Solar System

SOLAR, SYSTEM. — It is the whole of the Sun and of the celestial bodies (planets and their satellites, minor planets, comets and certain numerous swarms of meteors) that gravitate and move around it. The planets currently known are nine: Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto (in order of increasing distance from the Sun), whose characteristic data are summarized in the enclosed Table I, while the data of the 31 satellites that revolve around these planets are set out in the following Table II (except for the Moon [v.]).

In antiquity only the first five planets were known (because they are the only ones visible to the naked eye); only in 1781 was Uranus added (discovered by W. Herschel); in 1801 the first of the minor planets (Ceres, discovered by the abbot

Piazzi at Palermo) and these soon became a numerous family, then, with photography, a very numerous one; in 1846 Neptune was discovered (predicted by calculation by U. Le Verrier in Paris and found in the sky by the German Galle in Berlin), and finally in 1930 Pluto (also predicted theoretically by P. Lowell and found photographically by Tombaugh). All these bodies are subject to universal gravitation and move in obedience to Kepler's laws, mutually perturbing one another, but without however altering the stability of the system. Its boundary is represented — at least as far as is known so far — by the orbit of Pluto, surpassed only by the orbits of some of the periodic comets (v.) of longest revolution and, at aphelion, also by the orbit of Neptune.

The planets are distinguished into inner or inferior and outer or superior, according to whether their orbits around the Sun are inner or outer with respect to the Earth's orbit. The two inner planets, Mercury and Venus, never move too far in the sky from the position of the Sun, and are therefore visible only in the hours that follow sunset or that precede the rising of the day's star. With respect to densities and volumes, the planets are clearly divided into two groups, one of the planets similar to the Earth (Mercury, Venus, Earth, Mars and perhaps Pluto) with modest volumes and fairly high densities (terrestrial planets) and the other of the great planets (Jupiter, Saturn, Uranus and Neptune) of considerably greater dimensions and instead more moderate densities.

As regards the distribution of the planets' distances from the Sun, it seems that they are governed by a law of exponential type, which in its first form, called Bode's or Titius's law (Titius being the Latinized name of J. G. Tietz) of 1778, states that the distance of a planet in position n can be expressed by the formula D=0.4÷0.3 · 2ⁿ. The deviations obtained between the real distances and those supplied by this law are generally small for all the planets, except for Neptune and Pluto. Simpler is the recent law of G. Armellini: D=1.53^°, which contains only one parameter, instead of two, and yet gives the real planetary distances with greater approximation. In it one must put n=2 for Mercury, -1 for Venus, nere, 0 for the Earth, 1 for Mars, 2 and 3 for the minor planets (whose orbits are generally comprised between those of Mars and Jupiter), 4 for Jupiter, 5 for Saturn, 7 for Uranus and 8 for Neptune. This law supplies two distances for the minor planets; and this is in good agreement with reality, representing the beginning and the end of the asteroidal ring, but it unfortunately leaves a vacant place between Saturn and Uranus. It is important to note that all these laws, even if they leave room for some discussion, nevertheless all affirm the very remarkable fact that the distances of the planets from the Sun proceed according to an exponential law. Yet the intimate reason for this fact still escapes us, though it must undoubtedly have notable cosmological and cosmogonic repercussions. The planetary distances listed in Table I are the mean distances of the planets from the Sun; in reality they oscillate between a maximum and a minimum value fairly close to each other, as a result of the eccentricities of the elliptical orbits that they describe around the Sun, an eccentricity that is greatest for Pluto (0.25) and notable only for Mercury (0.21). Some descriptive notes are given here on the individual components of the s. s.

1. MERCURY

Extremely difficult to observe because of its great proximity to the Sun (from which it never departs angularly by more than 28°); on its surface only a few dark spots can be distinguished, which have nevertheless made it possible to reveal a rotation of 88 days, equal, that is, to the period of its revolution around the Sun, so that the planet always turns the same face toward the greater star, in the same way (and for the same reasons) as the Moon does with respect to the Earth. From the physical point of view, however, Mercury resembles the Moon much more than the Earth. Its dimensions and its mass are only slightly greater than those of our satellite. Its surface should present the same tormented nature as the Moon's, since Mercury reflects the Sun's light as the Moon does. And as for the latter, so too for Mercury the original atmosphere must have rapidly dissipated, because of the small values of gravity on Mercury. Mercury is the hottest planet of the s. s. in the hemisphere constantly turned toward the Sun (over 400° C according to the most recent measurements), and at the same time the most

TABLE I

Planets (of the s. s.)

