PLANETARY SUNS.
UTE presume we may ascribe to Mr. R. A. Proctor the very interesting paper in the new number of the Cortahili Illayazine, in which he pushes further the argument already advanced in his masterly book, "Other Worlds than Ours,"* for the partially solar character of the superior planets. The general character of the facts and arguments in the book and the article are too strikingly alike, to leave any substantial doubt on the authorship of the latter. But we find from it that Mr. Proctor's view has gained fresh probability in his eyes within the last two years, and this is no doubt due chiefly to new observations of Jupiter made by Mr. Browning during the autumn of 1870, the year in which his book was published,—and by Mr. Lassell and Father Seochi still more recently. The general drift of all their observations is to show that changes of extraordinary magnitude and violence take place from time to time in the cloud-belts of Jupiter,—that even the great creamy-white belt of the equatorial regions sometimes changes to a brilliant orange ; that Jupiter, under the influence of these potent causes, whatever they be, exhibits the most wonderful bands of prismatic colouring,—rose- colour strewn with yellow clouds, brown, orange, blue, purple, olive-green. Further, there is evidence of the most tremendous hurricanes taking place on his surface. Mr. Proctor—if it be he who writes in the Cornhill—says that in 1860 " a rift in one of the Jovian cloud-belts behaved in such a way as to demonstrate the startling fact that a hurricane was raging over an extent of Jovian territory equalling the whole surface of our earth, at a rate of fully 150 miles per hour. It is not too much to say that a hur- ricane of like velocity on our earth would destroy every building in the territory over which it raged, would uproot the mightiest forest trees, and would cause in fact universal desolation. At sea no ship that man ever made could withstand the fury of such a storm for a single minute. And yet this tremendous Jovian hurricane con- tinued to rage with unabated fury for at least six weeks,—or at least one hundred Jovian days." Now, taking into consideration these violent atmospheric storms, and also the far more potent causes to which the wonderful changes in the colouring of the planet's bands must in all probability be ascribed, and remembering that there is hardly any seasonal change in Jupiter to which great changes of local temperature and consequently of atmospheric disturbance could be referred, and finally, that the spectrum analysis of a powerful light shining through tubes filled with steam showed Jannsen the very same bands which he found in the analysis of the planet's spectrum, Mr. Proctor infers with a very high degree of probability that there must be an enormous generation of heat within the planet, not due to the sun's action upon it,—and that this is the explanation of the wonderful phenomena we witness. In fact, he concludes that the surface of Jupiter is probably at something like the temperature of red-hot iron,—not incandescent with white heat like the Solar surface, but still hot enough to convey a great deal of heat and perhaps not a little light to the four planets which circle round him. Mr. Proctor shows by the extreme darkness of the satellites in passing over Jupiter's surface,—a darkness which must mean that Jupiter shines with a relatively stronger light than that of the satellite passing across it, though both are equally reflecting back the sun's light,—that in all probability this conclusion is true, and that Jupiter, as well as reflecting the sun's light, emits some little light of his own. The most delicate measurements of light have established that Jupiter shines three times as brightly as his globe would shine if con- stituted like the planet Mars, and four times as brightly as his globe would shine if constituted like our moon,—from which, again, Mr. Proctor infers that the reflected solar light in Jupiter is rein- forced by a certain amount of intrinsic light of his own. There are some still more cogent reasons for attributing the same quasi-solar condition to the planet Saturn, the whole shape of which, as has been observed by many astronomers, alters at times in so remarkable a manner that it is hardly possible to doubt but that this planet also contains in itself an enormous
store of heat, under the influence of which either the sur- face of the planet itself or those great belts of cloud which are by our astronomers necessarily confounded with the body of the planet, undergo extraordinary convulsions. Hence, argues Mr. Proctor, the superior planets are most likely quasi-suns, sources of heat for the satellites which circle them, and more or
* Longmans. 1870. lees sources of light also, even in positions where they do not reflect to their satellites the son's light. Even when Jupiter lies between his satellites and the sun, even when he outs off from them the sun's light, as he generally does under those circumstances, since three of the satellites are almost always eclipsed when otherwise they would be fall, and even the fourth is very frequently so eclipsed, Mr. Proctor thinks that the inhabitants of these satellites, if there be any, see Jupiter glowing with a dull red or yellow glare,—a slow fire shining through an envelope of mist and cloud, something like what our sun gives out when seen on the horizon on a foggy November morning, but occupying, for the inhabitants of the nearest satellite, something like fourteen hundred times as much superficial area as our sun appears to us to occupy,—i.e., as much as a square filled up with suns of the same apparent magnitude as our own, and counting thirty-eight such seeming suns in every line and on every side of the square ;—or say that it would be a gigantic light covering at least as much of the horizon as subtends the angle made by the London and South-Western line with the Windsor branch. Further, Mr. Proctor calculates the density of the four little satellites of Jupiter as much higher than they have hitherto been calculated, giving to the three outer satellites a decidedly greater density than the planet itself,—and therefore a consistency much more suitable to creatures organised like ourselves,—and to the inner one nearly the same density. He infers, then, that if beings like ourselves exist, at present at least, in the Jovian and Saturnian systems at all, it must be on the satellites, not on the planets themselves, which are probably as uninhabitable to beings like ourselves as the sun ; and that these satellites are warmed, and partly lighted, by the heat which the giant planets give out.
