itself at the time of the catastrophe and may
conceivably be the stuff of which planetary systems are made.