THE EVOLUTION OF THE SOLAR SYSTEM.
The Primæval Nebula—A Planetary Nebula—The Progress of its
Evolution—Unsymmetrical Contraction—Centres of Condensation—The Form
ultimately assumed—Difference between Small Bodies and Large—Earth
and Sun—Acceleration of Velocities—Formation of the Subordinate
Systems—Special Circumstances in the case of the Earth and Moon—Vast
Scale of the Spirals—Spectra of the Spiral Nebulæ.
WE shall consider in this chapter what we believe to have been the
history of that splendid system, formed by the planets under the
presiding control of the sun. The ground over which we have already
passed will prepare us for the famous doctrine that the sun, the planets
and their satellites, together with the other bodies which form the
group we call the solar system, have originated from the contraction of
a primæval nebula.
As the ages rolled by, this great primæval nebula began to undergo
modification. In accordance with the universal law which we find obeyed
in our laboratories, and which we have reason to believe must be equally
obeyed throughout the whole extent of space, this nebula, if warmer than
the surrounding space, must begin to radiate forth its heat. We are to
assume that the nebula does not receive heat from other bodies, adequate
to compensate for that which it dissipates by radiation. There is thus a
loss of heat and consequently the nebula must begin to contract. Its
material must gradually draw together, and must do so under the
operation of those fundamental laws which we have explained in the last
chapter.
The contraction, or rather the condensation, of the material would of
course generally be greatest at the central portion of the nebula. This
is especially noticeable in the photograph of the great spiral already
referred to. But in addition to this special condensation at the centre,
the concentration takes place also, though in a lesser degree, at many
other points throughout the whole extent of the glowing mass. Each
centre of condensation which in this way becomes established tends
continually to increase. In consequence of this law, as the great nebula
contracted and as the great bulk of the material drew in towards the
centre, there were isolated regions in the nebula which became
subordinate centres of condensation. Perhaps in the primæval nebula,
from which the solar system originated, there were half-a-dozen or more
of these centres that were of conspicuous importance, while a much
larger number of small points were also distinguished from the
surrounding nebula. (Figs. 40 and 41.) And still the contraction went
on. The heat, or rather the energy with which the nebula had been
originally charged, was still being dissipated by radiation. We give no
estimate of the myriads of years that each stage of the mighty process
must have occupied. The tendency of the transformation was, however,
always in one direction. It did at last result in a great increase of
the density of the substance of the nebula, both in the central regions
as well as in the subordinate parts. In due time this increase in
density had reached such a point that the materials in the condensing
centres could be no longer described as retaining the gaseous form.
But though heat was incessantly being radiated from the great nebula, it
did not necessarily follow that the nebula was itself losing
temperature. This is a seeming paradox to which we have already had
occasion to refer in Chapter VI. We need not now further refer to it
than to remember that, in speaking of the loss of heat from the nebula,
it would sometimes not be correct to describe the operation as that of
cooling. Up to a certain stage in the condensation, the loss of heat
leads rather to an augmentation of temperature than to its decline.
We are thus led to see how the laws of heat, after being in action on
the primitive nebula for a period of illimitable ages, have at last
effected a marvellous transformation. That nebula has condensed into a
vast central mass with a number of associated subordinate portions. We
may suppose that the original nebula in the course of time does
practically disappear. It is absorbed by the attraction of those
ponderous centres which have gradually developed throughout its extent.
The large central body, and perhaps some of the other bodies thus
evolved, are at first of so high a temperature that a copious radiation
of heat still goes forth from the system. As they discharge their stores
of heat, the smaller bodies show the effects of loss of heat more
rapidly than those which are larger. It is indeed obvious that a small
body must cool more rapidly than a big one. It is sufficient to note
that the cooling takes place from the surface, and that the bigger the
body the larger the quantity of material that it contains for each unit
of superficial area. If the radius of a sphere be doubled, its volume is
increased eightfold, while its surface is only increased fourfold.
[Illustration: Fig. 38.—THE RING NEBULA IN LYRA (Lick Observatory).
