DURING the last few years the researches of Cham-berlin, F. R. Moulton and See on the evolution of our , system have greatly shaken the faith of astronomers in Laplace's well-known Nebular Hypothesis. More than a century ago Laplace, who more completely than any other had worked out the consequences of Newton's tneory of gravitation to the satisfactory explanation of almost every known feature of the motions of the planets, developed a hypothesis previously proposed by Swedenborg, Wright, and the great philosopher Kant. The solar system consists of a number of bodies arranged in an orderly manner, all moving in nearly circular paths round the central body, these paths being all nearly in the same plane, and their motion in the same direction, while there is a fairly regular progression of distances from the sun (Bode's Law), and the bodies are either spherical or spheroidal. These features are by no means a necessary consequence of gravitation, and seemed to imply an original connection or common origin. Laplace supposed that at one time the matter now forming the sun, earth and other planets was in the form of an intensely hot gas, perhaps hotter than the sun is now. This mass was of approximately spherical form and rotated slowly on its own axis, the rotation becoming swifter as the mass grew colder and contracted. In time rings of matter would be left behind the main mass (not thrown off, as is sometimes stated); each of these rings would gradually collect into a single globe, and thus the planets would be formed. A planet thus formed continuing to revolve might itself abandon rings in contracting; these rings would form into the satellites. The rings of Saturn, were at one time thought to be examples of this process, but we now know that they are composed of swarms of meteorites rather than of continuous substance. Plateau devised an experiment illustrating this formation of rings. He prepared a mixture of alcohol and water of a specific gravity as nearly as possible equal to that of oil. Some oil was then poured into the mixture. As the bottom of the mixture was slightly more dense than the oil, and the top slightly less dense, the oil sank half-way and floated in the middle as a round ball. By means of a disk attached to a wire the ball of oil was set rotating. The effect of the rotation caused the oil globe to expand into the form of a spheroid flattened at the poles, and this flattening increased with the speed, until at last a ring was formed which revolved round the globe. After a time the ring broke up and gathered into a smaller globe which rotated, besides revolving round the large globe. Laplace supposed that the rings would rotate as though solid, their outer edges thus moving more swiftly than the inner, and thus the planets formed therefrom would rotate in the same direction. The “exceptional cases” of our system—the fact that the satellites of Uranus and Neptune move in the opposite direction to that in which most of the other members do, and the swift revolution of the inner satellite of Mars—cannot be explained by this form of the hypothesis. M. Faye, however, by modifying the original idea of Laplace, and supposing that the planets were formed by local condensations (not by the detachment of rings) within the revolving nebula, and that the outer planets, Uranus and Neptune, have been more recently formed than the rest, has shown that these bodies would have retrograde rotation on their axes, which he supposed to be the case from the motion of their satellites. Since, however, Saturn's rotation is in the same direction as that of our own earth, and eight of its satellites move in one direction, but the last discovered (Phoebe) moves in the opposite direction, we have still a difficulty, unless we suppose this body to be a recent capture and not an original member of the Saturnian family. The same thing is the case also with the eighth satellite of Jupiter, whose motion is retrograde, while the other seven have direct motion. Prof. Sir George Darwin by his theory of tidal evolution has attempted to explain the swift motion of the inner satellite of Mars, the fact that the moon always turns the same face toward the earth, and that the rotation period of Mercury (and probably that of some of the satellites of Jupiter and Saturn) is the same as that of its revolution. He has given reasons for thinking that in former ages the period of rotation of Mars was much shorter than at present, but that by tidal action of the sun this period has been gradually lengthened to its present value; at the same time the satellite's period is supposed to be shortening, and its distance from the planet slowly diminishing. In the case of the moon, he considers that millions of years ago our earth was rotating much more quickly than at present. In contracting, a portion separated from the rest, and gradually receded, becoming the moon. The earth's tidal action upon the latter has resulted in the periods of rotation and revolution becoming