SOLARE, SISTEMA - Enciclopedia Cattolica, Vol. XI, p. 553

Machine translation of the 1948–1954 Italian original. The original text is authoritative.

SOLAR SYSTEM. — It is the aggregate of the Sun and the celestial bodies (planets and their satellites, minor planets, comets and some numerous meteor streams) which gravitate around it and move about II. 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 accompanying Table I, while the data of the 31 satellites that revolve around these planets are expressed in the following Table II (except for the Moon [q.v.]).

In antiquity only the first five planets were known (because they alone are 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 Abbot Piazza in Palermo) and these soon became a numerous family, then, with the aid of 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, yet without thereby altering the stability of the system. Its boundary is represented—at least so far as is known—by the orbit of Pluto, surpassed only by the orbits of some of the periodic comets (q.v.) of longest revolution and, in the aphelion, also by the orbit of Neptune.

The planets are distinguished as *interior* or *inferior* and *exterior* or *superior*, according as their orbits around the Sun are inside or outside the Earth’s orbit. The two interior planets, Mercury and Venus, never recede far in the sky from the position of the Sun, and are therefore visible only in the hours following sunset or preceding the rising of the orb of day. With respect to density and volume, the planets fall distinctly into two groups: one of 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 notably larger dimensions but more moderate densities.

As regards the distribution of the distances of the planets from the Sun, it seems that they are governed by a law of exponential type, which in its first form, known as Bode’s or Titius’s (the Latinized name of J. G. Tietz) of 1778, expresses that the distance of a planet in a given position can be expressed by the formula \(D = 0.4 + 0.3 \cdot 2^n\). The discrepancies obtained between the actual distances and those given by this law are generally small for all the planets, except for Neptune and Pluto. Simpler is the recent law of G. Armellini: \(D = 1.53^n\), which contains only one parameter instead of two, and yet gives the actual planetary distances with greater approximation. In it one must set \(n = 2\) for Mercury, \(n = 1\) for Venus, \(n = 0\) for the Earth, \(n = 2\) for Mars, \(n = 3\) for the planets (whose orbits are generally between those of Mars and Jupiter), \(n = 4\) for Jupiter, \(n = 5\) for Saturn, \(n = 6\) for Uranus and \(n = 7\) for Neptune. This law provides two distances for the planets, and this is in good agreement with reality, representing the beginning and end of the asteroid ring, but 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, all affirm the very notable fact that the distances of the planets from the Sun proceed according to an exponential law. However, the intimate reason for this fact still escapes us, though it must undoubtedly have notable cosmological and cosmogonical implications. 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 that are fairly close to each other, owing to the eccentricities of the elliptical orbits which they describe around the Sun, eccentricities that are greatest for Pluto (0.25) and notable only for Mercury (0.21). Here are some descriptive notes on the individual components of the solar system.

1. **Mercury.** — Very difficult to observe because of its great proximity to the Sun (from which it never departs angularly by more than 20°); on its surface only a few dark spots can be distinguished, which however have made it possible to reveal a rotation of 88 days, equal to the period of revolution around the Sun, so that the planet always turns the same face toward the greater luminary, 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 mass are only a little greater than those of our satellite. Its surface should present the same rugged nature as the Moon, since Mercury reflects the light of the Sun like the Moon. And as with the latter, so also with Mercury the original atmosphere must have rapidly dissipated, because of the small values of gravity on Mercury. Mercury is the hottest planet in the solar system in the hemisphere constantly turned toward the Sun (over 400° C according to the most recent measurements), and at the same time the planet with

