408 
The Solar System [ch. xvi 
indicated that if such a tidal break-up occurred, the masses of primary and 
satellites would be more nearly equal than if the planet had been wholly 
gaseous. Thus on passing from planets which were wholly gaseous at birth 
to planets which were wholly liquid either at or immediately after their 
birth, we ought first to find planets with large numbers of relatively small 
satellites, and, after passing through the boundary cases of planets with small 
numbers of relatively large satellites, come to planets with no satellites at 
all. This is exactly what we find in the solar system. Starting from Jupiter 
and Saturn, each with nine relatively small satellites, we pass Mars with only 
two satellites and the earth with its one relatively large satellite, and come to 
Venus and Mercury which have no satellites at all. Proceeding in the other 
direction from Jupiter and Saturn, we pass Uranus with four small satellites 
and come to Neptune with one comparatively big satellite. The earth and 
Neptune, which have only one satellite each, and those comparatively large 
ones, form the obvious division between planets which were originally liquid 
and those which were originally gaseous. Thus we may conjecture that 
Mercury and Venus must have become liquid or solid immediately after birth, 
that the Earth and Neptune were partly liquid and partly gaseous, and that 
Mars, Jupiter, Saturn, and Uranus were born gaseous and remained gaseous 
during the birth of their families of satellites. 
Confirmatory evidence is provided by the circumstance that the masses 
of Mars and Uranus are abnormally small for their positions in the sequence 
of planets. If, as we have supposed, the planets were born out of a continuous 
filament of matter, the mass of Mars at birth ought to have been intermediate 
between those of the Earth and Jupiter, while the mass of Uranus ought to 
have been intermediate between those of Neptune and Saturn. If however 
the two anomalous planets Mars and Uranus were the two smallest planets 
to be born in the gaseous state, they would be likely to suffer more than the 
other planets from dissipation of their outer layers. If we suppose that Mars 
and Uranus are only fragments of planets which were initially far more 
massive than they now are, then the anomalies disappear and the pieces of 
the puzzle begin to fit together in a very satisfactory manner. 
Effects of a Resisting Medium. 
383 . The matter which was ejected from the sun by the tidal cataclysm 
cannot all have immediately condensed into planets; a considerable amount 
of gas must at first have been scattered throughout the space surrounding 
the sun, forming a resisting medium through which the new-born planets 
had to fight their way. The effect of such a resisting medium can be studied 
by giving a negative sign to G in the analysis of § 261; we see that the 
presence of a resisting medium must lessen the eccentricity of the orbits of 
the planets, so that if the medium remains in existence for long enough, all 
planets and their satellites must acquire approximately circular orbits.