equal to one another. The ancient Arcadians are said to have boasted that their race came into existence before the moon, but they were probably unaware of the period they claimed for their ancestry (fifty-seven millions of years!). The observations of Schiaparelli having led him to the conclusion that the planet Mercury (which is the nearest known planet to the sun) rotates on its axis in a period equal to that of its revolution round the sun (88 days), Sir George Darwin considers this is due in a similar manner to the tidal action of the central body having lengthened the planet's period of rotation until the latter always presents the same hemisphere toward the sun, just as the moon does toward the earth. The same thing has been asserted of the planet Venus also, but it still seems probable that the shorter period of 231h hours, determined by the early Italian observers, is the true length of the “day” on the earth's “twin-sister." Further modifications, in consequence of increased knowledge of actual existing nebulas and the applications of the principles of energy and thermodynamics, have been proposed from time to time; and most supporters of the nebular hypothesis no longer believe that in its original condition the “nebula” was even at so high a temperature as that of the sun at present. It is considered probable that the original nebula was largely composed of meteorites, which by collisions during their gradual drawing together would grow hotter and hotter. After a time the central mass would become an intensely hot “white” star. Later on, the loss of heat from radiation exceeding the gain from contraction and condensation, the star would cool down and perhaps finally become a dark body like the companion to Algol. The planets, being smaller than the star or sun round which they revolved, would cool down at a much quicker rate, . losing more heat from their surfaces and becoming non-luminous bodies, while their interiors would be still very hot. Our earth and the inner planets seem to have reached this stage, while Jupiter and Saturn appear to be still, to a small extent, self-luminous. Estimates of the past and future duration of our system have been formed by Lord Kelvin, Helmholtz and others; but the very various lengths of time given, ranging from twenty to four hundred millions of years, alone show that these periods are little more than rough guesses, needing further knowledge to be of value. The discovery of the properties of radium has enormously extended the probable future duration of the sun's heat. “We have every reason to think,” says Arrhenius, “that the sun's chemical energy will suffice to maintain its heat during thousands of millions, perhaps billions of years." In 1861 Babinet proposed the application of a criterion, based on the mechanical principle of the “conservation of areas.” He showed that if o be the sun's angular velocity of rotation, with radius r, and ror represent these quantities when the globe is expanded so as to have the radius r, then &jr'=ro1r'2 [Moment of momentum, a constant quantity for a system rotating freely and subject to no external forces] or Suppose now the “solar nebula” extending to the earth's orbit, let us find its time of rotation. We get for this 25.3 days 23,445 109.5 = 3,192 years. For the case of Neptune, whose mean distance is thirty times that of the earth from the sun, the solar nebula when reaching to that distance will rotate in 25.3 days (30 X 23,445 )' 109.5 / 2,888,533 years. (These figures are taken from a paper by Dr. See.) Applying this criterion to the case of the various planets and satellites of our system, we find periods in every case much greater than the known periods of revolution of these bodies. The earth revolves in one year about the sun, Neptune in about a hundred and sixty-five years. Thus it follows that the “hypothetical solar nebula could not have rotated with sufficient speed to detach the masses,” when it extended to the orbits of the several planets, as Laplace supposed. The evolution of the planets by separation of rings of matter from the central condensation, through rotational instability, must therefore be abandoned. It is, however, possible that secondary condensation nuclei might be formed by gravitational instability within the gaseous nebula; and this has been pointed out by Mr. Jeans in papers which he has contributed to the Philosophical Transactions of the Royal Society. We turn now to the alternative hypothesis developed by the work of Prof. T. J. J. See. He has recently pointed out that some remarkable anticipations of his views as to the action of a resisting medium were made by Euler in 1749. The essential features of this hypothesis are that the Solar System has been formed from a spiral nebula, and that the planets have not been detached from the central mass through its rotation but have been captured or added on from the outer parts of the nebula. The roundness of the orbits of the planets and satellites in general