TABLE II

Satellites (of the s. s.)

cold in the hemisphere of eternal night, the temperature probably approaches absolute zero (—273° C), and is therefore lower than that of Pluto itself. The orbit of Mercury, except for that of Pluto, is the most inclined to the ecliptic (7°) and the most eccentric. Its distance from the Sun varies between 46 and 70 million km. Mercury is the planet that presents the greatest mean orbital velocity, ca. 48 km/sec. It has no satellites.

II. VENUS

It is the brightest of the planets and also of all celestial objects after the Sun and the Moon, sometimes becoming visible to the naked eye in broad daylight. Venus completes its revolution around the Sun at a mean distance of 107 million km, describing an orbit of small eccentricity in 225 days. Few details can be discerned on its surface (whence the value of its rotation period is still somewhat uncertain), which always appears of a nearly uniform white. It is thought that what can be observed is only the outer part of a thick layer of clouds that envelop the entire planet. Spectroscopic analysis has not, however, revealed in it any appreciable trace of water vapor or of oxygen, but only a large quantity of carbon dioxide. Bolometric measurements have shown a temperature of 55° C on the face illuminated by the Sun; but this is naturally the temperature of the cloud layers. The temperature of the ground should be around 100° C because of the greenhouse effect produced there by the carbon dioxide of the clouds, which blocks the infrared radiation re-emitted by the ground itself. The physical conditions on Venus, contrary to what was previously thought, must therefore be quite different from those on Earth. Venus has no satellites.

III. MARS

It is the first of the outer planets, which are much easier to observe than Mercury and Venus, since they present their disks fully illuminated by the Sun when they are at their minimum distance from the Earth. Unlike Mercury and Venus, they also show only small phase variations. Mars has a diameter slightly larger than that of the Earth, and its mass is 10 times smaller. Its distance from the Earth varies between 55 and 101 million km. The numerous observations that have been possible when the planet is at its minimum distance from the Earth (great oppositions, which recur about every 16 years, that is, when Mars is near its perihelion: the last occurred in 1939, the next will be in 1956), have shown that about half of the surface of Mars presents a red-orange coloration against which certain dark spots stand out, sometimes of a clear blue-greenish color, some fairly extensive, others thin and elongated, barely perceptible, in the form of canals. During the Martian winters a large white spot covers the poles of the planet. The period of rotation of the planet is known with great precision, and is appreciably equal to that of the Earth, just as the inclination of the Martian equator to the plane of its orbit is likewise equal (24°), so that Mars presents seasons entirely analogous to those of the Earth, but about twice as long because of the greater duration of the Martian year. The difference of the seasons between the two Martian hemispheres is, however, more pronounced than on the Earth because of the much greater eccentricity of the orbit. The principal sign of the Martian seasons is provided by the greater or lesser development of the polar caps, and by certain periodic and regular color variations that some spots present (green areas) and that could be an indication of seasonal development of vegetation.

