THEORY OF THE SOLAR SYSTEM.
1. We have given our views respecting the various planets which
constitute the Solar System;--views established, it would seem, by all
that we know, of the laws of heat and moisture, density and attraction,
organization and life. We have examined and reasoned upon the cases of
the different planets separately. But it may serve to confirm this view,
and to establish it in the reader's mind, if we give a description of
the system which shall combine and connect the views which we have
presented, of the constitution and peculiarities, as to physical
circumstances, of each of the planets. It will help us in our
speculations, if we can regard the planets not only as a collection, but
as a scheme;--if we can give, not an enumeration only, but a theory. Now
such a scheme, such a theory, appears to offer itself to us.
2. The planets exterior to Mars, Jupiter, and Saturn especially, as the
best known of them, appear, by the best judgment which we can form, to
be spheres of water, and of aqueous vapor, combined, it may be, with
atmospheric air, in which their cloudy belts float over their deep
oceans. Mars seems to have some portion at least of aqueous atmosphere;
the earth, we know, has a considerable atmosphere of air, and of vapor;
but the Moon, so near to her mistress, has none. On Venus and Mercury,
we see nothing of a gaseous or aqueous atmosphere; and they, and Mars,
do not differ much in their density from the Earth. Now, does not this
look as if the water and the vapor, which belong to the solar system,
were driven off into the outer regions of its vast circuit; while the
solid masses which are nearest to the focus of heat, are all
approximately of the same nature? And if this be so, what is the
peculiar physical condition which we are led to ascribe to the Earth?
Plainly this: that she is situated just in that region of the system,
where the existence of matter, both in a solid, a fluid, and a gaseous
condition, is possible. Outside the Earth's orbit, or at least outside
Mars and the small Planetoids, there is, in the planets, apparently, no
solid matter; or rather, if there be, there is a vast preponderance of
watery and vaporous matter. Inside the Earth's orbit, we see, in the
planets, no traces of water or vapor, or gas; but solid matter, about
the density of terrestrial matter. The Earth, alone, is placed at the
border where the conditions of life are combined; ground to stand upon;
air to breathe; water to nourish vegetables, and thus, animals; and
solid matter to supply the materials for their more solid parts; and
with this, a due supply of light and heat, a due energy of the force of
weight. All these conditions are, in our conception, requisite for life:
that all these conditions meet, elsewhere than in the neighborhood of
the Earth's orbit, we see strong reasons to disbelieve. The Earth, then,
it would seem, is the abode of life, not because all the globes which
revolve round the Sun may be assumed to be the abodes of life; but
because the Earth is fitted to be so, by a curious and complex
combination of properties and relations, which do not at all apply to
the others. That the Earth is inhabited, is not a reason for believing
that the other Planets are so, but for believing that they are not so.
3. Can we see any physical reason, for the fact which appears to us so
probable, that all the water and vapor of the system is gathered in its
outward parts? It would seem that we can. Water and aqueous vapor are
driven from the Sun to the outer parts of the solar system, or are
allowed to be permanent there only, as they are driven off and retained
at a distance by any other source of heat;--to use a homely
illustration, as they are driven from wet objects placed near the
kitchen-fire: as they are driven from the hot sands of Egypt into the
upper air: as they are driven from the tropics to the poles. In this
latter case, and generally, in all cases, in which vapor is thus driven
from a hotter region, when it comes into a colder, it may again be
condensed in water, and fall in rain. So the cold of the air in the
temperate zone condenses the aqueous vapors which flow from the tropics;
and so, we have our clouds and our showers. And as there is this rainy
region, indistinctly defined, between the torrid and the frigid zones on
the earth; so is there a region of clouds and rain, of air and water,
much more precisely defined, in the solar system, between the central
torrid zone and the external frigid zone which surrounds the Sun at a
greater distance.
4. _The Earth's Orbit is the Temperate Zone of the Solar System._ In
that Zone only is the play of Hot and Cold, of Moist and Dry, possible.