use is due to the action of a resisting medium which has reduced the size of their paths, and well nigh obliterated the deviations from circularity. Just as the planets have been captured by the sun's action, so in like manner the satellites have been captured by their several primaries, not detached by rotation of these latter. The moon too was originally a planet, which neared the earth and was finally captured and made a satellite. The asteroids or minor planets between the orbits of Mars and Jupiter are the surviving remains of millions of small planets, most of which have been swallowed up by colliding with larger ones, though many are still moving in independent paths round the sun. Our own earth frequently encounters some of these objects, and we have then a more or less brilliant “meteor shower.” The satellites having been captured in this way, it is not surprising that a few of them should revolve in the opposite direction to the rest. It is also remarkable that the paths of Phoebe (Saturn's 9th satellite) and of the 8th satellite of Jupiter are much more oval than those of any other known satellites, from which it would appear that the density of the resisting medium must have been very slight at the great distances from the planets at which they revolve. The planetary rotations have also been produced by the capture and absorption of small bodies; and thus the larger planets Jupiter and Saturn should rotate most rapidly, as is known to be the case. It has long been known that the effect of a resisting medium on the paths of bodies moving in it, in a manner analogous to the planets moving round the sun is (1) to reduce their distances from the central body; (2) to diminish the eccentricity of their orbits, i. e., to make these more nearly circular. The proof of this is given in works on analytical dynamics (Cheyne's Planetary Theory and other books) and was of course well known to Laplace, who says, “At the same time the planet approaches the sun, by the effect of the resisting medium the orbit also becomes rounder.” The well-known comet of Encke is thought to be gradually drawing nearer to the sun by such an action. Thus the present shape of the planetary paths is accounted for, the action of the resisting medium having changed their orbits. Around each planet circulates a vortex of cosmical dust, and the descent of this material upon the surfaces of sun and planets is considered to give rise to the accelerations of their equatorial regions—i. e., the fact that the parts of the sun, Jupiter and Saturn near their respective equators have a shorter period of rotation than those farther north or south. However, there is no perceptible difference of rotation in different regions of our own earth or of Mars, so far as, known and the amount of matter required to produce such an effect (at present) seems greater than can reasonably be supposed to fall upon the surfaces of the planetary bodies. A similar difficulty occurs in the meteoric theory of the sun's heat, attributing the latter to the impact produced by the fall of countless meteorites upon its surface. No doubt such bodies do fall upon the surface of the sun in considerable quantities, but the amount required to maintain the sun's output of heat is so enormous that there should be an enormously greater quantity in regions near the sun, so that our own earth ought to receive nearly half as much heat as she gets from the sun by impact with meteors. This is certainly not the case. The descent of matter upon the sun increasing its mass may also account for the small secular acceleration of the earth indicated by the observations of eclipses, and the outstanding motion of the perihelion of Mercury, which Leverrier attributed to a planet or ring of small planets lying between Mercury and the sun, may be also explained in this manner. The moon having suffered numerous collisions with smaller satellites has had its surface marked with the round sunken craters which are so distinctive a feature. So different a theory from the ordinary volcanic one, however, will not be easily accepted by selen-ologists. Prof. See considers that the almost perfect circularity of Neptune's orbit shows that it cannot be the outermost planet of our system, the roundness indicating that the nebulous medium was quite dense at that distance, and consequently the limits of the system are much farther out. Others planets lying beyond Neptune have been suspected and may yet be discovered by the telescope. It is remarkable that Prof. Forbes considers that one of these bodies, whose distance he supposes is about a hundred times that of the earth from the sun, and consequently would have a period of a thousand years (by Kepler's third law, squares of periodic times as cubes of distances from sun, 1,0002 = 1,000,000 = 1003) moves in a very eccentric orbit, whose plane makes a large angle to that of the ecliptic, the resisting medium at that distance apparently having had little effect on its motion. The solar