But we may consider it as a conceivable case that a planet of the exact dimensions of Jupiter may be revolving in an annual period of the same length as his, round some star that is capable of affording it adequate nourishment; and so with the three other giant planets. The actual Jupiter and Saturn of the solar system have, so far as we can tell, neither present nor future as habitable worlds, but we can consider what would be the case of imaginary bodies of similar dimensions in systems where the supply of heat would be sufficient. Or we can neglect the question of temperature altogether, as we did at first in the case of Mars. All the four planets must shrink much in volume before their solidification will take place. Their average density at present but little exceeds that of water; indeed, Saturn is not so dense as water; yet we must suppose that the same elements are in general common to the Earth and to them all. If we assume, then, that the four planets all cool to the point of solidification, their densities must be much increased, and their volumes correspondingly diminished. Since all four greatly exceed the Earth in mass, it is but natural to expect that, when they have assumed the terrestrial condition, they will be more closely compacted than the Earth, and their densities in consequence will be greater. It will, however, be simpler if we assume exactly the same density for them as for the Earth. Jupiter will then have shrunk to about one-fourth of its present volume, and the statistics for the four planets will run as in the following Table: STATISTICS OF THE FOUR OUTER PLANETS IF WITH THE SAME DENSITY AS THE EARTH PROPORTIONS OF THE PLANETS:-- Uranus Neptune Saturn Jupiter Diameter in miles 19300 20400 36000 54000 do [Symbol] = 1 2·44 2·57 4·56 6·82 Surface, [Symbol] = 1 6·0 6·6 20·8 46·6 Mass and Volume, [Symbol] = 1 14·6 17·0 94·8 317·7 Gravity at surface, [Symbol] = 1 2·44 2·57 4·56 6·82 Rate of Fall, Feet in the First Second 39·2 41·3 73·3 109·7 ATMOSPHERE, assuming the total mass of the atmosphere to be proportional to the mass of the planet:-- Pressure at the surface in lb. per square inch 88·2 97·0 305·8 685·0 Pressure at the surface in "atmospheres" 6·0 6·6 20·8 46·6 Level of half-pressure in miles 1·37 1·30 0·73 0·49 Boiling point of water at surface 127°C 129°C 148°C 164°C