The planets of the Solar System and numerous asteroids (or minor planets), all probably share a common origin with the Sun about 4600 million years ago (see Solar System, origin). But two very different classes of planet are found: four terrestrial planets, rocky bodies with shallow atmospheres, which orbit close to the Sun, and four giant planets, largely gaseous bodies, which circle it further out. Pluto, by tradition regarded as the ninth planet, is too small to be a terrestrial planet. It also has the greatest eccentricity and inclination to the planetary plane of any of the planets. The discoveries in 2002 of Quaoar, the largest Kuiper Belt object so far known, and in 2003 of Sedna, a mysterious object apparently from the inner Oort Cloud of comets, have prompted astronomers to reexamine their definition of the word ‘planet’ in respect of Pluto.
Planets of other stars (extrasolar planets) are too faint to be seen from Earth with present-day instruments, but very massive planets several times larger than Jupiter have been detected around the nearest stars by their perturbing effect on stellar motion. The slight wobble that would be produced in a star's motion can be determined by precise measurements of a star's position over many decades, or from characteristic changes in a star's radial velocity determined from its doppler shift (see also planet pulsar). Future spaceborne observatories such as ESA's Darwin project may be able to detect directly light reflected from a planet orbiting a nearby star by masking out the star's light. Extrasolar planets with orbits edge-on to the Earth are particularly hard to detect, but the development of spacebased instruments sensitive enough to detect the minute drop in a star's light output as a large planet transits its disk are in the pipeline. Far-infrared observations can already reveal dust shells around stars and it may be possible to detect a protoplanetary system in this way (see Beta Pictoris; Vega). See also Table 1, backmatter.
A planet may symbolize the exploration of another “world”—Of new dimensions of thought and creativity—or a new adventure.
a hand or horse-drawn plane-type implement used to loosen the soil and undercut weeds in the interrow areas of crops.
a large celestial body that moves around the sun and shines by reflected sunlight. The dimensions and masses of the planets are several orders of magnitude smaller than those of the sun. Seven celestial bodies that change their position (“wander”) among the stars were known even in remote antiquity: the sun, the moon, Mercury, Venus, Mars, Jupiter, and Saturn. It was believed that all these bodies, called planets, revolved around the earth. Only in the early 16th century did the originator of the heliocentric system of the world, N. Copernicus, demonstrate that the moon alone moves around the earth, while the other planets, like the earth, move around the sun, which thus is the central body of the system of planets—the solar system. The sun itself is not a planet but a star, since it is illuminated by its own light rather than by reflected light. The moon—the earth’s satellite—also was eliminated from the planets of antiquity. Three other planets have been discovered in modern times—Uranus (1781, W. Herschel), Neptune (1846, J. Adams, U. Leverrier, E. Halley), and Pluto (1930, P. Lowell, C. Tombaugh). Thus, nine principal planets are known. Moreover, several thousand asteroids, or minor planets, whose dimensions range from several hundred km to 1 km or less, have been discovered; they move for the most part between the orbits of Mars and Jupiter.
Even in ancient times the planets were divided into the inferior and the superior, according to the character of their motion among the stars. The inferior planets are Mercury and Venus, whose orbits lie between the sun and the earth; the superior planets are all the other planets, whose orbits lie beyond that of the earth. The division of the planets into the inner and the outer has greater scientific meaning. The inner planets are those planets that move in orbits within the asteroid belt, namely, Mercury, Venus, the earth, and Mars; they are also called the terrestrial planets. The outer planets are located beyond the asteroid belt and include Jupiter, Saturn, Uranus, Neptune, and Pluto. Except for Pluto, they are also called the major planets (also giant or Jovian planets) because of their large size.
A mutual attraction, described by Newton’s law of gravitation, exists between the planets and the sun. The planets follow elliptical orbits around he sun, essentially obeying Kepler’s comparatively simple laws. However, the mutual attraction of the planets complicates the motion, as a result of which the calculation of the position of the planets in the stellar sky and their distances from the sun constitutes a difficult task of celestial mechanics, especially if the calculation must be made for a period in the distant future or past. Nonetheless, modern mathematical theories of planetary motion make it possible to calculate the positions of the planets in the sky in the remote past, for example, several thousand years ago, with higher accuracy than the astronomers of that era could achieve by direct observation.
General description. The apparent brightness of all planets known since antiquity is comparable to that of the brightest stars, and Venus, Mars, and Jupiter surpass even the brightest stars. Of the planets discovered in modern times, only Uranus is visible to the naked eye. To the average human eye, all the planets appear to be luminous points, like the stars, but through a small telescope the disks of all the planets other than distant Pluto can be seen. This fact was first observed by Galileo in 1609. Phases similar to the phases of the moon—from full to a narrow crescent or to complete invisibility at inferior conjunction with the sun—can be seen for Venus and Mercury. The superior planets do not have a complete succession of phases. For example, Mars exhibits the gibbous phase at an angle that does not exceed 47°, and Jupiter at an angle that does not exceed 11°. The phases and angular dimensions of a planetary disk vary, depending on the relative positions of the planet, the sun, and the earth, as well as on the distance of the planet from the earth. Calculation of the linear dimensions of the planets on the basis of the angular dimensions of planets is not difficult, since the distance from the planets to the earth is known with sufficient accuracy. Telescopic measurements of the angular dimensions are encumbered by hard-to-eliminate systematic errors that may be as high as 1 percent of the measured quantity.