system, in the opinion of Prof. See, was formed from a spiral nebula, the latter arising from the meeting of two or more streams of cosmical dust. The system began to whirl about a central point and thus gave rise to a vortex. Great numbers of spiral nebulae are now known to exist scattered out over the heavens, millions of these objects being visible in the most powerful telescopes. On the other hand, it lias been pointed out that there are very few nebulm of the oblate spheroidal form, such as the hypothesis of Laplace assumed, to be met with in the sky. “Such nebulae as we see have, it seems, a greater analogy with the solar corona than with the fiery condensing mists conceived of by Laplace” (Proctor, “Old and New Astronomy,” § 1445). The rotation period of Mars being about 24 hours 37 minutes and that of our own earth 23 hours 56 minutes, Prof. See considers that the period 23 hours 21 minutes for Venus, obtained by the early Italian observers, is probably about its true value, and thus the planet is habitable and probably inhabited by intelligent beings. It is well known that periodic comets probably owe their present position as permanent members of our system to the action of the planets. When a comet coming from outer space in a parabolic orbit approaches a planet its motion is either accelerated or retarded. In the latter case the parabola becomes an ellipse, and the comet henceforth moves in a closed path around the sun, always coming at each revolution to (or near to) the point where this retardation commences. Thus arise the. planets' families of comets. A very large number of members of Jupiter's family of comets are known; Halley's famous comet is a member of Neptune's family. In a similar manner it is supposed that the asteroids and satellites have attained their present positions. The whirling of the gaseous matter of a spiral nebula is considered to be due to the unsymmetrical meeting of two streams or to the settling down of a nebula of unsymmetrical. figure. From this ultimately results a star surrounded by a system of planets and satellites. The two opposite branches of spiral nebulae often seen on photographs represent the “original streams of cosmical dust which are coiling up and forming spiral systems.” If the streams so converge that the nebulous mass becomes very concentrated, the nebula may divide at its center and give rise to a double star. This theory of the capture of the planets and the rounding of their orbits by the action of the resisting medium gives results in some cases the exact opposite to those which are given by the theory of tidal evolution, as investigated by Sir George Darwin. While tidal friction usually increases the major axis and eccentricity of an orbit, the resisting medium as regularly decreases both elements. “In the actual physical universe both causes are at work together, sometimes one influence preponderating and then the other.” With a large central sun and small planets, as in our system, the action of the resisting medium is most effective; for systems made up of two large masses, tidal friction is the predominating agency. There can be little doubt that these researches form a most important advance in our knowledge of the genesis of our system; and though answers more or less satisfactory may be found to parts of the criticism of Laplace's famous hypothesis, yet we may fairly say that, if not completely disproved, it has been very seriously undermined. It is not to be supposed, however, that the alternative hypothesis is free from difficulties, some of which have been slightly outlined, but we may still say that it gives a reasonable explanation of many remarkable peculiarities. Further evidence in its favor is no doubt wanted, as well as spectroscopic proofs cif motions derived from the study of actual existing spiral nebulre. Some recent work by Dr. Nolke on the effect of a resisting medium in the evolution of the solar system from a primitive nebulous condition has been published by him at Berlin. Sir George Darwin in his article on “The Genesis of Double Stars” gives an interesting historical account of work on the theory of the equilibrium of revolving liquid bodies, by Poin-care, Jeans, and others, together with an application-of their results to stars of the Algol type. Probably there is no subject more fascinating than the question as to the past and future of our system; and though from our limited experience, both in time and space, there is the greatest necessity for caution in drawing conclusions, yet the mind of man seems so constituted that it cannot help doing so. It remains for the future to show whether “the vast masses of observational data accumulated by the persevering industry- of self-denying men of science” can be put together in the manner indicated above to yield the laws of stellar evolution.—Science ProgresS.
This article was originally published with the title "New Theories of the Evolution of Stellar Systems" in s , , 202-203 (January 2012)