Planetoid

“ As ́teroids, Planetoids, or Minor Planets, a numerous group of very small planets revolving round the sun, in the great majority of cases at mean distances, intermediate between those of Mars and Jupiter, in orbits of large eccentricity at considerable inclination to the ecliptic. The diameter of the largest is not supposed to exceed 450 miles, while most of the others are very much smaller. Over one thousand are known, and new members are being constantly discovered. ”

“ The planets are all globes, more or less flat at the poles, like an orange, and each is turning and swaying on its axis, thus exposing every part to the light and warmth of the sun. They are divided by the planetoids into an inner and an outer band. The inner four are Mercury, Venus, the Earth, and Mars; the outer four are Jupiter, Saturn, Uranus, and Neptune. Moreover, a number of comets and swarms of meteoric stones or meteorites are circulating round the sun in eccentric paths, which cross those of the planets. ”

“ There are so many of the planetoids, and they are so minute, that even after they have been discovered they are frequently lost again. Hence it is sometimes uncertain when they register themselves on the photographic plates whether they are really new to us or have been known before. ”

“ Admitting, however, the hypothesis that these planetary bodies, are the remains of a larger body, which circulated round the sun, nearly in the orbit of the greatest fragment, the system resumes its order, and we discover a regular procession in the distances of the planets, and a general harmony in the form and position of their orbits. But, independently of analogical reasoning, the elements of the new planets furnish several direct arguments, drawn from the eccentricity and inclination of their orbits, and from the position of their perihelia and nodes ”

“ The planets are divided into “primary” and “secondary.” By a “primary” planet we mean one which directly circulates round the Sun; by 8 a “secondary” planet we mean one which in the first instance circulates round a primary planet, and therefore only in a secondary sense circulates round the Sun. The planets are also “major” or “minor”; this, however, is only a distinction of size.
The secondary planets are usually termed “satellites,” or, very often, in popular language, “moons,” because they own allegiance to their respective primaries just as our Moon—the Moon—does to the Earth. ”

“ In addition to the five or six large planets in each group, which, however, are many millions of miles apart, there is in some clusters a small planet that swings backward and forward across the common centre, like a pendulum, but in nearly a straight line; and while this multiplicity of motion goes on, the whole aggregation sweeps majestically around Sirius, its mighty sun. Our little solar system contains, as we know, about one thousand planets, satellites, and asteroids large enough to be dignified by the name of heavenly bodies. ”

“ As the planets have been evolved so as to describe their several orbits around the sun, so in like manner the smaller systems of satellites have been so evolved as to describe their orbits round the several planets that are their respective primaries. When a planet, or rather the materials which were drawing together to form a planet, had acquired a predominant attraction for the parts of the primæval nebula in their locality, a portion of the nebulous material became specially associated with the planet. ”

“ COMPARATIVE SIZE OF THE PLANETS.
"Yes, dear," replied his sister; "only some planets, like Jupiter and Saturn, are still wrapped up in a blanket of clouds and steam, and we cannot see them yet. They are very hot indeed, and all the water that will make the oceans and seas and bays is now steam and clouds hiding the true planet from view. Water could no more rest on the surface of the planets Jupiter and Saturn than it could rest on red-hot iron. Don't you remember, the other day, when nurse upset a cup of water on the hot stove, how the water sizzled and turned into steam in a moment? ”

“ But if we admit that the substance of which the solar system consists once existed in a nebulous condition, and was in rotation, all the above peculiarities follow as necessary mechanical consequences. Nay, more, the formation of planets, the formation of satellites and of asteroids, is accounted for. We see why the outer planets and satellites are larger than the interior ones; why the larger planets rotate rapidly, and the small ones slowly; why of the satellites the outer planets have more, the inner fewer. ”

“ If we consider the other bodies of our system, such, for instance, as Venus or Jupiter, we find that the orbit of Venus also lies in a plane, and that plane also passes through the centre of the sun. The orbit of Jupiter is found to be contained within a plane, and it, too, passes through the sun’s centre. Each of the remaining planets in like manner is found to revolve in an orbit which is contained in a plane, and all these planes have one common point, that point being the centre of the sun. ”

“ Several of these have one or more attendants, or satellites, which revolve around them as they revolve around the sun. The Earth has one satellite, namely, the Moon; Jupiter has four; Saturn, seven; and Uranus, six. These bodies are also planets, but, in distinction from the others, they are called secondary planets. Hence, the whole number of planets are twenty-nine, namely, eleven primary, and eighteen secondary, planets.
You need never look for a planet either very far in the north or very far in the south, since they are always near the ecliptic. ”

“ And now we observe that this star that we have drawn near to has attending it a number of minute satellites, faintly shining specks, that circle about it as if charmed, like night-wandering insects, by its splendor. It is manifest to us at the first glance that without the sun these obedient little planets would not exist; it is his attraction that binds them together in a system, and his rays that make them visible to one another in the abyss of space. Although they vary in relative size, yet we observe a striking similarity among them. ”