(_From the Royal Astronomical Society Series._)]
Let us now concentrate our attention on two of the bodies which, after
immense ages, have been formed from the condensation of the primæval
nebula. Let one of the two bodies be that central object, which
preponderates so enormously that its mass is a thousandfold that of all
the others taken together. Let the other be one of the smaller bodies.
As it parts with its heat, the smaller body, which has originally
condensed from the nebula, will assume some of the features of a mass of
molten liquid. From the liquid condition, the body will pass with
comparative rapidity into a solid state, at least on its outer parts.
The exterior of this body will therefore become solid while the interior
is still at an excessively high temperature. The outer material, which
has assumed the solid form, is constituted of the elements with which we
are acquainted, and is in the form of what the geologist would class as
the igneous rocks, of which granite is the best known example. The shell
of hard rocks outside encloses the material which is still heated and
molten inside. Such a crust would certainly be an extremely bad
conductor of heat. The internal heat is therefore greatly obstructed in
its passage outwards to the surface. The internal heat may consequently
be preserved in the interior of the body for an enormously protracted
period, a period perhaps comparable with those immense ages which the
evolution of the body from the primæval nebula has demanded. The smaller
body may have thus attained a condition in which the temperature
reigning on its surface is regulated chiefly by the external conditions
of the space around, while the internal parts are still highly charged
with the primitive heat from the original nebula.
The great central mass, which we may regard as thousands of times
greater than that of the subordinate body, cools much more slowly. The
cooling of this great mass is so enormously protracted in comparison
with that of the smaller body that it is quite conceivable the central
mass may continue to glow with intense fervour for immense ages after
the smaller body has become covered with hard rock.
It will, I hope, be clear that the two bodies to which I am here
alluding are not merely imaginary objects. The small body, which has so
far cooled down that its surface has lost all indication of internal
heat, is of course our earth. The great central mass which still glows
with intense fervour is the sun. Such is in outline the origin of the
sun and the earth as suggested by the nebular theory.
What we have said of the formation of the earth will equally apply to
the evolution of other detached portions of the primitive nebula. There
may be several of these, and they may vary greatly in size. The smaller
they are the more rapidly in general will the superabundant heat be
radiated away, and the sooner will the surface of that planet acquire
the temperature which is determined by the surrounding conditions. There
are, however, many modifying circumstances.
It is essential to notice that the primæval nebula must have had some
initial moment of momentum, unless we are to assume the occurrence of
that which is infinitely improbable. It would have been infinitely
improbable for the system not to have had some moment of momentum
originally. As the evolution proceeds, and as the energy is expended,
while this original endowment of moment of momentum is preserved, we
find, as explained in the last chapter, the system gradually settling
down into proximity to a plane, and gradually acquiring a uniform
direction of revolution. Hence we see that each of the subordinate
masses which ultimately consolidate to form a planet have a motion of
revolution around the central body. In like manner the central body
itself rotates, and all these motions are performed in the same
direction.
In addition to the revolutions of the planets around the sun, there are
other motions which can be accounted for as consequences of the
contraction of the nebula. We now refer to that central portion which is
to form the sun, and consider, in the first instance, only one of the
subordinate portions which is to form a planet. As these two bodies form
part of the same nebulous mass they will to a certain extent rotate
together as one piece. If any body is rotating as a whole, every part of
that body is also in actual rotation. We shall refer to this again later
on; but for the present it is sufficient to observe that as the planet
was originally continuous with the sun, it had a motion of rotation
besides its motion of revolution, and it revolved round its own axis in
a period equal to that of its revolution round the sun. In the beginning
the rotation of the planet was therefore an exceedingly slow movement.
But it became subsequently accelerated. For we have already explained
that each planet is by itself subjected to the law of the conservation
of moment of momentum. As each planet assumes a separate existence, it
draws to itself its share of the moment of momentum, and that must be
strictly preserved. But the planet, or rather the materials which are to
form the future planet, are all the time shrinking; they are drawing
more closely together. If, therefore, the area which each particle of
the planet describes when multiplied by the mass of that particle and
added to the similar products arising from all the other particles, is
to remain constant, it becomes necessary that just as the orbits of
these particles diminish in size, so must the speed at which they
revolve increase. We thus find that there is a tendency in the planet to
accelerate its rotation. And thus we see that a time will come when the
planet, having assumed an independent existence, will be found rotating
round its axis with a velocity which must be considered high in
comparison with the angular velocity which the planet had while it still
formed part of the original nebula.