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**TABLE I. — Planets (of the solar system)**

| Mercury | Venus | Earth | Mars | Jupiter | Saturn | Uranus | Neptune | Pluto |
| --- | --- | --- | --- | --- | --- | --- | --- | --- |
| Mean distance from the Sun [in astronomical units = mean Earth-Sun distance] | 0.39 | 0.72 | 1.00 | 1.52 | 5.20 | 9.54 | 19.19 | 30.07 | 39.46 |
| Period of revolution (in days or years) | 87.00 d | 224.70 d | 365.25 d | 686.00 d | 11.86 yr | 29.46 yr | 84.02 yr | 164.79 yr | 248.43 yr |
| Inclination of orbit to the ecliptic | 7° 00′ | 3° 24′ | 0° | 1° 51′ | 1° 18′ | 2° 29′ | 0° 46′ | 1° 47′ | 17° 09′ |
| Equatorial diameter [Earth = 1] | 0.37 | 0.97 | 1 | 0.54 | 11.14 | 9.4 | 4.0 | 4.3 | ? |
| Mass [Earth = 1] | 0.04 | 0.81 | 1 | 0.11 | 317. | 95. | 14.7 | 17.2 | 0.8? |
| Density [water = 1] | 3.8 | 4.86 | 5.52 | 3.96 | 1.34 | 0.71 | 1.27 | 1.58 | ? |
| Surface gravity [Earth = 1] | 0.27 | 0.85 | 1 | 0.38 | 2.64 | 1.17 | 0.92 | 1.12 | ? |
| Period of rotation [in days or hours] | 88 d | ? | 1 d | 24 h 37 m | 9 h 50 m | 10 h 14 m | 10 h 49 m | 15 h 48 m | ? |
| Inclination of equator to orbit | ? | 23° | 23° 27′ | 23° 59′ | 3° 07′ | 26° 45′ | 98° | 28° 48′ | 29°? |
| Stellar magnitude at maximum brightness | 1.2 | -4.3 | -2.8 | -2.5 | -2.4 | +0.7 | +5.7 | +7.6 | +15 |

Great precision, and it is noticeably equal to that of the Earth, just as equal is the inclination of the Martian equator to the plane of its orbit (24°), so that Mars presents seasons entirely analogous to those of the Earth, but about twice as long due to the greater length 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 main sign of the Martian seasons is provided by the more or less extensive development of the polar caps, and by some periodic and regular variations in color that certain markings display (green areas) and which might be indicative of seasonal vegetation growth.

The current state of areography, that is, the study of the surface of Mars (analogous to "geography" for the Earth), is, however, in a phase of skepticism and debate, since new optical theories (initiated by V. Cerulli and M. Maggini) suggest that many of the appearances discerned on the surface of Mars should rather be considered as simple optical illusions. Indeed, when the human eye strains to distinguish details of an object seen poorly (due to 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. Some years ago, this tendency was also shown with regard to the famous "canals" (a term first introduced by Fr. Angelo Secchi, S.J., and later adopted by G. V. Schiaparelli and P. Lowell and disseminated worldwide) and their supposed "germination." Recent photographs, obtained with new techniques at the Pic-du-Midi Observatory in the high Pyrenees, have instead actually shown kinds of canals, though in varied forms different from the geometric shapes that visual observers tended to measure. In short, even if these canals exist, their nature remains completely unknown. Astronomers expect new results from the giant telescope at Mount Palomar, which in 1956 will for the first time be used for observing Mars. More certain, however, seem 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 of oxygen or water vapor have been found (if these two elements exist, they cannot exceed 0.15% of the quantities present in the Earth's atmosphere). The polar caps, however, must consist of water ice, though probably in a very thin layer on the order of a tenth of a millimeter in thickness, 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, 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, especially the latter for 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. The orbital period of Phobos is 7h 39m and is less than the planet's rotational period, so that, 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 large group (over 1,600 known so far) of small bodies orbiting the Sun in generally elliptical orbits between Mars and Jupiter. Their dimensions are on the order of fifty km or even less. Only one, Vesta, is just visible to the naked eye. Their number is very large (estimated at over fifty thousand), and several new ones are discovered each year. The first, Ceres, was discovered by Abbot G. Piazzi in Palermo on January 1, 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, 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 is estimated at 1/8000 of the Earth's mass. It has been estimated, however, that the total mass of the minor planets is certainly less than the Earth's mass and perhaps even less than the Moon's. As for their origin, it seems that at least a considerable group of them must have come from the explosion of some larger planets. But astronomers are not all in agreement on this question, which still presents many obscure points. The orbits of the minor planets show a much greater variety than those of the major planets. For the majority, the average distance from the Sun is between 2 and 3.6 astronomical units. The orbital period has an average 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 approaches to within 1.5 million km.

