Nebular Hypothesis, the celebrated speculation of Sir "William Herschel, adopted and developed by Laplace, assigning the genesis of the heavenly bodies to the gradual aggregation and condensation of a highly attenuated self-luminous substance diffused through space. (See "Philosophical Transactions," 1802 and 1811.) To this hypothesis Herschel was led by his conclusion that there were nebulosities not composed of stars. The Rosse telescope having decomposed nebulas hitherto considered to be irresolvable, and exhibited symptoms of resolvability in others still more intractable, it was assumed that all nebulas are stellar, their nebulosity being solely a question of distance; and thus, the basis of Herschel's reasoning failing, the fabric of his hypothesis was thought to be demolished. Mr. Herbert Spencer came to its support in the " Westminster Review," No. cxxxvii. (July, 1858). The argument in its favor is substantially as follows. The assumption that all nebulas are remote galaxies does not invalidate the indications furnished by the structure of the solar system, which still points to a nebular origin just as significantly as before. But the assumption is inadmissible.
The mode of distribution of the nebulas furnishes evidence of a physical connection with our stellar system; and this evidence is confirmed by the fact of their re-solvability with telescopic power which fails to make individually visible the most distant stars of our own milky way. If they are remote galaxies, it may be assumed that, speaking generally, the largest are the nearest, and therefore the most resolvable. But the fact is, the smallest are the most resolvable. Another difficulty is presented by the Magellanic clouds. (See Nebula.) Sir John Herschel, considering the structure of the larger of these clouds, concludes that "it must be taken as a demonstrated fact that stars of the seventh or eighth magnitude, and irresolvable nebula, may coexist within limits of distance not differing in proportion more than as 9 to 10." (" Outlines of Astronomy," London, 1851, p. 615.) This clearly supplies a reductio ad ab-surdum of the popular doctrine. Assuming, for the sake of the argument, a rare, homogeneous, nebulous matter, widely diffused through space, the following successive changes will, on physical principles, take place in it: 1, mutual gravitation of its atoms: 2, atomic repulsion; 3, evolution of heat, by overcoming this repulsion; 4, molecular combination, at a certain stage of condensation, followed by, 5, sudden and great disengagement of heat; 6, lowering of temperature by radiation and consequent precipitation of binary atoms, aggregating into irregular flocculi and floating in the rarer medium, just as water when precipitated from air collects into clouds; 7, each flocculus will move toward the common centre of gravity of all; but being an irregular mass in a resisting medium, this motion will be out of the rectilinear, that is to say, not directly toward the common centre of gravity, but toward one or other side of it; and thus, 8, a spiral movement will ensue, which will be communicated to the rarer medium through which the flocculus is moving; and, 9, a preponderating momentum and rotation of the whole mass in some one direction, converging in spirals toward the common centre of gravity.
Certain subordinate actions are to be noticed also. Mutual attraction will tend to produce groups of flocculi concentrating around local centres of gravity, and acquiring a subordinate vertical movement. These conclusions are shown to be in entire harmony with the observed phenomena. In this genetic process, when the precipitated matter is aggregating into flocculi, there will be found here and there detached portions, like shreds of cloud in a summer sky, which will not coalesce with the larger internal masses, but will slowly follow without overtaking them. These fragments will assume characteristics of motion strikingly correspondent to those of the comets, whose physical constitution and distribution are seen to be completely accordant with the hypothesis. - The physical characters resulting from the hypothesis are found to tally with the facts. In a rotating spheroid of aëriform matter in the latter stages of concentration, but before it has begun to take a liquid or solid form, the following actions will go on: 1, more and more rapid aggregation of its atoms into a smaller and denser mass, as the common centre of gravity is approached; 2, development of oblateness; 3, evolution of heat, greatest at the central parts; and, as a consequence, 4, circulation - currents setting from the centre toward the poles and thence to the equator, and counter currents from the equator to the centre.
In the course of this round there will be, 5. an oscillation of temperature: first, from the centre outward - expansion by diminished pressure and other causes, and consequent lowering of temperature; secondly, from the equator inward - rise in tempera:ure for converse reasons. 0. As a corollary to 4 and 5, external condensation will occur according to the laws of precipitation from gases, resulting in a belt of vapor about the equator, gradually widening and condensing into a fluid; 7, this fluid film will gradually extend itself till it eventually closes over at the poles, thus forming a thin hollow spheroid tilled with gaseous matter; 8, at length the liquid shell will become very thick, the outer surface will experience a fall of temperature and begin to harden into a solid crust. This hypothesis explains the relative specific gravities of the planetary bodies, the formation of the asteroids, the earth's sup-posed interior structure, indications of past or present high temperature throughout the solar system, and the sun's incandescence. - These considerations relate chiefly to the physical changes undergone by a forming system. Laplace's nebular hypothesis deals with the changes of arrangement in the distribution of matter forming into a system under the action of dynamical laws.
