Solar System

SOLARE, SISTEMA. — It is the ensemble comprising the Sun and the celestial bodies (planets and their satellites, minor planets, comets, and several numerous swarms of meteors) that revolve and move around II. The planets currently known number nine: Mercurio, Venere, Marte, Giove, Saturno, Urano, Nettuno, and Plutone (in order of increasing distance from the Sun); their characteristic data are summarized in the accompanying Table I, while the data for the 31 satellites that revolve around these planets are given in the following Table II (except for the moon [v.]).

In antiquity only the first five planets were known (because they were the only ones visible to the naked eye); only in 1781 was Urano added (discovered by W. Herschel); in 1801 the first of the minor planets (Cerere, discovered by the abbot Piazzi in Palermo), and these soon became a numerous family, then, with photography, a very numerous one; in 1846 Nettuno was discovered (predicted by calculation by U. Le Verrier in Paris and located in the sky by the German Galle in Berlin), and finally, in 1930, Plutone (also theoretically predicted by P. Lowell and located photographically by Tombaugh). All these bodies are subject to universal gravitation and move in obedience to Kepler’s laws, perturbing one another but nevertheless without altering the stability of the system. Its boundary is represented — at least insofar as is known to date — by the orbit of Plutone, surpassed only by the orbits of some of the periodic comets (v.) with the longest periods of revolution and, at aphelion, also by the orbit of Nettuno.

The planets are divided into interior or inferior and exterior or superior, according as their orbits around the Sun lie inside or outside the orbit of the Earth. The two interior planets, Mercurio and Venere, never move very far in the sky from the position of the Sun, and are therefore visible only during the hours following sunset or preceding the rising of the star of day. With regard to density and volume, the planets fall clearly into two groups: one consisting of planets similar to the Earth (Mercurio, Venere, Terra, Marte, and perhaps Plutone), with modest volumes and fairly high densities (terrestrial planets), and the other consisting of the giant planets (Giove, Saturno, Urano, and Nettuno), which are considerably larger and instead have more moderate densities.

As regards the distribution of the planets’ distances from the Sun, they appear to be governed by a law of exponential type which, in its first form, called the law of Bode or Titius (the Latinized name of J. G. Tietz), dating from 1778, states that the distance of a planet in position n may be expressed by the formula D=0,4÷0,3 · 2ⁿ. The deviations obtained between the actual distances and those supplied by this law are generally slight for all the planets, except Nettuno and Plutone. The recent law of G. Armellini is simpler: D=1,53^°, containing only one parameter instead of two, and nevertheless giving the actual planetary distances with greater approximation. In it, n must be set at 2 for Mercurio, -1 for Venere,

nere, 0 for Terra, 1 for Marte, 2 and 3 for the minor planets (whose orbits generally lie between those of Marte and Giove), 4 for Giove, 5 for Saturno, 7 for Urano, and 8 for Nettuno. This law supplies two distances for the minor planets; and this agrees well with reality, representing the beginning and the end of the asteroid belt, but unfortunately leaves an unoccupied position between Saturno and Urano. It is important to note that all these laws, even if they leave room for some discussion, nevertheless all affirm the highly remarkable fact that the distances of the planets from the Sun proceed according to an exponential law. The intrinsic reason for this fact, however, still eludes us, although it must undoubtedly have notable cosmological and cosmogonic implications. The planetary distances listed in Table I are the mean distances of the planets from the Sun; in reality, as a result of the eccentricities of the elliptical orbits that they describe around the Sun, they oscillate between a maximum and a minimum value that are fairly close to one another; the eccentricity is greatest for Plutone (0.25) and significant only for Mercurio (0.21). Here are given a few descriptive notes on the individual components of the s. s.

1. MERCURIO

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; consequently, the planet constantly turns the same face toward the larger star, in the same way (and for the same reasons) as the Moon does with respect to the Earth. From a physical point of view, however, Mercurio resembles the Moon much more than the Earth. Its dimensions and mass are only slightly greater than those of our satellite. Its soil should present the same rugged nature as that of the Moon, since Mercurio reflects the light of the Sun like the Moon. And, as with the latter, Mercurio’s original atmosphere must also have rapidly dissipated, owing to the low values of gravity on Mercurio. Mercurio is the hottest planet in the s. s. in the hemisphere constantly facing the Sun (over 400^° C according to the most recent measurements), and at the same time the planet most TABLE I. – Planets (of the solar system) TABLE II. — Satellites (of the solar system)

Cold in the hemisphere of eternal night, its temperature probably approaches absolute zero (—273° C), and is therefore lower than that of Pluto itself. Mercury’s orbit, apart from that of Pluto, is the most highly 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 with the greatest mean orbital velocity, approximately 48 km/sec. It has no satellites.

