THE BEGINNING OF THINGS
LAPLACE'S NEBULAR HYPOTHESIS
Dwelling upon the fact that all the motions of revolution and rotation
in the solar system, as known in his day, took place in the same
direction and nearly in the same plane, the great French astronomer,
Laplace, about the year 1796, put forward a theory to account for the
origin and evolution of that system. He conceived that it had come into
being as a result of the gradual contraction, through cooling, of an
intensely heated gaseous lens-shaped mass, which had originally occupied
its place, and had extended outwards beyond the orbit of the furthest
planet. He did not, however, attempt to explain how such a mass might
have originated! He went on to suppose that this mass, _in some manner_,
perhaps by mutual gravitation among its parts, had acquired a motion of
rotation in the same direction as the planets now revolve. As this
nebulous mass parted with its heat by radiation, it contracted towards
the centre. Becoming smaller and smaller, it was obliged to rotate
faster and faster in order to preserve its equilibrium. Meanwhile, in
the course of contraction, rings of matter became separated from the
nucleus of the mass, and were left behind at various intervals. These
rings were swept up into subordinate masses similar to the original
nebula. These subordinate masses also contracted in the same manner,
leaving rings behind them which, in turn, were swept up to form
satellites. Saturn's ring was considered, by Laplace, as the only
portion of the system left which still showed traces of this
evolutionary process. It is even probable that it may have suggested the
whole of the idea to him.
Laplace was, however, not the first philosopher who had speculated along
these lines concerning the origin of the world.
Nearly fifty years before, in 1750 to be exact, Thomas Wright, of
Durham, had put forward a theory to account for the origin of the whole
sidereal universe. In his theory, however, the birth of our solar system
was treated merely as an incident. Shortly afterwards the subject was
taken up by the famous German philosopher, Kant, who dealt with the
question in a still more ambitious manner, and endeavoured to account in
detail for the origin of the solar system as well as of the sidereal
universe. Something of the trend of such theories may be gathered from
the remarkable lines in Tennyson's _Princess_:--
"This world was once a fluid haze of light,
Till toward the centre set the starry tides,
And eddied into suns, that wheeling cast
The planets."
The theory, as worked out by Kant, was, however, at the best merely a
_tour de force_ of philosophy. Laplace's conception was much less
ambitious, for it did not attempt to explain the origin of the entire
universe, but only of the solar system. Being thus reasonably limited in
its scope, it more easily obtained credence. The arguments of Laplace
were further founded upon a mathematical basis. The great place which
he occupied among the astronomers of that time caused his theory to
exert a preponderating influence on scientific thought during the
century which followed.
A modification of Laplace's theory is the Meteoritic Hypothesis of Sir
Norman Lockyer. According to the views of that astronomer, the material
of which the original nebula was composed is presumed to have been in
the meteoric, rather than in the gaseous, state. Sir Norman Lockyer
holds, indeed, that nebulæ are, in reality, vast swarms of meteors, and
the light they emit results from continual collisions between the
constituent particles. The French astronomer, Faye, also proposed to
modify Laplace's theory by assuming that the nebula broke up into rings
all at once, and not in detail, as Laplace had wished to suppose.
The hypothesis of Laplace fits in remarkably well with the theory put
forward in later times by Helmholtz, that the heat of the sun is kept up
by the continual contraction of its mass. It could thus have only
contracted to its present size from one very much larger.
Plausible, however, as Laplace's great hypothesis appears on the
surface, closer examination shows several vital objections, a few of
those set forth by Professor Moulton being here enumerated--
Although Laplace held that the orbits of the planets were sufficiently
near to being in the one plane to support his views, yet later
investigators consider that their very deviations from this plane are a
strong argument against the hypothesis.
Again, it is thought that if the theory were the correct explanation,
the various orbits of the planets would be much more nearly circular
than they are.
It is also thought that such interlaced paths, as those in which the
asteroids and the little planet Eros move, are most unlikely to have
been produced as a result of Laplace's nebula.
Further, while each of the rings was sweeping up its matter into a body
of respectable dimensions, its gravitative power would have been for the
time being so weak, through being thus spread out, that any lighter
elements, as, for instance, those of the gaseous order, would have
escaped into space in accordance with the principles of the kinetic
theory.
_The idea that rings would at all be left behind at certain intervals
during the contraction of the nebula is, perhaps, one of the weakest
points in Laplace's hypothesis._
Mathematical investigation does not go to show that the rings, presuming
they could be left behind during the contraction of the mass, would have
aggregated into planetary bodies. Indeed, it rather points to the
reverse.
Lastly, such a discovery as that the ninth satellite of Saturn revolves
in a _retrograde_ direction--that is to say, in a direction contrary to
the other revolutions and rotations in our solar system--appears
directly to contradict the hypothesis.
Although Laplace's hypothesis seems to break down under the keen
criticism to which it has been subjected, yet astronomers have not
relinquished the idea that our solar system has probably had its origin
from a nebulous mass. But the apparent failure of the Laplacian theory
is emphasised by the fact, that _not a single example of a nebula, in
the course of breaking up into concentric rings, is known to exist in
the entire heaven_. Indeed, as we saw in Chapter XXIV., there seems to
be no reliable example of even a "ring" nebula at all. Mr. Gore has
pointed this out very succinctly in his recently published work,
_Astronomical Essays_, where he says:--"To any one who still persists in
maintaining the hypothesis of ring formation in nebulæ, it may be said
that the whole heavens are against him."