If Mr. Proctor is right, as seems likely, the inhabitants of these four little Jovian satellites, of which the biggest has a sur- face less than one-fifth of that of the earth,—say (at most) as much as our continents alone, excluding the oceans and the islands, —must have had incitements to astronomical study such as we can hardly conceive. The satellite nearest to Jupiter must see the glowing colours of the brilliant prismatic rings on the huge planet with a splendour altogether overwhelming. It revolves round the planet in a period of forty-two and a half hours of terrestrial time, a period during which there have been four complete revolutions of the planet itself, four Jovian days and four Jovian nights. So that, without taking into account the length of its own solar day and night, which is not known (though it is now generally believed that the satellites do not always turn the same face to Jupiter, as our moon does to the earth, but turn on their own axes either slower or faster than they revolve round Jupiter, so as to give all their inhabitants their turn at receiving light and heat from the planet), the astronomers of the satellite must see this enormous planet once in every twenty-one hours reflecting sun- light, and once in every twenty-one hours shining solely by its own light, as well as being able in the interval to compare a part of Jupi- ter under sunlight with a part irradiated only by its own clouded glare. Together with these wonderful visions,—occupying spaces in their horizon of which we can hardly conceive,—they have the constant and curious experience of watching the motions of the other satellites, and of chasing some of them round the great planet with a speed almost comparable to that with which one hand of a watch chases the other; and in addition to all this, they have great helps for studying the constitution of our Sun in the changes which they have most wonderful opportunities for observing in that miniature sun, their own red-hot primary, which is not much further off them than our earth is from the moon, but which is a spectacle indefinitely mightier and grander than even the earth would be as seen from the moon, and even though the earth were, as Jupiter seems to be, red-hot, and with all its seas floating above it in the shape of belts of glowing vapour. No doubt the astronomers of Jupiter's satellites are not in nearly so good a position for observing the Sun itself as we are ; for the Sun's diameter seen from Jupiter is five times less than it is as seen from the earth, and consequently their direct observations of the solar spots, red prominences, corona, and so forth, must; caeteris paribus, be far less striking than ours. But then in the changes taking place on the surface of Jupiter itself,—the dark spots which appear and disappear, and into the depths of which they must be able to gaze with far more minuteness than we can even into the craters of our moon,—they must have a sort of easy introduction to the theory of the solar surface itself. Indeed, the enormous grandeur and great variety of the astronomical phenomena visible within the Jovian system itself must have fixed attention on them, and led to the discovery of the great natural laws, if the inhabitants of
the satellites have any faculties like ours, at a relatively much earlier period of their history. A world with a face, not like that of our moon, dead and perpetually the same, but occupying 1,400 times as much space on the horizon, and changing colour so that a white band turns brilliant rose-colour, and a rose-colour band turns purple, while here and there a great dark spot spreads over its surface, or a tornado changes the coast-line of the huge belts of clouds till you seem to see the innermost processes of nature's chemistry taking place on the most portentous scale before your eyes, must present a spectacle of both scientific and popular interest such as no dweller on our planet can easily realize.
After reading Mr. Proctor's essays on Jupiter, one can hardly help asking oneself whether the highest kind of life does not per- haps belong to the minutest worlds,—such worlds as these satellites of Jupiter, where it seems as if life must be intenser and more rapid, though in many respects smaller in scale than even on our own little planet. Certainly the big places of the universe, the Suns, and the biggest planets of which we know anything, seem absolutely unfitted for any life sharing the smallest analogies to our own ; and it would seem as if these huge centres of heat and light were subsidiary to the life of relatively almost infinitesimal spheres. If the gigantic Jupiter be, as Mr. Proctor evidently believes, a miniature sun, only auxiliary to the life on those minute beads of light which we see as mere pin-heads in the sky, circling round him, and if the same be true, as he thinks, of Saturn and Uranus, then clearly, as far as we have any means of knowing, the great things of the universe, from the San downwards, seem to be rather subservient to the small things, than the small things to the great. And if we consider that, after all, moral life grows out of experience, and that experience depends more or less on the rapidity of human conscioneness, on the number of true discrimina- tions which we are taught to make within a given time, it is quite probable that that Jovian system, with its wonderfully short months,—(the furthest and slowest satellite completes the whole vast circle of its orbit round Jupiter in sixteen days and a half, rather more than half our month, while the nearest and quickest satellite makes its circuit in less than two days),—with its enormous year,—Jupiter's year is as long as nearly twelve of our terrestrial years,—with the crowds of astronomical phenomena which happen within that time, the miniature sun going through its revolutions and showing the whole circle of its gorgeous belts every ten hours, and eclipses of all the satellites by Jupiter's shadow, and transits of the inner satellites over Jupiter's disc, happening with great frequency,—it is quite probable, we say, that a system such as this is one favourable to a more rapid and complete mental development than that of any world of larger dimen- sions in which the natural divisions of time are fewer and the phenomena crowded into them fewer too. Of course these conjectures must be very uncertain, after all ; but it is hardly possible to conceive the brilliancy and vast variety of astronomical spectacles exhibited to the inhabitants, if there be inhabitants, of those tiny satellites of Jupiter—(and still more of Saturn, where the magnificent spectacles presented by the rings and also by a variety of seasonal changes are added to all that is strange in the astronomical phenomena of Jupiter),—without hazarding the speculation that it is the smallest worlds for which, after all, the great globes are made, and that it may well be in the smallest worlds that the highest education of reasoning beings belonging to our solar system really takes place.
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