The Torrid Zone of the Earth is not free from moisture; it has its
rains, for it has its upper colder atmosphere. But how much hotter are
Venus and Mercury than the Torrid Zone? There, no vapors can linger;
they are expelled by the fierce solar energy; and there is no cool
stratum to catch them and return them. If they were there, they must fly
to the outer regions; to the cold abodes of Jupiter and Saturn, if on
their way, the Earth did not with cold and airy finger outstretched
afar, catch a few drops of their treasures, for the use of plant, and
beast, and man. The solid stone only, and the metallic ore which can be
fused and solidified with little loss of substance, can bear the
continual force of the near solar fire, and be the material of permanent
solid planets in that region. But the lava pavement of the Inner Planets
bears no superstructure of life; for all life would be scorched away
along with water, its first element. On the Earth first, can this
superstructure be raised; and there, through we know not what graduation
of forms, the waters were made to bring forth abundantly things that had
life; plants, and animals nourished by plants, and conspiring with them,
to feed on their respective appointed elements, in the air which
surrounded them. And so, nourished by the influences of air and water,
plants and animals lived and died, and were entombed in the scourings of
the land, which the descending streams carried to the bottom of the
waters. And then, these beds of dead generations were raised into
mountain ranges; perhaps by the yet unextinguished forces of
subterraneous fires. And then a new creation of plants and animals
succeeded; still living under the fostering influence of the united
pair, Air and Water, which never ceased to brood over the World of Life,
their Nurseling; and then, perhaps, a new change of the limits of land
and water, and a new creation again: till at last, Man was placed upon
the Earth; with far higher powers, and far different purposes, from any
of the preceding tribes of creatures: and with this, for one of his
offices;--that there might be an intelligent being to learn how
wonderfully the scheme of creation had been carried on, and to admire,
and to worship the Creator.
5. But we have a few more remarks to make on the structure of the Solar
System, in this point of view. When we say that the water and vapor of
the System were driven to the outer parts, or retained there, by the
central heat of the Sun, perhaps it might be supposed to be most simple
and natural, that the aqueous vapor, and the water, should assume its
place in a distinct circle, or rather a spherical shell, of which the
Sun was the centre; thus making an elemental sphere about the centre,
such as the ancients imagined in their schemes of the Universe. Nor will
we venture to say that such an arrangement of elements might not be;
though perhaps it might be shown that no stable equilibrium of the
system would be, in this way, mechanically possible. But this at least
we may say; that a rotatory motion of all the parts of the universe
appears to be a universal law prevalent in it, so far as our observation
can reach: and that, by such rotation of the separate masses, the whole
is put in a condition which is everywhere one of stable equilibrium. It
was, then, agreeable to the general scheme, that the excess of water and
vapor, which must necessarily be carried away, or stored up, in the
outer regions of the System, should be put into shapes in which it
should have a permanent place and form. And thus, it is suitable to the
general economy of creation, that this water and vapor should be packed
into rotating masses, such as are Jupiter and Saturn, Uranus and
Neptune. When once collected in such rotating masses, the attraction of
its parts would gather it into spheroidal forms; oblate by the effect of
rotation, as Jupiter, or perhaps into annular forms, like the Ring of
Saturn;[1] for such also is a mechanically possible form of equilibrium,
for a fluid mass. And these spheroids once formed, the water would form
a central nucleus, over which would hang a cover of vapor, raised by
the evaporating power of the Sun, and forming clouds, where the rarity
of the upper strata of vapor allowed the cold of the external space to
act; and these clouds, spun into belts by the rotation of the sphere.
And thus, the vapor, which would otherwise have wandered loose about the
atmosphere, was neatly wound into balls; which, again, were kept in
their due place, by being made to revolve in nearly circular orbits
about the Sun.
6. And thus, according to our view, water and gases, clouds and vapors,
form mainly the planets in the outer part of the solar system; while
masses such as result from the fusion of the most solid materials, lie
nearer the sun, and are found principally within the orbit of
Jupiter.[2] To conceive planetary systems as formed by the gradual
contraction of a nebular mass, and by the solidification of some of its
parts, is a favorite notion of several speculators. If we adopt this
notion, we shall, I think, find additional proofs in favor of our view
of the system. For, in the first place, we have the zodiacal light, a
nebulous appendage to the Sun, as Herschel conceives, extending beyond
the orbits of Mercury and Venus. These planets, then, have not yet fully
emerged from the atmosphere in which they had their origin:--the
_mother-light_ and _mother-fire_, in which they began to crystallize, as
crystals do in their mother-water. Though they are already opaque, they
are still immersed in luminous vapor: and bearing such traces of their
chaotic state being not yet ended, we need not wonder, if we find no
evidence of their having inhabitants, and some evidence to the contrary.
