Are the Planets Inhabited?

ter sundown in the evening-its "eastern elongation"-and still more as it attains its greatest brilliancy, it has attracted attention everywhere and in all a

of the Sun; it can even assert itself in the presence of the Lord of Day, for it has ofte

of the planet yields little detail, and that little is illusive and ill-defined. The clear-cut outlines and black shadows of the Moon have no place here, nor do the ruddy plains and blue-grey "seas" of Mars find any analogues. All that can be observed beyond

in atmospheric equipment. If we assume that the atmosphere of any planet is in direct proportion to its mass-and as Venus is so nearly the twin of the Earth there is no reason to expect any great difference between the two in this respect-the atmosphere of Venus would

er as a liquid is just passed, but, for the planet in general, a fairly safe margin is maintained. Here then is sufficient explanation why the topography of Venus is concealed. The atmosphere will always be abundantly charged with water-vapour, and an almost unbroken screen of clouds be spread throughout its upper re

h the cloud veil; there is no moon and the stars must be almost always hidden. The Earth with its Moon might form a beautiful ornament at times in the midnight sky if the cloud-shell should occasionally open, but o

and be abundant. The mean temperature would not, in general, be high enough to drive off the

e to life on Venus, a hindrance that a

re sunrise or after sunset. In the telescope it is not so attractive as Venus, partly because it is smaller, partly because, though it receives more than three times as much light from the Sun, it

dently is, very slight and transparent; offering little or no hindrance to an observer scanning it from another world. The other necessary consequences of small size and

uniformly, for Mercury moves round the Sun in a very eccentric orbit, so that when in aphelion it receives, surface for surface, only about four times as much heat as the Earth, but some six weeks later whe

dition the exact reverse of Mars. Water as a liquid will be rare on Mercury, not because it is congealed, but because it is evaporated; on the dark side of the planet it may, inde

the dark side of Mercury at one time is always dark; t

rof. G. H. Darwin in his development of the theory of tidal friction, puts the condition of Mercury in quite a new light. No alternation of day or night refreshes and restores the little world; one hemisphere is for ever exposed to the blasting heat of the Sun, seven times hotter for it than for the Earth; the other hemisphere is for ever exposed to the darkness and cold of outer space, a range from something like 390° C. above freezing-point, to 270° C.

ood, it would still be impossible to regard it as a world for habitation. But its case shows that a further condition of habitability has to

assisted by the clear Italian sky, believed that he had demonstrated that Venus, like Mercury, rotates once in her y

o for ever; one on the night side of Venus can never have a sunrise, or gladden in the daylight. The side exposed to the Sun will wither in a temperature of about 227° C., in which all m

ult one. The spectroscopic method of determining the speed of rotation has been equally indecisive. Until, therefore, the rotation period has been decided, the habitability of Venus must remain in question. If it always turns the same face to the Sun, there

would be sufficient to put it out of court. In the six weeks in which Mercury moves from aphelion to perihelion, it approaches the Sun by fourteen millions of miles, and the heat received by it is increased 2? times. Then, in the next six w

ears. At perihelion it is 31 millions of miles from the Sun; one-third the distance of the Earth. It receives, therefore, at this part of its orbit, 9 times as much light and heat as the Earth. But at aphelion it retreats deep into the region of the asteroids, and is much more than four times the mean distance of the Earth. At this part of its orbit it receives but 1?17th as much heat as the Earth. By far the most famous of all the comets is that known by the name of Halley, and its mean period is 7

of small stones. But even if comets were of planetary size it is clear that life could not be supported on them;

minor planets. Minor planets indeed they are, for the one first discovered-Ceres-probably outweighs all the rest, known and u

le conditions resulting from smallness of size that the Moon presents are therefore exaggerated exceedingly in the case of Ceres; its atmosphere must approach in tenuity what we should regard as a vacuum in a terrestrial laboratory, and water as a liquid be entirely unknown. Its distance fr

t on the Earth only on the upper face of its crust, and in a very thin film of air and water; but the enormous solid bulk within, inert though it be, that supports the stage on whi