He takes as the basis of his theory certain features of our solar system which are not explained by the theory of gravitation. Gravity accounts for Kepler's laws, which are shown to be among its necessary consequences. No system could circulate in any manner around a centre, for instance, without the law holding that the numbers representing the cubes of the mean distances would be proportional to the numbers representing the squares of the periodic times. But a system could exist under gravity in which the plaints would travel in widely eccentric orbits or in planes largely inclined to each other. Nor has it been proved that the planets might not safely circulate in different directions. Assuredly, if revolution in different directions, or in planes largely inclined to each other, or in very eccentric paths, might in the long run result in collisions and therefore in the destruction of the system as such, there is yet no reason to believe that all the axial rotations need take place in the same direction as the motions of revolution.
But, to say the truth, none of those laws of harmony in our solar system, except the laws depending directly on gravity, can be regarded as essential to the well being of the system; nor, as will presently appear, would the difficulty of regarding the system as other than a product of evolution be appreciably diminished by supposing that without those laws the destruction of the system must inevitably have occurred in the course of time. For it would be manifestly unreasonable to regard our system as one in which the original arrangements were fortuitously so happy that it has continued to exist as a system, if we find that the probability of these arrangements so existing by mere coincidence is exceedingly minute. Now, how small this probability is may be inferred by considering only the motion of the planets in one common direction. There are known at the present time 8 major planets and 137 minor planets (the number of these is increasing year by year). Thus there are 145 known planets. Taking the earth's direction of revolution as a standard direction, the chance that any one of the remaining 144 planets would have this direction as a result of mere chance is of course one half, since a planet must revolve in one of two ways.
Therefore, by the laws of probability, the chance that all the 144 other planets would revolve in that direction is represented by a fraction whose numerator is unity and its denominator 2 raised to the 144th power. Now 144 times the logarithm of 2, (or.3010300) = 43.3483200, showing that the above mentioned denominator is a number of 44 digits, beginning 2230077 with 37 digits to follow. This inconceivably enormous number represents the odds to 1 against the observed arrangement being the result of chance, even considering only one relation out of several mentioned above, all of which present the same order of antecedent improbability. Thus Laplace was led to his conception of a vast rotating nebulous disk, from the gradual contraction of which, and the consequent throwing off of rings, breaking up into globes, all revolving and rotating in one common direction and nearly in the same level, the solar system was formed. This hypothesis, however, does not explain the distribution of the masses of the solar system; one planet (Jupiter), for example, containing nearly 5/7 of all the matter outside the sun, and Saturn and Jupiter together containing about 1/1 ½ of all that matter.
Accordingly, the present writer has suggested a modification of it, in which, starting from some such primary condition as that assumed by Herbert Spencer, the various parts of the solar system were formed by processes of aggregation such as are still going on (though now with extreme slowness). For the motions of the flocculi of Spencer (or of the parts, whatever their nature, from which the system was to be formed) would be more and more rapid with proximity to the central aggregation, according to well known dynamical laws. Accordingly, subordinate aggregations would form with difficulty close by the sun; and hence we can understand the Smallness of the members of the interior family of planets, comprising Mercury, Venus, the earth, and Mars. Again, with extreme distance from the centre, the gravity of available material whence aggregations could form would be so far reduced, that for that reason the planets so formed would be smaller. Hence we can understand why Uranus and Neptune are so far inferior to Jupiter and Saturn. These two giant orbs are thus seen to occupy the space where the conditions were most favorable to the rapid development of subordinate aggregations.
In this intermediate region there was abundance of material while yet the motions were not so rapid that a subordinate aggregation could not readily master, so to speak, the matter rushing past toward the aphelion of its orbital motion round the sun. The theory also explains well the existence of a zone of discrete bodies next within the path of Jupiter, that is, in a region disturbed at once by his attraction and that of the sun. - It is not improbable, as remarked in the article Meteor, that the study of co-metic and meteoric astronomy may before long throw considerable light on the interesting question of the evolution of our solar system, and may enable us to form a nebular hypothesis on safer grounds than those on which the theories now in vogue have been based.