The present situation of areography, that is, of the study of the surface of Mars (similar to "geography" for the Earth) is, however, in a phase of skepticism and discussion, since the new optical theories (initiated by V. Cerulli and M. Maggini) suggest that many of the appearances that one seems to discern and identify on the surface of Mars must rather be considered as simple optical illusions. In fact, when the human eye strives to distinguish the details of an object that is poorly seen (because of poor illumination or great distance, and the latter is the case of Mars), one has unconsciously the tendency to join ("integrate") the most notable points with straight-line segments. Some years ago this tendency also manifested itself with regard to the famous canals (a name first introduced by Fr. Angelo Secchi S.J. and later taken up by G. V. Schiaparelli and by P. Lowell and spread throughout the world) and their presumed "gemination." Recent photographs, obtained with a new technique at the Pic-du-Midi observatory in the high Pyrenees, have instead actually shown kinds of canals, though under aspects varied and different from the geometric forms that visual observers had a tendency to measure. In short, even if these canals exist, their nature is completely unknown. Astronomers expect new results from the giant telescope of Mount Palomar, which in 1956 will be able for the first time to be used for the observation of Mars. More certain, on the other hand, seem to be our knowledge of other physical characteristics of the Martian surface derived from spectroscopic and polarimetric observations, especially in the infrared. Mars possesses a tenuous atmosphere, which should have a height of about 80 km and in which no traces have been found of either oxygen or water vapor (if these two elements exist, they cannot exceed 0.15% of the quantities present in the Earth's atmosphere). Nevertheless the polar caps must be composed of frozen water, probably however in a very thin layer on the order of a tenth of a millimeter in thickness, in suspension in the very cold atmosphere of the planet. Bolometric measurements of the temperature of the Martian surface have shown that it reaches +10° C at the equator in summer, while it is close to -70° C at the pole in winter. The green areas would be due to elementary forms of vegetation such as mosses or lichens, particularly the latter because of the possibility of fixing nitrogen directly from the soil rather than from the atmosphere.

Mars has two satellites (Phobos and Deimos), both very small and very close to the planet. The period of revolution of Phobos is 7h39m, that is, less than the period of rotation of the planet, so that it, seen from Mars, would appear to rise in the west and set in the east.

IV. THE MINOR PLANETS OR ASTEROIDS

This is the name given to a conspicuous group (more than 1,600 known so far) of small bodies circling the Sun on elliptical orbits generally comprised between Mars and Jupiter. Their dimensions are on the order of about fifty km or even less. Only one, Vesta, is barely visible to the naked eye. Their number is very great (it is thought to be more than fifty thousand) and every year several new ones are discovered. The first, Ceres, was discovered by the abbot G. Piazzi at Palermo on 1 January 1801; in 1802 H. W. Olbers discovered the second, Pallas; in 1804 K. L. Harding discovered the third, Juno; in 1807 Olbers again discovered Vesta. These are the four largest minor planets, which have diameters of 770 km for Ceres, 480 for Pallas, 385 for Vesta and 195 for Juno. All the minor planets are too small to possess an atmosphere. Their masses are little known: that of Ceres would be 1/8000 of the mass of the Earth. But it has been possible to estimate that the total mass of the minor planets is certainly less than the Earth's mass, and perhaps even than that of the Moon. As for their origin, it seems that, at least for a notable group of them, they must have come from explosions of some larger planets. But astronomers are not all in agreement on this question, which still presents several obscure points. The orbits of the minor planets show a far greater variety than those of the large planets. For the majority the mean distance from the Sun is between 2 and 3.6 astronomical units. The period of revolution has a mean value of 4.5 years, but the extreme values are 2.5 and 12 years. Because of the very elongated shape of their orbits some minor planets pass very close to the Earth; thus Hermes, which comes as close as 1.5 million km.

V. JUPITER

Jupiter is the largest planet of the s. s., with a diameter 11 times greater than that of the Earth and 1/10 that of the Sun. Its mass, which is more than 300 times that of the Earth, is by itself greater than the mass of all the planets taken together. It is also the planet that exhibits the highest value of the velocity of rotation (period 9h50m); whence there results a strong polar flattening, clearly visible even directly through a telescope. This flattening is, however, less than what would obtain if the planet were similar to that of the Earth. Hence it is thought that an important part of its volume consists of a gaseous atmosphere. This is the only one that is observed directly, appearing with characteristic bands parallel to the equator, of a shape and coloration that vary from time to time (among them the famous "red spot"). Direct radiometric measurements have revealed a mean temperature of -135° C, and this indicates that the surface of the planet is heated solely by solar radiation. The spectrum of Jupiter (as, for that matter, that of the other large distant planets) shows characteristic bands in the red and infrared region, bands that have been recognized as belonging to ammonia and methane, of which the former must be found condensed into crystals at the temperature of -135° C, while the latter is still gaseous. Hence the thick clouds of Jupiter that are observed must be formed of ammonia crystals suspended in an atmosphere composed of methane and probably also of hydrogen. The various colorations would then be due to metallic compounds.