As the planets have been evolved so as to describe their several orbits
around the sun, so in like manner the smaller systems of satellites have
been so evolved as to describe their orbits round the several planets
that are their respective primaries. When a planet, or rather the
materials which were drawing together to form a planet, had acquired a
predominant attraction for the parts of the primæval nebula in their
locality, a portion of the nebulous material became specially associated
with the planet. As the planet with this nebulous material became
separated from the central contracting sun, or became, as it were, left
behind while the sun was drawing into itself the material which
surrounded it the planet and its associated nebula underwent on a
miniature scale an evolution similar to that which had already taken
place in the formation of the sun and the planets as a whole. In this
manner secondary systems seem sometimes to have had their origin.
We should, however, say that though what we have here indicated appears
to explain fully the evolution of some of the systems, such, for
instance, as that of Jupiter and his four moons, or Saturn and his eight
or nine, the circumstances with regard to the earth and the moon are
such as to require a very different explanation of the origin of our
satellite. In the first place we may notice that the great mass of the
moon, in comparison with the earth, is a wholly exceptional feature in
the relations between the planets and their satellites in the other
parts of the system. In no other instance does the mass of a satellite
bear to the mass of the planet a ratio anything like so great as the
ratio of our moon to the earth. The moon has a mass which is about
one-eightieth of the mass of the earth, while even the largest of
Jupiter’s satellites has not one ten-thousandth part of the mass of the
planet itself. The evolution of the earth and moon system has been
brought about in a manner very different from that of the evolution of
the other systems of satellites. We do not here enter into any
discussion of the matter. We merely remind the reader that it is now
known, mainly by the researches of Professor G. H. Darwin, that in all
probability the moon was originally part of the earth, and that a
partition having occurred while the materials of the earth and moon were
still in a plastic state, a small portion broke away to form the moon,
leaving behind the greater mass to form the earth. Then, under the
influence of tides, which may agitate a mass of molten rock, as the moon
was once (Fig. 39), just as they may agitate an ocean, the moon was
forced away, and was ultimately conducted to its present orbit.
[Illustration: Fig. 39.—LUNAR CRATERS: HYGINUS AND ALBATEGNIUS.
(_Photographed by MM. Loewy and Puiseux._)]
It was at first tempting to imagine that a theory which accounted so
satisfactorily for the evolution of the moon from the earth might also
account in a similar manner for the evolution of the earth from the sun.
Had this been the case, it is needless to say that the principles we now
accept in the nebular theory would have needed large modification, if
not actual abandonment. A close examination into the actual statistics
brings forcibly before us the exceptional character of the earth-moon
system. It can be demonstrated that the earth could not have been
evolved from the sun in the same manner as there is every reason to
believe that the moon has been evolved from the earth. The evolution of
the satellites of Jupiter has proceeded along lines quite different from
those of the evolution of the moon from the earth, so that we may,
perhaps, find in the evolution of the satellites of Jupiter an
illustration in miniature of the way in which the planets themselves
have been evolved in relation to the sun.
We must not forget that the only spiral nebulæ which lie within the
reach of our powers of observation, whether telescopic or photographic,
appear to be objects of enormously greater cosmical magnificence than
was that primæval nebula from which so insignificant an object as the
solar system has sprung. The great spirals, so far as we can tell at
present, appear to be thousands of times, or even millions of times,
greater in area than the solar system. At this point, however, we must
speak with special caution, having due regard to the paucity of our
knowledge of a most important element. Astronomers must confess that no
efforts which have yet been made to determine the dimensions of a nebula
have been crowned with success. We have not any precise idea as to what
the distance of the great spiral might be. We generally take for granted
that these nebulæ are at distances comparable with the distances of the
stars. On this assumption we estimate that the spiral nebulæ must
transcend enormously the dimensions of the primæval nebula from which
the solar system has sprung. The spiral nebulæ that have so far come
within our observation seem to be objects of an order of magnitude
altogether higher than a solar system. They seem to be engaged on the
majestic function of evolving systems of stars like the Milky Way,
rather than on the inconsiderable task of producing a system which
concerns only a single star and not a galaxy.
