It is curious that these four outer planets, that resemble each other so strikingly in many of their conditions—in their vast size, high albedo, low density, and vaporous envelopes, that show, in their spectra, not merely the lines of reflected sunlight, but also special lines due to their own atmospheres (the chief of these being common to all the four planets)—should yet, in the inclination of their axes to the plane of their orbits, display every possible variety. The axis of Jupiter is almost normal to its orbit, that of Uranus lies almost in the plane of its orbit. The axes of Saturn and Neptune have a mean inclination, but it would appear that the rotation of Neptune is in the reverse direction to that of planets in general, so that the true inclination is usually taken as being the complement of the observed angle, as if the axis were turned right over. It is uncertain whether this would have any important effect upon the habitability of the planet, but it supplies the fourth possible case for the position of the axis.
CHAPTER XII
THE FINAL QUESTION
In passing in review the various members of the solar system, it has been seen that there are many conditions that have to be fulfilled before a planet can be regarded as the possible abode of life, because there are many conditions necessary in order that water may exist on its surface in the liquid state. The size and mass of the planet are restricted within quite narrow limits; and a world much larger or much smaller than our own is necessarily excluded. The supply of light and heat received from the Sun must not fall much below that received by the Earth, nor greatly exceed it; in other words, the distance of the planet from its Sun is somewhat precisely fixed, since the light and heat vary inversely not as the distance, but as its square. Of course, in different systems, with suns of different power, the most favourable distance will not be the same in each; but in any system there will be one most advantageous distance, and no great departure from it will be possible. This condition further implies that the planetary orbits must be nearly circular; pronounced eccentricity, such as the orbits of even our short-period comets display, would be fatal to the persistence of water in the liquid state, and hence to the continuance of life. A wide discordance between the planes of the planet’s equator and of its orbit, by rendering the seasons extravagantly diverse, would act as prejudicially as an eccentric orbit, and a rotation period equal to that of revolution would mean that one hemisphere was eternally frozen while the other was exposed to perpetual heat.
It follows that in any given system there can be at most only one or two planets upon which life can find a home, and this only where the right conditions of size and mass, of rotation period, inclination of axis, and shape of orbit, all co-exist in a globe at the proper distance. But the type of system offered by our Sun and his planets is not the only one that exists. A very large proportion of stars are binaries—two suns revolve round their common centre of gravity. In many cases the two suns are separable in the telescope, and their relative movements can be measured; in other cases, termed “spectroscopic binaries,” we only learn that a star which appears absolutely single has two components from the evidence of its spectrum; the spectroscope revealing two sets of lines that vibrate to and fro with respect to each other. Yet, again, a third class of double stars has made itself known in the “Algol variables.” The optical double stars are cases where the two components are far distant from each other, and hence can be distinguished in our telescopes as separate points of light. The “spectroscopic binaries” are cases where the two components are too close to be separately perceived, but where the two are not greatly unequal in brightness, so that the spectrum of the one does not overpower that of the other. The “Algol variables” are cases where the two components are of very unequal brightness, and, being very close to each other, are so placed with respect to the Earth that the fainter partly eclipses the brighter in its revolution round it, and so causes a temporary diminution in its light at regular intervals. All these three classes of binary systems are now known to be very numerous. Prof. Campbell estimates that fully one star in six is a spectroscopic binary. But there must be many binary systems that do not reveal themselves—double stars where the companion is too faint or too close to be detected, Algol systems where the companion does not pass before its primary—and it seems almost certain that simple systems, like that of which our Sun is the unchallenged autocrat, must be comparatively rare.
But the problem of the movements of a planet attendant upon two or more suns is one of amazing complexity, and our greatest mathematicians have as yet only been able to deal with the approximate solution of a few very special cases. These are, however, sufficient to show that the orbit of a planet so placed would be most irregular; the variations in the supplies of light and heat received would be as great as even comets experience within the solar system, and, what would be more disastrous still, these variations would not be periodic but irregular. One year would be unlike that which preceded it, and would be followed by changed conditions in the next. Plants and animals would never have the chance of acclimatizing themselves to these ever-changing vicissitudes. The stability of condition essential for the maintenance of water in a liquid state would be wanting; and, in consequence, Life could neither come into existence, nor persist if it once appeared.
So far, therefore, our line of thought has led us to recognize that Life can exist in comparatively few of the innumerable stellar systems strewn through infinite space, and in any given system it can at best find only one or two homes. The conditions for a Life-bearing planet are thus both numerous and stringent—there is no elasticity about them. It is not sufficient that a planet might fulfil many or even most of these conditions; failure in one is failure altogether; “one black ball excludes;” the candidate who fails in a single subject is “ploughed” without mercy. And in most cases the failure is final; no opportunity is given to the candidate to “sit” again.
But Space is not the only horizon along which our thought must be directed; there is also the horizon of Time. Every world must have its Past and its Future, as well as its Present. For some worlds the conditions are so fixed that, like Jupiter and Saturn, they are not now worlds that can be dwelt in, they never were in that condition, and they never can be; their enormous mass forbids it. Mercury and the Moon at the other end of the planetary scale are also permanently disabled; their insignificant size excludes them. There was also a time when the Earth was not a world of habitation; it was “without form and void”; hot and vaporous, even as the four outer planets are now. Now it is inhabited, but there may come a time when this phase of its history has run its course, and either from a falling off in the tribute of light and heat rendered to it by the Sun, or from the gradual desiccation of the surface, or, perchance, from the slow loss of its atmosphere, it may approach the condition of Mars, and in its turn be no longer an abode of life. Many planets are essentially debarred from ever entering on the vital stage; but of those to which such a stage is possible, it can only form an incident in the entire duration of the orb. And if our Earth is any type or example of the vital stage in general, vast aeons must run their course from the first appearance of the humblest germs of life up to the bringing forth of Life in conscious Intelligence. One hundred million years are freely spoken of in this connection by those who study the crust of the Earth and those who are occupied with the relations of the varied forms of life. Man is the latest arrival on this planet, and however far back we try to push the time of his earliest appearance, it is beyond question that that time, relatively to the entire duration of the Earth since a solid crust began to form, is but as yesterday. If, from some other globe in the depths of space, this world of ours could have been watched during the long aeons that elapsed from its first separation from the solar nebula down to the time when it first possessed a surface of land and water, and from that time, again, throughout the hypothetical one hundred million years that preceded the advent of man, then, during all those aeons, those imagined observers would have had under their scrutiny a world as yet without inhabitant. The Earth now is in the inhabited condition, but science gives us no clue as to how long that condition will endure; rather such hints as are afforded us would seem to point to its lasting but for a brief season as compared with the indefinite duration which preceded it, and the indefinite duration which shall follow.