“ In Jupiter you would see creatures similar to those in Saturn, but with different powers of locomotion; in Mars and Venus you would find races of created forms more analogous to those belonging to the Earth; but in every part of the planetary system you would find one character peculiar to all intelligent natures, a sense of receiving impressions from light by various organs of vision, and toward this result you can not but perceive that all the arrangements and motions of the planetary bodies, their satellites and atmospheres, are subservient. ”

“ There are thousands of millions of suns in our galaxy, and not less than one in three has planets, and among these myriads of unknown worlds there will be thousands with seas and land and clouds and continents, fit for men to enter upon, there to rear their cities. There will be starships roaming distant sun-clusters, and landing on planets in the Milky Way. We ourselves will see freight-lines to Rigel and Arcturus, and journey on passenger-liners singing through the void to Andromeda and Aldebaran! ”

“ We shall begin with the planets nearest to the sun, and then we shall gradually proceed outwards to one planet after another, until we reach the confines of the system. We shall find much to occupy our attention. Each planet is itself a globe, and it will be for us to describe as much as is known of it. The satellites by which so many of the planets are accompanied possess many points of interest. The circumstances of their discovery, their sizes, their movements, and their distances must be duly considered. ”

“ Yet if a planet is fortunate enough to have one satellite or more, the astronomer's method of weighing the planet is exceedingly simple; and all the major planets have satellites except the two interior ones, Mercury and Venus. As the satellite travels round its primary, just as the moon does round the earth, two elements of its orbit need to be ascertained, and only two. ”

“ Indeed, we may say at once that this little object was not a star at all; it was a planet. That such was its true nature was confirmed, after a little further observation, by perceiving that the body was shifting its place on the heavens relatively to the stars. The organist at the Octagon Chapel at Bath had, therefore, discovered a new planet with his home-made telescope.
I can imagine some one will say, "Oh, there was nothing so wonderful in that; are not planets always being discovered? ”

“ It would thus appear that many great luminous exhibitions would attend the awful paroxysms with which a large planet passes away from the stage of existence in a solar or in a secondary system. Though a small satellite, if fluid, may meet its final doom in an obscure manner, yet, if solid, it would be likely to maintain a planetary form until it came very close to the primary ”

“ In the ample zone bounded on the inside by the orbit of Mars and on the outside by the orbit of Jupiter it was thought at one time that no planet revolved. Modern research has shown that this region is tenanted, not by one planet, but by hundreds. The discovery of these planets is a charge which has been undertaken by various diligent astronomers of the present day, while the discussion of their movements affords labour to other men of science. ”

“ When they first sprang into existence, in the outer zone of our system, each of these great planets was attended with a large companion which subsequently incorporated with the superior orb by a series of paroxysms, and thus occasioned the birth of a family of minor worlds. Accordingly, in a system of classification based on their modes of origin, neither our moon nor perhaps that of Neptune could be assigned to the same class which includes the satellites of Mars, Jupiter, Saturn, and Uranus. ”

“ A single moment's reflection upon the solar system will fully demonstrate the difficulties that have attended this subject; the principal planets are attracted by the sun, and the sun by the planets; the satellites are also attracted by their principal planets, and each planet attracts all the rest, and is attracted by them. All these actions and reactions vary according to the quantities of matter and the distances, and produce great inequalities and irregularities. ”

“ As a group, our solar system is comparatively isolated in space; the distance of the nearest fixed star being at least seven thousand times that of Neptune, the most remote known planet. Besides the central or controlling orb, it contains, so far as known at present, ninety-nine primary planets, eighteen satellites, three planetary rings, and nearly eight hundred comets. ”

“ The solution is—there are nine planets in this solar system! Neptune and Uranus are each far vaster than Earth; they are utterly impossible for life as we know it, but a small colony might be established there to refine metals for the distant Earth. We might be able to build domed and sealed cities. But first we could try the nearer planets—Mars, Venus, or some satellites such as our Moon. I certainly hope that this machine will make it possible. ”

“ For days and days, nothing seemed to change in the endless depths of black space as the Rigel sped toward Pluto. There were the same monotonous patterns of stardust and the eternally broad sweep of the Milky Way and other remote galaxies. Only the distant planets grew and shrank in size. As the space ship neared Neptune, the big green world enlarged importantly. Rob and Duff, in an off-duty hour, watched the frigid, lonely planet. ”

“ Ron Val, busy at the telescope, was too excited to look up from the eye-piece. "There are at least two planets circling Vega!" he said quickly. "There may be other planets farther out, but I can see two plainly. And Jed, the nearest planet, the one we are approaching, has an atmosphere. The telescope reveals a blur that could only be caused by an atmosphere. And—Jed, this may seem so impossible you won't believe it—but I can see several large spots on the surface that are almost certainly lakes. They are not big enough to be called oceans or seas. But I am almost positive they are lakes!" ”

“ In the smaller systems the largest outer planet sometimes draws to itself its neighboring worlds, while those planets near the sun begin their terminal plunge. With your solar system, such an end would mean that the four inner planets would be claimed by the sun, while the major planet, Jupiter, would be greatly enlarged by capturing the remaining worlds. ”