V. JUPITER

Jupiter is the largest planet in the solar system, with a diameter 11 times that of the Earth and 1/10 that of the Sun. Its mass, which is more than 300 times that of the Earth, alone exceeds the combined mass of all the other planets. It is also the planet with the highest rotational speed (period 9h 50m), resulting in a pronounced polar flattening, clearly visible even through a telescope. This flattening is, however, less than would be expected if the planet were solid like the Earth. It is therefore thought that a significant part of its volume consists of a gaseous atmosphere, the only one directly observable, appearing with characteristic bands parallel to the equator, of variable shape and coloration 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. The spectrum of Jupiter (as indeed that of the other distant giant planets) shows characteristic bands in the red and infrared regions, which have been identified as belonging to ammonia and methane; the former must be condensed into crystals at -135° C, while the latter remains gaseous. Thus, the thick clouds of Jupiter observed must be composed of ammonia crystals suspended in an atmosphere of methane and probably also hydrogen. The various colorations would then be due to metallic compounds.

Jupiter has 12 satellites, numbered according to the order of their discovery, except for the first five, which also have proper names. The four largest, Io, Europa, Ganymede, and Callisto (discovered by Galileo and known as the "Medicean planets"), have diameters comparable to that of the Moon and would be visible to the naked eye were they not lost in the planet's glare. Since they move near the planet's equatorial plane, they are liable to pass in front of (transit) or behind (eclipse) Jupiter's disk. The other eight satellites are difficult to observe, and some of them may be planets captured by the larger body. The fifth satellite is the closest to the planet, around which it revolves very rapidly, with a speed exceeding one thousand km per minute.

VI. SATURN

It is the most distant planet known to the ancients and the only one endowed with a magnificent system of rings, concentric to the planet and situated in its equatorial plane. Glimpsed by Galileo («altissimum planetam tergeminum observavi») and recognized in their nature as rings by C. Huygens in 1655, they appear divided into 3 separate rings, of which the outermost has an external diameter of 280,000 km, i.e., 2.3 times the equatorial diameter of the planet. Their thickness is extremely small, perhaps only about a dozen km. The rings consist of a collection of very small satellites rotating separately around the planet according to the laws of gravitation, so that those closer in rotate more rapidly than the outer ones. According to the research of G. Kuiper (1950), they would consist of small fragments of water ice. The total mass of the rings is very small, on the order of one millionth of the mass of the planet.

Another notable feature of Saturn is its low density (0.71, less than that of water), the smallest among all bodies in the solar system. Indeed, while its mean diameter is 9 times larger than that of the Earth, and thus 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 a planetary disk with bands parallel to the equator, also composed of ammonia and methane, but the ammonia bands are weaker and those of methane more intense.

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

VII. URANUS AND NEPTUNE

These two planets, very similar to each other, are the last and most distant of the group of giants. Both have a diameter that is approximately 4 times the terrestrial diameter; Uranus is slightly larger than Neptune, but the latter is instead more massive. Because of their great distance, little can be discerned on the disk of Uranus and practically nothing on that of Neptune. Both appear with a slightly greenish-blue tint. The spectroscope reveals in them the same spectral bands of ammonia and methane; but the ammonia bands are much weaker, while those of methane are 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 thus they also have a notable polar flattening. A peculiarity of Uranus 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 slightly exceeds 90°, the rotational motion of Uranus is considered retrograde. Uranus has 5 satellites and Neptune 2.

VIII. PLUTO

Very little is known about this most distant planet, which appears in the telescope with a slight yellowish color. Much is hoped to be learned when it reaches 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 large telescope at Mount Palomar (Kuiper and Humason) and deduce from it the actual diameter, and thus, through the value of its mass—obtained in turn from perturbations on Neptune—the value of its volume. The mass of Pluto 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. Based on its apparent brightness, Pluto must have a low reflective power (or albedo), similar to that of the Moon, or even less; 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 due to the extremely low temperature prevailing on the planet's surface.