LAPLACE'S NEBULAR HYPOTHESIS
LaPlace's nebular hypothesis is the name given to an ingenious
hypothesis proposed by LaPlace, a celebrated French astronomer, in an
endeavor to explain how the solar system has been evolved.
You will notice that this is called a hypothesis and not a theory.
The word hypothesis is properly applied to a more or less intelligent
guess or assumption, that has been made for the purpose of trying
to find out in the cause of any natural phenomenon. A theory is an
expression of a physical truth based on natural laws and principles
that have been independently established. A theory, therefore, is much
more complete than a hypothesis. A hypothesis, as Silliman remarks,
bears the same relation to a theory or law, that a scaffolding does to
a completed building, since it forms a convenient means for erecting
the building. LaPlace's work is properly called a hypothesis, because
it is not to be considered as any more than a means for enabling one
intelligently to inquire into the probable manner in which the solar
system has reached its present condition, by gradual steps or stages
during the almost inconceivable length of time since its creation.
Before describing LaPlace's hypothesis it will be necessary to give
you some ideas concerning what is known by astronomers as the solar
system.
The solar system consists of the sun, and the eight large bodies
called planets that revolve around the sun. It also includes a number
of moons or satellites revolving around the planets, a number of small
bodies, called planetoids or asteroids, together with numerous comets
and meteorites. Besides these there is probably a system of meteoric
bodies that are believed to revolve around the sun, and to produce, by
the reflection of the light from their surfaces, what is known as the
_zodiacal light_.
The principal bodies of the solar system are the planets. These
constitute eight large bodies named in their order from the sun,
beginning with the nearest: Mercury, Venus, Earth, Mars, Jupiter,
Saturn, Uranus, and Neptune. The last four planets, Jupiter, Saturn,
Uranus, and Neptune are much larger than the others, and are therefore
known as the _major planets_ in order to distinguish them from
Mercury, Venus, Earth, and Mars, which are called the _minor planets_.
You can remember the order in which the last three planets come by
their initial letter, S-aturn, U-ranus, and N-eptune, spelling the
word SUN, around which they all revolve.
It may be interesting to state here that the ancients knew of seven
only of these planets. Since, as they asserted, there were only seven
days in the week, and seven openings into the head; i. e., two for the
eyes, two for the nostrils, two for the ears, and one for the mouth,
it was natural that there should be but seven planets. During later
years, however, an eighth planet was discovered and named Neptune. It
would be interesting to explain to you how the position of this planet
was reasoned out by mathematical calculations, that is, in other
words, how, as a result of such calculations, an astronomer was told
that if he would point his telescope to a certain part of the heavens
he would discover a new planet. He did this and located the planet
Neptune. However interesting this story may be it belongs properly
to astronomy, and will be described in full in the Wonder Book of
Astronomy.
In the opinion of some astronomers it is quite probable that a ninth
planet will be found far beyond the orbit of Neptune. There may also
be some additional planets discovered between Mercury and the Sun.
Besides the eight known planets there exist, somewhere between the
orbits of Mars and Jupiter, many smaller planets called _asteroids_,
or _minor planets_. A long time ago it was pointed out by Bode that
a curious relation exists between the distances of the planets from
the sun. This relation or law is generally known, after the name of
the astronomer who first called attention to it, as _Bode's Law_.
No reason has been discovered for this arrangement of the planets,
so that Bode's Law may be regarded as empirical. It may, however,
be mentioned here that the distances of all the planets from the
sun agrees with the law very closely, with the single exception of
Neptune, which is quite at variance with the law.
It was noticed at an early date, that a gap existed between Mars
and Jupiter, so that astronomers began to believe that there was
probably a missing planet in that space, and this belief was greatly
strengthened when Neptune was discovered in 1781. Without going any
further into this story in this book, it may be said that it is the
general opinion of astronomers that the planetoids or asteroids were
formed possibly from the fragments of the missing planet, or, more
probably, from the breaking up of some of the outer rings on the
planet Mars.
The distances of the planets from the central sun vary from the
nearest planet, Mercury, which is about 36,000,000 miles from the sun,
to the furthest, or Neptune, which is 2,766,000,000 miles from the sun.
All the major planets have a single moon, or more, revolving around
them. For example, Jupiter has four moons; Uranus, six; Saturn, eight;
Neptune, one. As to the minor planets, Mars has two moons; and, as far
as is known, neither Mercury or Venus has a moon. Our earth has one
moon, but, as we shall afterwards see, this is not to be regarded as
a moon or satellite of the earth, but rather as a twin planet to the
earth.
LaPlace's nebular hypothesis was proposed by LaPlace during the year
1796. While there are many objections that can be brought against
it, since it fails to account for all of the phenomena of the solar
system, yet it is a significant fact now, in the year 1907, nearly a
century and a quarter after the hypothesis was first announced, that
although modified in many respects, there has not been any hypothesis
proposed to entirely replace it.
While the nebular hypothesis of LaPlace is necessarily a matter that
belongs to astronomy, yet it will be advisable to consider it here,
since it explains the source of the original heat of both the earth
and the moon, which we believe is the true cause of volcanoes.
In his nebular hypothesis, LaPlace assumes that all the materials of
which the solar system is formed, were originally scattered throughout
space in the shape of an exceedingly rare form of matter known as
nebulous matter. He points out that if it be granted that this medium
began to accumulate around a common centre, so as to form a huge globe
or sphere, and if a motion of rotation on its axis from west to east
were given to this sphere that, on strictly mechanical principles, a
system of heavenly bodies corresponding to the solar system might have
been evolved. Let us, therefore, try to understand how this might have
been brought about.
