The Nebular Hypothesis

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How LaPlace Explained the Origin of the World

Nature Magazine, June 1929

When LaPlace, the French Newton, as he has often been called, advanced his celebrated Nebular Hypothesis to explain the origin of the solar system it was in the form of a note in a remarkable popular treatise on astronomy entitled "Exposition du Systems du Monde", published in the year 1796. He warned his readers that it was propounded "with that doubt which everything should arouse which is not a result of observation or calculation". Time has proved the wisdom of this statement, for fundamental weaknesses in the theory have been gradually exposed, partly as a result of the discovery of members of the solar system unknown in the day of LaPlace, and these defects have led to its gradual abandonment by the leading mathematicians and astronomers of modern times. Yet so clear, plausible and easily understood was this famous hypothesis that it met with unusual favor even from the beginning and was widely accepted and taught during the nineteenth century and almost up to the present time. Many astronomers of today first became acquainted with the nebular hypothesis in textbooks of physical geography in their high school days, and some are even now reluctant to part with it.

Briefly stated in its most essential form, the nebular hypothesis; assumes that the solar system originally consisted of a nebulous mass of highly heated gases extending beyond the orbit of the outermost planet, Neptune, and rotating like a rigid body, in the direction in which the planets now revolve around the sun and in which the sun turns on its axis. In the process of condensation the mass contracted and its angular velocity increased and finally became so great that the equatorial portion could no longer be retained by gravity and was detached from the rotating mass in the form of a ring. This ring then became unstable and broke up and the material of which it was composed collected in the form of a planet. By successive repetitions of this process eight planets were formed and in a similar manner satellites were also formed from the planets. An extremely simple and satisfying theory it was, destined to be received with favor for many years to come and then to be abandoned reluctantly. According to his biographers LaPlace's initial distrust of his theory was gradually replaced by a feeling of complacent interest in his own creation. For years no criticism of the theory was heard. Few theories have met with such universal favor.

Toward the end of the nineteenth century the nebular hypothesis began to get involved in considerable difficulties. Two small satellites of Mars, Deimos and Phobos, were discovered and the inner of the two satellites, Phobos, proved to be a most peculiar little object. It was found that it was making more than three revolutions around the planet Mars, while Mars was turning once on its axis! Such a state of affairs was inadmissible according to the nebular hypothesis which assumed that the planet threw off its satellites originally in the form of rings which of course could not revolve more rapidly than the planet was rotating. The mathematician Poincare tried to extricate the nebular hypothesis from its difficulty by the rather far-fetched explanation that solar tides had greatly slowed down the rotation of Mars after the little satellite had been formed. But then, as Moulton later pointed out, it had been discovered that the inner particles of Saturn's rings were revolving in only one half the period of Saturn's rotation and in this case tidal friction was too slight to have any appreciable effect on the planet's rotation period.

Again, according to the nebular hypothesis, all planets and all satellites should revolve in the west-to-east, or counter-clockwise, direction in which the original nebula was rotating. But a ninth satellite of Saturn named Phoebe was discovered, which also caused trouble for the theory because it was found to be revolving around Saturn in retrograde direction. Later two satellites of Jupiter were discovered which were also revolving in the east-to-west direction and this did not make the outlook any brighter for the nebular hypothesis.

As an additional serious objection it was shown by the astronomer Kirkwood in the later part of the nineteenth century that if any unstable condition should set in the equatorial zone of the rotating nebula, such as was assumed to result in the detachment of a ring, it would persist continuously. In other words it would be impossible for rings to be detached except at certain intervals. Moreover, if a ring should form, it could never condense into a planet because expansion of the gases of which it was composed and tidal attraction of the solar nebula would overbalance the gravitation of its parts.

A most serious objection to the LaPlacian theory pointed out by Prof. F. R. Moulton lay in the fact that it was possible to calculate what the speed of rotation of the solar nebula would be when it extended to the distance of anyone planet and the calculation showed that the speed of rotation must have been far too small to result in the detachment of any rings.