Radiolocation of the planets, namely, Mercury, Venus, Mars, and Jupiter, makes it possible to establish quite accurately the distance to a planet’s surface, and celestial mechanical calculations based on an analysis of radar measurements over a period of years make it possible to calculate the distance to a planet’s center. The difference between the distances is equal to the planet’s radius. This method of calculating planetary radii provides an accuracy greater than 0.1 percent. The radii are also
| Table 1a. Geometrical and mechanical characteristics of the planets (1973 data) | ||||||
|---|---|---|---|---|---|---|
| Planet | Diameter (equatorial) | Angular diameters (equatorial) minimum and maximum (seconds of arc) | Oblateness | Volume (Earth = 1) | Mass (Earth = 1) | |
| (km) | (Earth = 1) | |||||
| 1 Highly unreliable value | ||||||
| Mercury………… | 4,86 5 | 0.38 | 4.7-12.9 | 0.0 | 0.055 | 0.055 |
| Venus…………. | 12,105 | 0.95 | 9.9-65.2 | 0.0 | 0.861 | 0.815 |
| Earth………….. | 12,756 | 1.00 | – | 1/298.2 | 1.000 | 1.000 |
| Mars………… | 6800 | 0.53 | 3.5-25.5 | 1/190 | 0.150 | 0.107 |
| Jupiter………… | 141 700 | 11.11 | 30.5-50.1 | 1/15.3 | 1,344.8 | 317.82 |
| Saturn………… | 120,200 | 9.41 | 14.7-20.7 | 1/10.2 | 770 | 95.28 |
| Uranus………… | 50,700 | 3.98 | 3.4-4.3 | 1/33 | 61 | 14.56 |
| Neptune………… | 49,500 | 3.88 | 2.2-2.4 | 1/60 | 57 | 17.28 |
| Pluto………… | 6,000 1 | 0.47 | 0.5 | – | 0.1 | 0.111 |
determined from observations of the occultation of a planet’s satellite during its passage behind the planetary disk. The results are especially successful when applied to planets with a rarefied atmosphere, such as Mars. Measurements of the apparent diameters of a planet make it possible to determine the planet’s figure, or at least its oblateness. The figures of planets can be ascertained fairly reliably by a planet’s dynamical flattening, which is determined by analyzing perturbations in the motion of the planet’s satellites, assuming that the planet is in hydrostatic equilibrium.
The geometrical, mechanical, and physical characteristics of the planets are given in Tables la and lb and 2a and 3b.
Surface details and rotation; mapping. Various details are visible on the surface of a planet that completely, or almost completely, lacks an atmosphere. They are often conditionally assigned the names of terrestrial formations, although their physical nature may not correspond to the names. Such, for example, are the dark maria on Mars, which are not at all seas in the meaning of the word for earth. They stand out against the background of other features only because of their lower reflectance of sunlight. For a planet such as Venus, which has a thick atmosphere, surface details are not visible; only details of the cloud layer are accessible to observation. The surface of Venus was photographed partially from the spacecraft Mariner 10, through openings between clouds. The periodic shifts of features on the planet’s disk indicate that the planet rotates; by measuring the position of a specific feature at different times, the period of rotation of the planet about its axis and the position of the axis of rotation in space can be determined. This makes it possible to determine the planetographic coordinates of various features and to compile a map of the planet; such maps exist for Mars and Mercury. This method is inapplicable to Venus and all the outer planets, since we can observe continuously only their cloud cover, in which strong systematic motions coincident with the rotation of the planet itself may be present.
The rotation of a planet can be studied by the methods of radio astronomy. Because the planet is rotating, a radar signal transmitted from earth is reflected by points on the planet’s surface that are moving toward an observer on earth and by points moving away from the observer. Because of the Doppler effect, the mode of the signal changes, and the faster the planet rotates, the greater this effect. Soviet radiophysicists, including V. A. Kotel’nikov and co-workers, and American radiophysicists have determined by this method that Venus rotates with a period of 243 earth days in the direction opposite its revolution around the sun. Subsequent observations have revealed that Venus’ cloud layer rotates with a period slightly longer than four days. Study of the natural radio-frequency radiation of Jupiter in decimeter waves has shown that the sources of the radiation, which are associated with the body of the planet, rotate with a period of 9 hr 55 min 29.4 sec, while the cloud layer at the planet’s equator has a period of rotation of 9 hr 50 min 30.00 sec.
