According to both the nebular and the planetesimal hypotheses, the earth's crust appears to be colder now than it was hundreds or thousands of millions of years ago.
The Planetesimal Theory suggests that these thickened knots are embryo planets and the central portion of the nebulæ an embryo sun.
Within a few years a Planetesimal Theory has been announced, and is gaining considerable prominence, although it is too early yet to say whether it will supersede La Place's idea.
Among the many that have been proposed the most elaborate is the ``Planetesimal Hypothesis'' of Professors Chamberlin and Moulton.
Even as an explanation of the spiral nebulæ, not as solar systems in process of formation, but as the birthplaces of stellar clusters, the Planetesimal Hypothesis would be open to many objections.
Such, then, is the mechanism of the first phase in the history of a spiral nebula according to the Planetesimal Hypothesis.
To deal at length with this theory, to which the name of "Planetesimal Hypothesis" has been given, would not be possible in a book of this kind.
Possibly the planetesimal hypothesis of Chamberlin and Moulton affords the true explanation, as the result of a rupture due to excessive tidal strain.
As is apparent, these investigations lead to conclusions very different from those derived by Chamberlin and Moulton from the planetesimal hypothesis.
The absence of a dominating nucleus beyond Mars accounts for the zone of the asteroids remaining in some sense in the original planetesimal condition.
The point is not necessarily in opposition to the planetesimal hypothesis; but whatever the explanation, it ought to apply to the planet as well as to the Sun.
The evidence, except as to the asteroids and Mercury, is not favorable to the planetesimal hypothesis, unless we make special assumptions as to the distribution of materials in the spiral nebulae.
The inner satellite of Mars and the rings of Saturn make no special difficulty under the planetesimal hypothesis.
But the planetesimal hypothesis has no room for this enormous percentage of carbon-dioxide in the primitive atmosphere.
On one point the new planetesimal theory differs from the other theories.
In deference to the opinion of a number of geologists we must glance once more at the alternative view of the planetesimal school.
The planetesimal hypothesis, advanced in recent years by Professor Moulton and Professor Chamberlin, contends that the nebula is a vast cloud of liquid or solid (but not gaseous) particles.
Let us now inquire briefly into the bearing of this planetesimalhypothesis upon the early geological history of the earth.
Anything like a full understanding of the planetesimal hypothesis would be difficult to obtain, and, in the brief space at our disposal, we shall attempt to make clear only a few of the salient points.
Diagram showing the origin and character of a spiral nebula according to the planetesimal hypothesis of the origin of the solar system.