The conclusions of Keeler already alluded to, that the spiral is the
normal type of nebula, has led during the past few years to a new theory
by the American astronomers, Professors Chamberlin and Moulton. In the
detailed account of it which they have set forth, they show that those
anomalies which were stumbling-blocks to Laplace's theory do not
contradict theirs. To deal at length with this theory, to which the name
of "Planetesimal Hypothesis" has been given, would not be possible in a
book of this kind. But it may be of interest to mention that the authors
of the theory in question remount the stream of time still further than
did Laplace, and seek to explain the _origin_ of the spiral nebulæ
themselves in the following manner:--
Having begun by assuming that the stars are moving apparently in every
direction with great velocities, they proceed to point out that sooner
or later, although the lapse of time may be extraordinarily long,
collisions or near approaches between stars are bound to occur. In the
case of collisions the chances are against the bodies striking together
centrally, it being very much more likely that they will hit each other
rather towards the side. The nebulous mass formed as a result of the
disintegration of the bodies through their furious impact would thus
come into being with a spinning movement, and a spiral would ensue.
Again, the stars may not actually collide, but merely approach near to
each other. If very close, the interaction of gravitation will give rise
to intense strains, or tides, which will entirely disintegrate the
bodies, and a spiral nebula will similarly result. As happens upon our
earth, two such tides would rise opposite to each other; and,
consequently, it is a noticeable fact that spiral nebulæ have almost
invariably two opposite branches (see Plate XXII., p 314). Even if not
so close, the gravitational strains set up would produce tremendous
eruptions of matter; and in this case, a spiral movement would also be
generated. On such an assumption the various bodies of the solar system
may be regarded as having been ejected from parent masses.
The acceptance of the Planetesimal Hypothesis in the place of the
Hypothesis of Laplace will not, as we have seen, by any means do away
with the probability that our solar system, and similar systems, have
originated from a nebulous mass. On the contrary it puts that idea on a
firmer footing than before. The spiral nebulæ which we see in the
heavens are on a vast scale, and may represent the formation of stellar
systems and globular clusters. Our solar system may have arisen from a
small spiral.
We will close these speculations concerning the origin of things with a
short sketch of certain investigations made in recent years by Sir
George H. Darwin, of Cambridge University, into the question of the
probable birth of our moon. He comes to the conclusion that at least
fifty-four millions of years ago the earth and moon formed one body,
which had a diameter of a little over 8000 miles. This body rotated on
an axis in about five hours, namely, about five times as fast as it does
at present. The rapidity of the rotation caused such a tremendous strain
that the mass was in a condition of, what is called, unstable
equilibrium; very little more, in fact, being required to rend it
asunder. The gravitational pull of the sun, which, as we have already
seen, is in part the cause of our ordinary tides, supplied this extra
strain, and a portion of the mass consequently broke off, which receded
gradually from the rest and became what we now know as the moon. Sir
George Darwin holds that the gravitational action of the sun will in
time succeed in also disturbing the present apparent harmony of the
earth-moon system, and will eventually bring the moon back towards the
earth, so that after the lapse of great ages they will re-unite once
again.
In support of this theory of the terrestrial origin of the moon,
Professor W.H. Pickering has put forward a bold hypothesis that our
satellite had its origin in the great basin of the Pacific. This ocean
is roughly circular, and contains no large land masses, except the
Australian Continent. He supposes that, prior to the moon's birth, our
globe was already covered with a slight crust. In the tearing away of
that portion which was afterwards destined to become the moon the
remaining area of the crust was rent in twain by the shock; and thus
were formed the two great continental masses of the Old and New Worlds.
These masses floated apart across the fiery ocean, and at last settled
in the positions which they now occupy. In this way Professor Pickering
explains the remarkable parallelism which exists between the opposite
shores of the Atlantic. The fact of this parallelism had, however, been
noticed before; as, for example, by the late Rev. S.J. Johnson, in his
book _Eclipses, Past and Future_, where we find the following passage:--
"If we look at our maps we shall see the parts of one Continent that jut
out agree with the indented portions of another. The prominent coast of
Africa would fit in the opposite opening between North and South
America, and so in numerous other instances. A general rending asunder
of the World would seem to have taken place when the foundations of the
great deep were broken up."
Although Professor Pickering's theory is to a certain degree anticipated
in the above words, still he has worked out the idea much more fully,
and given it an additional fascination by connecting it with the birth
of the moon. He points out, in fact, that there is a remarkable
similarity between the lunar volcanoes and those in the immediate
neighbourhood of the Pacific Ocean. He goes even further to suggest that
Australia is another portion of the primal crust which was detached out
of the region now occupied by the Indian Ocean, where it was originally
connected with the south of India or the east of Africa.
Certain objections to the theory have been put forward, one of which is
that the parallelism noticed between the opposite shores of the Atlantic
is almost too perfect to have remained through some sixty millions of
years down to our own day, in the face of all those geological movements
of upheaval and submergence, which are perpetually at work upon our
globe. Professor Pickering, however, replies to this objection by
stating that many geologists believe that the main divisions of land and
water on the earth are permanent, and that the geological alterations
which have taken place since these were formed have been merely of a
temporary and superficial nature.