They are within a nebular region, which may easily be conceived to be
uninhabitable. And where this nebular region, marked by the zodiacal
light, terminates, the world of life begins, namely at the Earth.
7. But further, outside this region of the Earth, what do we find in the
solar system? Of solid matter, if our views are right, we find nothing
but an immense number of small bodies; namely, first, Mars, who, as we
have said, is only about one-eighth the earth in mass: the twenty-six
small planetoids, (or whatever number may have been discovered when
these pages meet the reader's eye,[3]) between Mars and Jupiter; the
four satellites of Jupiter; the eight satellites of Saturn; the six (if
that be the true number,) satellites of Uranus; and the one satellite of
Neptune, already detected. It is very remarkable, that all this array of
small bodies begins to be found just outside the Earth's orbit.
Supposing, as we have found so much reason to suppose, that Jupiter, and
the other exterior planets, are not solid bodies, but masses of water
and of vapor; the existence of great solid planetary masses, such as
exist in the region of the Earth's orbit, is succeeded externally by the
existence of a vast number of smaller bodies. The real quantity of
matter in these smaller bodies we cannot in general determine. Perhaps
the largest of them, (after Mars,) may be Jupiter's third satellite;
which[4] is reckoned, by Laplace, to have a mass less than 1-10,000th of
that of Jupiter himself; and thus, since Jupiter, as we have seen, has a
mass 333 times that of the Earth, the satellite would be above 1-30th of
the Earth's mass.[5] That none but masses of this size, and many far
below this, are found outside of Mars, appears to indicate, that the
_planet-making_ powers which were efficacious to this distance from the
sun, and which produced the great globe of the Earth, were, beyond this
point, feebler; so that they could only give birth to smaller masses; to
planetoids, to satellites, and to meteoric stones. Perhaps we may
describe this want of energy in the planet-making power, by saying, that
at so great a distance from the central fire, there was not heat enough
to melt together these smaller fragments into a larger globe;[6] or
rather, when they existed in a nebular, perhaps in a gaseous state, that
there was not heat enough to keep them in that state, till the
attraction of the parts of all of them had drawn them into one mass,
which might afterwards solidify into a single globe. The tendency of
nebular matter to separate into distinct portions, which may afterwards
be more and more detached from each other, so as to break the nebulous
light into patches and specks, appears to be seen in the structure of
the resolvable nebulae, as we have already had occasion to notice. And
according to the view we are now taking, we may conceive such patches,
by further cooling and concentration, to remain luminous as comets, and
perhaps shooting stars; or to become opaque as planets, planetoids,
satellites, or meteoric stones. And here we may call to mind what we
have already said, that the meteoric stones consist of the same elements
as those of the earth, combined by the same laws; and thus appear to
bring us a message from the other solid planets, that they also have the
same elements and the same chemical forces as the earth has.
8. It has already been supposed, by many astronomers, that shooting
stars, and meteoric stones, are bodies of connected nature and origin;
and that they are cosmical, not terrestrial bodies;--parts of the solar
system, not merely appendages to the earth. It has been conceived, that
the luminous masses, which appear as shooting stars, when they are
without the sphere of terrestrial influences, may, when they reach our
atmosphere, collapse into such solid lumps as have from time to time
fallen upon the earth's surface: many of them, with such sudden
manifestations of light and heat, as implied some rapid change taking
place in their chemical constitution and consistence. If shooting stars
are of this nature, then, in those cases in which a great number of them
appear in close succession, we have evidence that there is a region in
which there is a large collection of matter of a nebulous kind,
collected already into small clouds, and ready, by any additional touch
of the powers that hover round the earth, to be further consolidated
into planetary matter. That the earth's orbit carries her through such
regions, in her annual course, we have evidence, in the curious fact,
now so repeatedly observed, of showers of shooting stars, seen at
particular seasons of every year; especially about the 13th of November,
and the 10th of August. This phenomenon has been held, most reasonably,
to imply that at those periods of the year, the earth passes through a
crowd of such meteor-planets, which form a ring round the sun; and
revolving round him, like the other planets, retain their place in the
system from year to year.[7] It may be that the orbits of these
meteor-planets are very elliptical. That they are to a certain extent
elliptical, appears to be shown, by our falling in with them only once a
year, not every half year, as we should do, if their orbit, being nearly
circular, met the earth's orbit in two opposite points. That the
shooting stars, thus seen in great numbers when the earth is at certain
points of her orbit, are really planetoidal bodies, appears to be
further proved by this;--that they all seem to move nearly in the same
direction.[8] They are, each of them, visible for a short time only,
(indeed commonly only for a few seconds), while they are nearest the
earth; much in the same way in which a comet is visible only for a small
portion of its path: and this portion is described in a short time,
because they move near the earth. They are so small that a little change
of distance removes them beyond our vision.