The nebulous matter that LaPlace assumed originally constituted all
the matter in the solar system, was highly heated gaseous matter. In
other words, it consisted of ordinary matter raised to a very high
temperature; LaPlace thought at a temperature very much hotter than
that of the sun.
As this great mass of matter commenced to cool, it began to collect
around a centre and slowly rotate. Its contraction or shrinkage, while
cooling, must have caused an increase in the speed with which it spun
around or rotated on its axis. At first it spun but sluggishly, but as
it cooled and began to shrink this rate of rotation began slowly to
increase.
Now you must bear in mind that the huge rotating mass, as imagined
by LaPlace, was very many times larger than the size of our present
sun. Indeed, instead of having a diameter of only 866,500 miles, its
temperature was so high that the nebulous matter of which it was
composed had expanded it so much that it extended far beyond the orbit
of Neptune, or had a diameter twice as great as 2,766,000,000 miles.
As the huge mass continued to shrink or contract, its rotation
began to gradually increase until at last its centrifugal force was
sufficiently great to cause it to bulge out at the equator, so as
at last to separate a ring of gaseous matter. This ring was left
behind by the sun, as it continued cooling, and formed the first
planet that was born into the solar system. The ring might have
continued to revolve around the sun for a time, and would, of course,
revolve in the same direction as that in which the sun was rotating,
that is, from west to east. Eventually, however, it broke up into
smaller fragments, that afterwards collected in a single body, and,
assuming a globe-like shape of the planet, formed the planet Neptune.
Necessarily, too, the planet so formed not only would revolve in its
orbit from west to east in the same direction in which the sun was
revolving on its axis, but would also rotate or spin on its axis in
the same direction.
After, in this way, throwing off the first planet, the central sun
continued to cool and grow smaller, until the increase in the rate
of its rotation was again such as to permit its centrifugal force to
form a second ring around its equator, which being left as the sun
continued to contract, gave rise to another planet, or to Uranus, and
so on until the four major planets and the four minor planets were
born.
According to this hypothesis, the planet that was first born was the
planet that is farthest from the sun, that is, Neptune, and the planet
last born must have been the nearest planet, Mercury.
But while all this planet forming was going on, the separate planets
also continued to shrink, and, therefore, began to rotate more rapidly
on their axes. Under the influence of the centrifugal force, ring-like
masses began to form around their equators, and these masses left by
the planet constituted their moons or satellites. As you can see,
according to this hypothesis, just as the planets would all revolve in
their orbits from west to east, and rotate on their axes in the same
direction as the sun, so, too, the moons or satellites of the planets
would also rotate on their axes, from east to west, and revolve in
their orbits in the same direction.
In order to show the extent to which LaPlace's nebular hypothesis
explains the peculiarities of the solar system, we must inquire what
are the most important of these peculiarities. We will take these from
Young's general book on Astronomy, from which most of the facts in
this chapter have been condensed. They are as follows:
The orbits of nearly all the planets and their satellites are nearly
circular; they are all in the same plane; and all revolve in the same
direction. They are, moreover, with the single exception of Neptune,
arranged at distances from the sun in accordance with Bode's Law.
All the planets increase in both directions, towards and from the sun,
in density from Saturn, the least dense.
All the planets, with the exception probably of Uranus, rotate in a
plane that is nearly the same as the plane of the orbit in which they
revolve. Moreover, with the exception of probably both Uranus and
Neptune, all the planets rotate in the same direction as that in which
they revolve.
The satellites revolve in orbits whose planes nearly coincide with the
plane of the planets' rotation, while the direction of the revolution
of the satellites is the same as that in which their planets revolve.
Finally, the largest planets rotate most swiftly.
Now, LaPlace's nebular hypothesis explains nearly all of the above
facts. The following modifications of the hypothesis, however, are
necessary. Let us briefly examine some of these modifications.
In the first place it can be shown that the original nebulous mass
instead of being at a higher temperature than that of the sun was
probably at a much lower temperature, since the condensation of
the gaseous matter must have increased the temperature. Instead,
therefore, of the original nebulous mass being purely gaseous it was,
as Young expressed it: "Rather a cloud of ice cold meteoric dust than
an incandescent gas or a fire mist." Or in other words, the original
nebulous mass from which the solar system was evolved, consisted of
finely divided particles of solid or liquid matter surrounded by an
envelope of permanent gaseous matter.
A doubt, too, has been raised as regards the manner in which the
planets were liberated from the central sun. Instead of separating in
the form of a regular ring, it has been thought that probably in most
cases this separation assumed the shape of a lump. It might, however,
have occurred at times in the ring-like form as may be seen in the
case of the planet Saturn.
Again, instead of the outer rings being separated first, and the
others in regular order, so that the outer planets are much the older,
it would seem possible, or, as Young states, even probable, that
several of the planets may be of the same or nearly the same age, as
they would be if more than one ring had been separated at one time,
or, indeed, several planets may have been formed from different zones
of a single ring.
As you will see, LaPlace's nebular hypothesis assumes that both
sun and moon were in a highly heated condition when they were
separated from the nebulous sun, so that we can understand that the
former molten condition of their interiors was due to the heat they
originally possessed.