Man's physical universe

xanabras

128 THE EARTH AS MAN'S ABODE

As evidence piled up against it, this theory was finally given up even

by its fondest adherents.

The Planetesimal Hypothesis. The planetesimal hypothesis, advanced

by Chamberlin and Moulton of the University of Chicago, superseded

the nebular hypothesis. This theory, like all others, retains the

central idea that the whole solar system began as one large body but

differs from the nebular theory in the belief that the force which caused

the separation of the planets and other smaller bodies came from without.

Another star is thought to have come near the star of which our

sun is the residue.

It did not come close enough to produce a collision

but did come close enough to exert such a force of attraction on the

outer portions of the sun that large arms were shot out as it revolved

near the passing star. These gases then cooled to form solid meteorites,

or planetesimals. The planets and their satellites were originally

just extra-large meteorites that steadily grew by attracting to themselves

the still smaller particles.

This theory differs from the nebular hypothesis in that it assumes

that the earth was built up from smaller solid particles which were

cold, that the earth grew hotter due to increasing pressures developed

by the gradual accumulation of mass, and that the earth originally

had no atmosphere.

The Tidal Theory. The tidal theory, advanced by the British astronomers.

Sir James Jeans and Harold Jeffreys, in 1919, is a variation of

the planetesimal hypothesis that seems to be satisfactory today.

According to this theory, the planets were torn from the sun, possessing

masses approximately as we know them today, as the result of tides

produced in the surfaces of the two suns as they came very close

to each other.

In any event, it is probable that the origin of our earth was no

ordinary, everyday cosmic affair. Stars do not come near each other

very often, and when they do it is unlikely that they do so in such a way

as to form planets like those of our sun. But here again we may be all

wrong. Perhaps many stars have planets too distant to be observed.

For instance, a man on the nearest star. Alpha Centauri, would have to

have a telescope a thousand times more powerful than our best telescope

to be able to see the sun's largest planet, Jupiter.

Undoubtedly the present hypotheses will be revised from time to time

as additional knowledge becomes available. It is important to note

that scientists are just as anxious to disprove these hypotheses as they

are to prove them. Their acceptance of them is real, but tentative,

because they are ready to reject them for better hypotheses at any

time. There is no more fundamental test of the degree to which one is