9. Perhaps these revolving specks of nebulae are the outriders of the
zodiacal light; portions of it, which, being external to the permanently
nebulous central mass, have broken into patches, and are seen as stars
for the moment that we are near to them. And if this be true, we have to
correct, in a certain way, what we have previously said of the zodiacal
light;--that no one had thought of resolving it into stars: for it would
thus appear, that in its outer region, it resolves itself into stars,
visible, though but for a moment, to the naked eye.
10. And thus, all these phenomena concur in making it appear probable,
that the Earth is placed in that region of the solar system in which the
planet-forming powers are most vigorous and potent;--between the region
of permanent nebulous vapor, and the region of mere shreds and specks of
planetary matter, such as are the satellites and the planetoidal group.
And from these views, finally it follows, that the Earth is really the
largest planetary body in the Solar System. The vast globes of Jupiter
and Saturn, Uranus and Neptune, which roll far above her, are still only
huge masses of cloud and vapor, water and air; which, from their
enormous size, are ponderous enough to retain round them a body of
small satellites, perhaps, in some degree at least, solid; and which
have perhaps a small lump, or a few similar lumps, of planetary matter
at the centre of their watery globe. The Earth is really the domestic
hearth of this Solar System; adjusted between the hot and fiery haze on
one side, the cold and watery vapor on the other. This region only is
fit to be a domestic hearth, a seat of habitation; and in this region is
placed the largest solid globe of our system; and on this globe, by a
series of creative operations, entirely different from any of those
which separated the solid from the vaporous, the cold from the hot, the
moist from the dry, have been established, in succession, plants, and
animals, and man. So that the habitation has been occupied; the domestic
hearth has been surrounded by its family; the fitnesses so wonderfully
combined have been employed; and the Earth alone, of all the parts of
the frame which revolves round the Sun, has become a World.
11. Perhaps it may tend still further to illustrate, and to fix in the
reader's mind, the view of the constitution of the solar system here
given, if we remark an analogy which exists, in this respect, between
the Earth in particular, and the Solar System in general. The earth,
like the central parts of the system, is warmed by the sun; and hence,
drives off watery vapors into the circumambient space, where they are
condensed by the cold. The upper regions of the atmosphere, like the
outer regions of the solar system, form the vapors thus raised into
clouds, which are really only water in minute drops; while in the solar
system, the cold of the outer regions, and the rotation of the masses
themselves, maintain the water, and the vapor, in immense spheres. But
Jupiter and Saturn may be regarded as, in many respects, immense clouds;
the continuous water being collected at their centres, while the more
airy and looser parts circulate above. They are the permanent
receptacles of the superfluous water and air of the system. What is not
wanted on the Earth, is stored up there, and hangs above us, far removed
from our atmosphere; but yet, like the clouds in our atmosphere, an
example, what glorious objects accumulations of vapor and water,
illuminated by the rays of the sun, may become in our eyes.
12. These views are so different from those hitherto generally
entertained, and considered as having a sort of religious dignity
belonging to them, that we may fear, at first at least, they will appear
to many, rash and fanciful, and almost, as we have said, irreverent. On
the question of reverence we may hereafter say a few words; but as to
the rashness of these views, we would beg the reader, calmly and
dispassionately, to consider the very extraordinary number of points in
the solar system, hitherto unexplained, which they account for, or, at
least reduce into consistency and connection, in a manner which seems
wonderful. The Theory, as we may perhaps venture to call it, brings
together all these known phenomena;--the great size and small density of
the exterior planets;--their belts and streaks;--Saturn's
ring;--Jupiter's oblateness;--the great number of satellites of the
exterior planets;--the numerous group of planetoid bodies between
Jupiter and Mars;--the appearance of definite shapes of land and water
on Mars;--the showers of shooting stars which appear at certain periods
of the year;--the Zodiacal Light;--the appearance of Venus as different
from Mars;--and finally, the material composition of meteoric stones.