II. VENERE

It is the brightest of the planets and indeed of all celestial objects after the Sun and the Moon, at times becoming visible to the naked eye in broad daylight. Venus completes its revolution around the Sun at an average distance of 107 million km, describing an orbit of slight eccentricity in 225 days. Few details can be discerned on its surface (hence the value of its rotation period remains somewhat uncertain), which always appears almost uniformly white. It is thought that what can be observed is merely the outer part of a thick layer of clouds enveloping the entire planet. Spectroscopic analysis, however, has revealed no appreciable trace of water vapour or oxygen, but only a great 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 in the clouds, which prevents the infrared radiation re-emitted by the ground itself from escaping. Physical conditions on Venus, contrary to what was previously thought, must therefore be very different from those on Earth. Venus has no satellites.

III. MARTE

It is the first of the outer planets, which are much easier to observe than Mercury and Venus, since they present their disks completely illuminated by the Sun when they are at their minimum distance from the Earth. Unlike Mercury and Venus, they also exhibit only slight variations in phase. Mars has a diameter only slightly greater than that of the Earth, while its mass is ten 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 approximately every 16 years, that is, when Mars is near its perihelion: the last occurred in 1939, and the next will be in 1956) have shown that approximately half of the surface of Mars presents a reddish-orange coloration, against which several dark patches stand out, sometimes of a distinct bluish-green color, some fairly extensive, others thin and elongated, barely perceptible in the form of channels. During the Martian winters, a large white patch covers the planet’s poles. The planet’s period of rotation is known with

great precision, and is essentially equal to that of the Earth; likewise, the inclination of the Martian equator to the plane of its orbit (24°) is equal, so that Mars has seasons entirely analogous to those of the Earth, but approximately twice as long because of the greater duration of the Martian year. The difference between the seasons in the two Martian hemispheres is, however, more pronounced than on Earth because of the much greater eccentricity of the orbit. The principal indication of the Martian seasons is provided by the greater or lesser development of the polar caps, and by certain periodic and regular variations in color that some patches exhibit (green areas) and which could indicate the seasonal development of vegetation.

The present situation of aerography, that is, the study of the surface of Mars (corresponding to “geography” for the Earth), is nevertheless one of skepticism and debate, since the new (optical theories (initiated by V. Cerulli and M. Maggini) suggest that many of the appearances that seem to be discerned and identified on the surface of Mars should rather be regarded as simple optical illusions. Indeed, when the human eye endeavors to distinguish the details of an object that is poorly seen (because of insufficient illumination or great distance, the latter being the case with Mars), there is an unconscious tendency to connect (“integrate”) the most notable points with straight-line segments. A few years ago this tendency also manifested itself with regard to the famous channels (a designation first introduced by Fr. Angelo Secchi S. J. and subsequently taken up by G. V. Schiaparelli and P. Lowell, and disseminated throughout the world) and their presumed “duplication.” Recent photographs, obtained with a new technique at the Pic-du-Midi Observatory in the high Pyrenees, have instead effectively revealed forms resembling channels, although with aspects varied and different from the geometric forms that visual observers had tended to measure. In the final analysis, even if these channels exist, their nature is completely unknown. Astronomers expect new results from the giant telescope at Monte Palomar, which in 1956 will be available for the first time for the observation of Mars. Our knowledge of other physical characteristics of the Martian surface, derived from spectroscopic and polarimetric observations, especially in the infrared, seems more certain. Mars possesses a tenuous atmosphere, which should have a height of approximately 80 km and in which no traces have been found either of oxygen or of 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 consist of frozen water, probably, however, in an extremely thin layer approximately one-tenth of a millimeter thick, suspended in the planet’s very cold atmosphere. Bolometric measurements of the temperature of the Martian surface have shown that it reaches +10° C at the equator in summer, whereas 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 their ability to fix 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. Phobos’s period of revolution is 7h39m, that is, shorter than the planet’s period of rotation; consequently, as seen from Mars, it 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 considerable group (more than 1,600 so far known) of small bodies revolving around the Sun in elliptical orbits generally lying between Mars and Jupiter. Their dimensions are on the order of fifty kilometres or less. Only one, Vesta, is just visible to the naked eye. Their number is extremely large (it is thought to exceed fifty thousand), and several new ones are discovered every year. The first, Ceres, was discovered by Abbot G. Piazzi in Palermo on 1 Jan. 1801; in 1802 H. W. Olbers discovered the second, Pallas; in 1804 K. L. Harding discovered the third, Juno; in 1807 Olbers discovered Vesta as well. These are the four largest minor planets, with 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 mass of the Earth, and perhaps even less than that of the Moon. As for their origin, it seems that at least a considerable group of them must have arisen from the explosions of certain larger planets. Astronomers, however, are not all in agreement on this question, which still presents several obscure points. The orbits of the minor planets display a far greater variety than those of the major 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. Owing to the highly elongated form of their orbits, some minor planets pass very close to the Earth; such is the case with Hermes, which approaches to within 1.5 million km.