[Illustration: Fig. 40.—A REMARKABLE SPIRAL (n.g.c. 628; in Pisces).
(_Photographed by Dr. Isaac Roberts, F.R.S._)]
The spiral form of structure is one in which Nature seems to delight. We
find it in the organic world allied with objects of the greatest
interest and beauty. The ammonite, a magnificent spiral shell sometimes
exceeding three feet in diameter, belongs to a type which dominated the
waters of the globe in secondary times, and which still survives in the
nautilus. The same form is reproduced in minute creations totally
different from ammonites in their zoological relations. Among the
exquisite foraminifera which the microscopist knows so well may be found
most delicate and beautiful spirals. Just as we see every range of
spiral in the animal world, from an organism invisible to the naked eye,
up to an ammonite a yard or more across, so it would seem that there are
spiral nebulæ ranging from such vast objects as the great spiral in
Canes Venatici down to such relatively minute spirals as those whose
humble function it is to develop a solar system. It is no more than a
reasonable supposition that the great spirals in the heavens are
probably only the more majestic objects of an extremely numerous class.
The smaller objects of this type—among which we might expect to find
nebulæ like, in size and importance, to the primæval nebula of our
system—are so small that they have not yet been recognised.
It should at this stage be mentioned that several curious small
planetary nebulæ have in these modern days been discovered by their
peculiar spectra. If the nebulous character of these most interesting
objects had not been accidentally disclosed by characteristic lines in
their spectra, these undoubted nebulæ would each have been classified
merely as stars. This fact will lead us to the surmise that there must
be myriads of nebulæ in the heavens, too small to come within the range
of our telescopes or of our most sensitive photographic plates. Suppose
that a facsimile of the primæval nebula of our system, precisely
corresponding with it in size and identical with it in every detail,
were at the present moment located in space, but at a distance from our
standpoint, as great as the distance of, let us say, the great spiral;
it seems certain that this nebula, even though it contained the
materials for a huge sun and a potential system of mighty planets, if
not actually invisible to us here, would in all probability demand the
best powers of our instruments to reveal it, and then it would be
classed not as a nebula at all but as a star of perhaps the 12th or
15th, or even smaller magnitude.
[Illustration: Fig. 41.—A CLEARLY CUT SPIRAL (n.g.c. 4321; in Coma
Berenices).
(_Photographed by Dr. Isaac Roberts, F.R.S._)]
It is to be remembered that the class of minute planetary nebulæ make
themselves known solely by the fact that they exhibit the bright line
indicative of gaseous spectra. If these objects (though still nebulæ)
had not displayed gaseous spectra, it is certain they would have escaped
detection, at least by the process which has actually proved so
successful. The continuous band of light which they would then have
presented could not be discriminated from the band of light from a star.
It is therefore not improbable that among the star-like bodies which
have been represented on our photographs, there may be some which are
really minute spiral nebulæ. In general a star is a minute point of
light which no augmentation of telescopic power and no magnification
will show otherwise than as a point, granted only good optical
conditions and good opportunity so far as the atmosphere is concerned.
It has, however, been occasionally noted that certain so-called stars
are not mere points of light; they do possess what is described as a
disc. It is not at all impossible that the objects so referred to are
spiral nebulæ. We may describe them as formed on a small scale in
comparison with the great spiral or the nebula in Andromeda. But the
smallness here referred to is only relative. They are in all probability
quite as vast as the primæval spiral nebula from which the solar system
has been evolved, though not so large as those curious ring-shaped
nebulæ of which the most celebrated example lies in the constellation
Lyra (Fig. 38).
Such is an outline of what we believe to have been the history of our
solar system. We have already given the evidence derived from the laws
of heat. We have now to consider the evidence which has been derived
from the constitution of the system itself. We shall see how strongly it
supports the belief that the origin of sun and planets has been such as
the nebular theory suggests.
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