13. Perhaps there are other phenomena which more readily find an
explanation in this theory, than in any other: for instance, the recent
discovery of a dim half-transparent ring, as an appendage to the
luminous ring of Saturn, which has hitherto alone been observed. Perhaps
this is the ring of vapor which may naturally be expected to accompany
the ring of water. It is the annular atmosphere of the aqueous annulus.
But, the discovery of this faint ring being so new, and hitherto not
fully unfolded, we shall not further press the argument, which,
hereafter, perhaps, may be more confidently derived from its existence.
14. There are some other facts in the Solar System, which, we can hardly
doubt, must have a bearing upon the views which we have urged; though we
cannot yet undertake to explain that bearing fully. Not only do all the
planetary bodies of the solar system, as well as the Sun himself,
revolve upon their axes; but there is a very curious fact relative to
these revolutions, which appears to point out a further connection among
them. So far as has yet been ascertained, all those which we, in our
theory, regard as solid bodies, Mercury, Venus, the Earth, and Mars,
revolve in very nearly the same time: namely, in about twenty-four
hours. All those larger masses, on the other hand, which we, in our
theory, hold to be watery planets, Jupiter, Saturn, Uranus, revolve, not
in a longer time, as would perhaps have been expected, from their
greater size, but in a shorter time; in less than half the time; in
about ten hours. The near agreement of the times of revolution in each
of these two groups, is an extremely curious fact; and cannot fail to
lead our thoughts to the probability of some common original cause of
these motions. But no such common cause has been suggested, by any
speculator on these subjects. If, in this blank, even of hypotheses, one
might be admitted, as at least a mode of connecting the facts, we might
say, that the compound collection of solid materials, water, and air, of
which the solar system consists, and of which our earth alone, perhaps,
retains the combination, being, by whatever means, set a spinning round
an axis, at the rate of one revolution in 24 hours, the solid masses
which were detached from it, not being liable to much contraction,
retained their rate of revolution; while the vaporous masses which were
detached from the fluid and airy part, contracting much, when they came
into a colder region, increased their rate of revolution on account of
their contraction. That such an acceleration of the rate of revolution
would be the result of contraction, is known from mechanical principles;
and indeed, is evident: for the contraction of a circular ring of such
matter into a narrower compass, would not diminish the linear velocity
of its elements, while it would give them a smaller path to describe in
their revolutions. Such an hypothesis would account, therefore, both for
the nearly equal times of revolution of all the solid planets, and for
the smaller period of rotation, which the larger planets show.
15. In what manner, however, portions are to be detached from such a
rotating mass, so as to form solid planets on the one side, and watery
planets on the other, and how these planets, so detached, are to be made
to revolve round the Sun, in orbits nearly circular, we have no
hypothesis ready to explain. And perhaps we may say, that no
satisfactory, or even plausible, hypothesis to explain these facts, has
been proposed: for the Nebular Hypothesis, the only one which is likely
to be considered as worthy any notice on this subject, is too
imperfectly worked out, as yet, to enable us to know, what it will or
will not account for. According to that hypothesis, the nebular matter
of a system, having originally a rotatory motion, gradually contracts;
and separating, at various distances from the centre, forms rings; which
again, breaking at some point of their circumference, are, by the mutual
attraction of their parts, gathered up into one mass; which, when
cooled down, so as to be opaque, becomes a planet; still revolving round
the luminous mass which remains at the centre. That such a process, if
we suppose the consistency, and other properties, of the nebulous matter
to be such as to render it possible, would produce planetary masses
revolving round a sun in nearly circular orbits, and rotating about
their own axes, seems most likely; though it does not appear that it has
been very clearly shown.[9] But no successful attempt has been made to
deduce any laws of the distances from the centre, times of rotation, or
other properties of such planets; and therefore, we cannot say that the
nebular hypothesis is yet in any degree confirmed.