V. GIOVE

Jupiter is the largest planet of the s. s., with a diameter 11 times greater than that of the Earth and one-tenth 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 with the highest rotational velocity (period 9 h 50 m); hence it has a pronounced polar flattening, clearly visible even directly through a telescope. This flattening is nevertheless less than it would be if the planet were similar to the Earth. It is therefore thought that an important part of its volume consists of a gaseous atmosphere. This is the only one that can be observed directly, appearing in the form of parallel bands along the equator, whose shape and coloration vary from time to time (including the famous “red spot”). Direct radiometric measurements have revealed an average temperature of –135° C, indicating that the planet’s surface is heated solely by solar radiation. Jupiter’s spectrum (as, indeed, do those of the other large distant planets) shows characteristic bands in the red and infrared regions; these bands have been identified as belonging to ammonia and methane. The former must be condensed into crystals at a temperature of –135° C, whereas the latter is still gaseous. Hence the thick clouds of Jupiter that are observed must consist of ammonia crystals suspended in an atmosphere composed of methane and probably also hydrogen. The various colors would then be due to metallic compounds.

Jupiter has 12 satellites, numbered according to the order in which they were discovered, except for the first five, which also have proper names. The four largest—Io, Europa, Ganimede, and Callisto—discovered by Galileo (“Medicean planets”), have diameters approaching that of the Moon and would also be visible to the naked eye if they were not lost in the planet’s glare. Since they move near the plane of the ecliptic, they periodically pass in front of (transit) or behind (ecilisi) Jupiter’s disk. The other eight satellites are difficult to observe, and one or more 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 very rapidly, at a speed exceeding one thousand kilometers 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. First glimpsed by Galileo (“attissimum planetam tergeminum observavi”) and recognized as rings by G. Huygens in 1655, they appear divided into 3 separate rings, of which the outermost has an external diameter of 280,000 km, or 2.3 times the planet’s equatorial diameter. Their thickness is extremely small, perhaps only a dozen kilometres. The rings consist of an assemblage of tiny satellites rotating independently around the planet according to the laws of gravitation, so that the innermost rotate faster than the outer ones. According to the research of G. Kuiper (1950), they would consist of small fragments of frozen water. The total mass of the rings is very small, of the order of one-millionth of the planet’s mass.

Another notable characteristic of Saturn is its low density (0.71, lower than that of water), the lowest among all the bodies of the s. s. Indeed, while its mean diameter is 9 times greater than that of Earth, and its volume therefore 750 times greater, Saturn’s mass is only 95 times greater than that of Earth. Like Jupiter, Saturn must be surrounded by a thick, opaque atmosphere, whose appearance gives the planetary disk an aspect marked by bands parallel to the equator; these too are composed of ammonia and methane, but the ammonia bands are weaker there and the methane bands more intense.

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

VII. URANO AND NETTUNO

These two planets, very similar to one another, are the last and most distant members of the group of giants. Both have a diameter approximately four times that of Earth; Urano is somewhat larger than Nettuno, and therefore the latter is instead more massive. Because of their great distance, little can be seen on the disk of Urano and practically nothing on that of Nettuno. Both appear with a slightly greenish-blue hue. The spectroscope reveals in them the same spectral bands of ammonia and methane; but those of ammonia are much weaker, whereas those of methane are intensified, covering almost completely the red and yellow regions of their spectrum; this explains the greenish coloration of these two planets. Like the other giant planets, Urano and Nettuno rotate very rapidly around their respective axes and consequently also have considerable polar flattening. A particular feature of Urano is that its equatorial plane is almost perpendicular to the plane of the orbit described around the Sun. And since the angle between the two planes exceeds 90° by only a small amount, the rotational motion of Urano is considered retrograde. Urano has 5 satellites and Nettuno 2.

VIII. PLUTONE

Very little is known about this extremely distant planet, which appears in the telescope with a faint yellowish color. Much is hoped to be learned when it passes through perihelion in 1989, since it will then 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 large telescope at Mount Palomar (Kuiper and Humason) and to deduce from it the actual diameter, and then, through the value of its mass, obtained in turn from the perturbations it produces on Neptune, the value of its volume. Pluto’s mass would be on the order of 0.8 that of the Earth. Thus 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), similar, that is, to that of the Moon, or even lower; hence it is deduced that it should not be surrounded by any opaque atmosphere. Most gases must in fact be in a liquid or solid state because of the extremely low temperature prevailing on the planet’s surface. Lucio Gialanella