Jupiter has 12 satellites, which are numbered according to the order of discovery, except for the first five, which also have a proper name. The four largest, Io, Europa, Ganymede, Callisto, discovered by Galileo ("medicean planets"), have diameters that reach that of the Moon and would be visible even to the naked eye if they were not drowned in the glare of the planet. Since they move near the plane of the ecliptic, they are periodically subject to passing in front of (transit) or behind (eclipse) the disk of Jupiter. The other 8 satellites are difficult to observe, and some of them may be a minor planet captured by the larger body. The fifth satellite is the closest to the planet, around which it revolves quite rapidly with a velocity that exceeds a thousand km per minute.

VI. SATURNO

It is the most distant planet known to the ancients and the only one endowed with a magnificent system of rings, concentric with the planet and situated in its equatorial plane. Glimpsed by Galileo ("altissimum planetam tergeminum observavi") and recognized in their nature as rings by G. Huygens in 1655, they appear divided into 3 separate rings, of which the outermost has an outer diameter of 280,000 km, that is, 2.3 times the equatorial diameter of the planet. Their thickness is extremely small, perhaps only a dozen km. The rings are made up of a collection of very small satellites revolving separately around the planet according to the laws of gravitation, so that the innermost ones rotate faster than the outer ones. According to the research of G. Kuiper (1950) they would be composed of small fragments of frozen water. The total mass of the rings is very small, on the order of one millionth of the mass of the planet.

Another notable characteristic of Saturn is its low density (0.71, less than that of water), the smallest among all the bodies of the s. s. In fact, while its mean diameter is 9 times greater than that of the Earth, and therefore its volume 750 times greater, the mass of Saturn is only 95 times greater than that of the Earth. Like Jupiter, Saturn must be surrounded by a thick opaque atmosphere whose appearance shows an aspect of the planetary disk with bands parallel to the equator, composed likewise of ammonia and methane, but the ammonia bands there are weaker and those of methane more intense.

Saturn has 9 satellites, of which the largest, Titan, is also surrounded by an atmosphere of methane, the only example of the presence of an atmosphere among all the satellites of the s. s.

VII. URANUS AND NEPTUNE

These two planets, very similar to each other, are the last, the most distant, of the group of the giants. Both have a diameter that is about 4 times the Earth's diameter; Uranus is a little larger than Neptune, and the latter is therefore instead more massive. Because of their great distance, little is seen on the disk of Uranus and practically nothing on that of Neptune. Both appear with a slightly greenish-bluish tint. The spectroscope reveals in them the same spectral bands of ammonia and methane; but those of ammonia are much weaker there, and those of methane are instead strengthened, covering almost completely the red and yellow regions of their spectrum, and this explains the greenish coloration of these two planets. Like the other giant planets, Uranus and Neptune rotate very rapidly around their respective axes, and therefore they too have a notable polar flattening. A peculiarity of Uranus is that its equatorial plane is almost perpendicular to the plane of the orbit it describes around the Sun. And since the angle of the two planes slightly exceeds 90°, the rotational motion of Uranus is considered retrograde. Uranus has 5 satellites and Neptune 2.

VIII. PLUTONE

Very little is known about this very distant planet, which appears in the telescope with a faint yellowish color. Much is hoped to be learned when it passes perihelion in 1989, since at that time it will be at a distance from the Sun equal to that of Neptune. For now, it has only been possible to measure its diameter with the great telescope of Mount Palomar (Kuiper and Humason) and from this to deduce its actual diameter, and then, through the value of its mass, obtained in turn from the perturbations on Neptune, the value of its volume. The mass of Pluto would be on the order of 0.8 that of the Earth. Hence it does not resemble the giant planets at all, but rather the terrestrial ones. On the basis of its apparent luminosity, Pluto should have a weak reflecting power (or albedo), that is, similar to that of the Moon, or even less; hence it is deduced that it should not be surrounded by any opaque atmosphere. Most of the gases must in fact be in a liquid or solid state because of the very low temperature that prevails on the surface of the planet.

Lucio Gialanella