16. The Theory which we have ventured to propose, of the Solar System,
agrees with the Nebular Hypothesis, so far as that hypothesis goes; if
we suppose that there is, at the centre of the exterior planets,
Jupiter, Saturn, Uranus, and Neptune, a solid nucleus, probably small,
of the same nature as the other planets. Such an addition to our theory
is, perhaps, on all accounts, probable: for that circumstance would seem
to determine, to particular points, the accumulation of water and
vapors, to which we hold that those planets owe the greater part of
their bulk. Those planets then, Jupiter, Saturn, and the others, are
really small solid planets, with enormous oceans and atmospheres. The
Nebular Hypothesis, in that case, is that part of our Hypothesis, which
relates to the condensation of luminous nebular matter; while _we_
consider, further, the causes which, scorching the inner planets, and
driving the vapors to the outer orbs, would make the region of the earth
the only habitable part of the system.
17. The belief that other planets, as well as our own, are the seats of
habitation of living things, has been entertained, in general, not in
consequence of physical reasons, but in spite of physical reasons; and
because there were conceived to be other reasons, of another kind,
theological or philosophical, for such a belief. It was held that Venus,
or that Saturn, was inhabited, not because any one could devise, with
any degree of probability, any organized structure which would be
suitable to animal existence on the surfaces of those planets; but
because it was conceived that the greatness or goodness of the Creator,
or His wisdom, or some other of His attributes, would be manifestly
imperfect, if these planets were not tenanted by living creatures. The
evidences of design, of which we can trace so many, and such striking
examples, in our own sphere, the sphere of life, must, it was assumed,
exist, in the like form, in every other part of the universe. The
disposition to regard the Universe in this point of view, is very
general; the disinclination to accept any change in our belief which
seems, for a time, to interfere with this view, is very strong; and the
attempt to establish the necessity of new views discrepant from these
has, in many eyes, an appearance as if it were unfriendly to the best
established doctrines of Natural Theology. All these apprehensions will,
we trust, be shown, in the sequel, to be utterly unfounded: and in order
that any such repugnance to the doctrines here urged, may not linger in
the reader's mind, we shall next proceed to contemplate the phenomena of
the universe in their bearing upon such speculations.
FOOTNOTES:
[1] Other speculators also have regarded Saturn's Ring as a ring of
cloud or water. See _Cosmos_, III. 527 and 553.
[2] Humboldt has already remarked _(Cosmos_, I. 95, and III. 427), that
the inner planets as far as Mars, and the outer ones beginning with
Jupiter, form two groups having different properties. Also Encke. (See
Humboldt's Note.)
[3] Printed Oct. 19, 1853.
[4] Herschel, 540.
[5] It is probable, from the small density of Jupiter's satellites, that
they also consist in a great measure of water and vapor. Only one of
them is denser than Jupiter himself.--_Cosmos_.
[6] It has, in our own day, even in the present year, been regarded as a
great achievement of man to direct the fiery influences which he can
command, so as to cast a colossal statue in a single piece, instead of
casting it in several portions.
[7] Herschel, 900-905.
[8] Herschel, 901.
[9] Besides the curious relation of the times of rotation of the
planets, just noticed, there is another curious relation, of their
distance from the Sun, which any one, wishing to frame an hypothesis on
the origin of our Solar System, ought by all means to try to account
for.
The distances from the Sun, of the planets, Mercury, Venus, Earth, Mars,
the Planetoids, Jupiter, Saturn, Uranus, are nearly as the numbers,
4, 7, 10, 16, 28, 52, 100, 196:
now the excesses of each of these numbers above the first are,
3, 3, 6, 12, 24, 48, 96:
a series in which each term (after the first,) is double of the
preceding one. Hence, the distances of the planets conform to a series
following this law, (_Bode's law_, as it is termed.) And though the law
is by no means exact, yet it was so far considered a probable expression
of a general fact, that the deviation from this law, in the interval
between Mars and Jupiter, was the principal cause which led first to the
suspicion of a planet interposed in the seemingly vacant space; and thus
led to the discovery of the planetoids, which really occupy that region.
It is true, that the law is found not to hold, in the case of the
newly-discovered planet Neptune; for his distance from the Sun, which
according to this law, should be 388, is really only 300, 30 times the
Earth's distance, instead of 39 times. Still, Bode's law has a
comprehensive approximate reality in the Solar System, sufficient to
make it a strong recommendation of any hypothesis of the origin of the
system, that it shall account for this law. This, however, the nebular
hypothesis does not.