In addition the nebular hypothesis could not explain the existence of the asteroids with their intertwining orbits and high inclinations. It would not tolerate the existence of such an object as the planetoid Eros which extends in a greatly elongated orbit from a point near the earth to beyond the orbit of Mars. It would have difficulty explaining also the high inclination and eccentricity of the orbit of Mercury. Finally, it assumes that the outer planets are much older than the inner planets while as a matter of fact the inner planets appear to be fully as old as the outer ones.

The contraction theory of the sun's heat which was developed by Helmholtz in 1854 was looked upon as favorable to the nebular hypothesis. It showed that as the solar nebula contracted it would become hotter so long as it remained in a gaseous state, as it is at present. Since its mass, volume and rate of radiation are known, it could be shown, according to Helmholtz theory, that a contraction in the radius of the sun of only 120 feet in a year, an amount that could not be detected with the most powerful telescopes for some thousands of years, would be sufficient to supply as much heat as it is now radiating. When the sun ceased to behave as a perfect gas it would begin to lose heat more rapidly than it gained it. Its rate of radiation would decrease, its temperature and therefore that of its planets would begin to fail and eventually a cold and invisible sun would speed through space attended by a family of lifeless planets. The great weakness of the contraction theory as the sole source of the sun's heat lay in the fact that it gave far too short an interval of time for all the vast geological changes that are known to have taken place upon the earth. According to the contraction theory the earth could have received heat from the sun at its present rate for only about twenty-five million years. From many other sources it is known that the age of the earth is at least one hundred times as great as this.

It is now very evident that contraction of the solar nebula cannot be regarded as the only source of solar heat or even of a very considerable portion of it. Fortunately the discovery that a tremendous amount of energy is locked up in the atom and is continuously being released in the transmutation of elements within the sun itself has made it unnecessary to draw upon the contraction theory for an explanation of the source of the light and heat that the sun is so extravagantly pouring forth into space. Subatomic energy alone is sufficient to keep up the present rate of solar radiation for thousands of millions of years.

In spite of all the serious and fundamental objections that have been raised against the nebular hypothesis there are many who are slow to abandon it even today. Yet it is undoubtedly doomed to be entirely discarded before long in favor of modern theories against which fewer objections can be raised. The most noted of these theories and the one which has greatest weight with astronomers at the present time is The Planetesimal Theory of Chamberlin and Moulton which we will consider in our next article.

Two planets will be found in the evening sky this month. Mars is now in The Sickle in Leo, a little to the northwest of Regulus. The ruddy planet has greatly decreased in brightness since last winter and is now not much brighter than a second magnitude star. It is noticeably inferior to Regulus in brightness. Saturn is in Sagittarius and will be in opposition to the sun on June 19. It will then rise at sunset and will be visible all night.

Venus is now a brilliant morning star rising about two and a half hours before the sun. It will be at greatest western elongation or its greatest distance west of the sun on June 29. Jupiter will be too close to the sun to be seen the first part of the month. It will rise about one and a half hours before the sun in the constellation of Taurus on June 15. Mercury will not be favorably placed for observation in June as it will be in inferior conjunction with the sun on June 9.

Summer will begin on June 21 at 5:01 p.m, Eastern Time when the sun will reach its highest northern declination in the constellation of Gemini.

Although summer constellations are not as brilliant as those of winter many beautiful and interesting objects will be found in the evening sky this month. Coma Berenices is composed of a coarse cluster of fifth and sixth magnitude stars that gives the impression of a filmy veil or cloud of stars to the naked eye but is easily resolved into a multitude of faint stars with the aid of a field glass.

In Hercules on a dark, moonless night, one can faintly glimpse without telescopic aid a faint patch of light on the western side of the quadrilateral that outlines the giant's body, in the position shown on the chart. This is the great Hercules Cluster, a universe composed of hundreds of thousands of suns, at a distance of thirty-six thousand light years from the earth.

Arcturus and Vega, the two finest stars of the summer evening skies, are now prominently in view. Although almost exactly equal in apparent brightness these two stars differ strikingly in color. Both are comparatively near neighbors of the solar system. Arcturus is less than thirty light years distant and Vega about forty. Vega also is an extremely hot sun while Arcturus is comparatively cool with a temperature of about 40,000 degrees Centigrade.