Radiolocation makes it possible to construct a detailed map of a planet’s radiometric albedo by singling out in the signal returning to earth the parts reflected by different places on the planet’s surface. Moreover, by virtue of the exceptional accuracy of distance calculations by radar techniques, the planet’s relief can be determined, at least near the center of the planet’s visible disk. The reliefs of Venus and Mars have been determined in this manner.
Mass and density. The study of regularities in the motion of planetary satellites on the basis of the law of universal gravitation makes it possible to determine planetary masses. For Mercury, Venus, and Pluto, which do not have satellites, the masses are determined from the perturbations that the planets cause in the motions of other celestial bodies, chiefly comets and space probes; in the latter, the accuracy is particularly great. The mass of Mars has also been determined in this manner, using as a basis the motion of its natural satellites. Knowing the mass of a planet and its size makes it possible to calculate the mean density, the
| Table 1b. Geometrical and mechanical characteristics of the planets (1973 data) | |||||
|---|---|---|---|---|---|
| Planet | Mean density (gm/cm3) | Surface gravity (Earth = 1) | Escape velocity (km/sec) | Mean distance to sun (astronomical units) | Period of revolution |
| 1Highly unreliable value 2Varies greatly with time | |||||
| Mercury………….. | 5.52 | 0.38 | 4.3 | 0.387 | 88 days |
| Venus…………… | 5.22 | 0.90 | 10.3 | 0.723 | 224.7 days |
| Earth…………… | 5.517 | 1.00 | 11.2 | 1.000 | 365.3 days |
| Mars…………… | 3.97 | 0.38 | 5.0 | 1.524 | 1.881 years |
| Jupiter………….. | 1.30 | 2.35 | 57.5 | 5.203 | 11.862 years |
| Saturn………….. | 0.68 | 0.92 | 37 | 9.539 | 29.458 years |
| Uranus………….. | 1 32 | 0.92 | 22 | 19.19 | 84.015 years |
| Neptune………….. | 1.84 | 1.15 | 23 | 30.06 | 164.79 years |
| Pluto………….. | 61 | 0.51 | 5 | 39.752 | 250.62 years |
| Table 2a. Physical characteristics of the planets (1973 data) | ||||||
|---|---|---|---|---|---|---|
| Planet | Period of rotation about axis with respect to stars | Inclination of equator to orbit | Solar constant for planet | Illumination from sun at boundary of atmosphere (phots) | Brightness at mean opposition (stellar magnitude) | |
| mW/cm2 | (Earth’;s solar constant = 1) | |||||
| 1, at the equator 2II, at middle latitudes 3Unreliable value 41.95 cal/cm2min 5At elongation, depending on distance from sun 6At elongation; maximum possible brightness is –4.45 7Visible from the sun 8Saturn’s rings at maximum exposure give a value of -0.28 for this quantity | ||||||
| Mercury………… | 58.65 days | 0°3 | 910 | 6.7 | 90.1 | -0.3 to + 0.65 |
| Venus……….. | 243. 0 days | 178 | 261 | 1.9 | 25.8 | -0.076 |
| Earth……….. | 23 hr 56 min4.1 sec | 23.5 | 1364 | 1.0 | 13.5 | -3.877 |
| Mars………. | 24 h r 37min22.7 sec | 25.2 | 59 | 0.43 | 5.8 | -2.01 |
| Jupiter | I1 9 hr 50 min 30.0 sec II2 9 hr 55 min 40.6 sec | 3.1 | 5.0 | 0.037 | 0.50 | -2.55 |
| Saturn……… | I1 10 hr 14 min II2 10 hr 40 min | 26.4 | 1.5 | 0.011 | 0.15 | + 0.678 |
| Uranus…….. | 10. 8 hr | 98 | 0.37 | 0.0027 | 0.037 | + 5.52 |
| Neptune……. | 15. 8hr | 29 | 0.15 | 0.0011 | 0.015 | + 7.84 |
| Pluto…….. | 6.3 9 days | ? | 0.08 | 0.0006 | 0.0085 | + 14.9 |
surface gravity, and the escape velocity, that is, the critical velocity at which a body leaves the planet forever; the escape velocity is calculated for the surface of a planet.
Atmosphere. The presence of a gaseous envelope around a planet can be easily established through observations from the earth of the darkening of the planetary disk toward the edges, the gradual, but not instantaneous, fading of a star when a planet passes in front of it (occultation of the star by the planet), and the presence of cloud formations. Photometric measurements make it possible to obtain the value of the albedo of the whole planet or any part of the planet. Many planets have a high albedo, indicating the presence of a thick atmosphere. The value of the albedo and the nature of the variation in a planet’s brightness with changing phase make it possible to determine, by means of the theory of light scattering, the quantitative characteristics of a planet’s atmosphere, particularly optical thickness and extent. Important results have been obtained in this area in the 20th century by the Soviet astronomers N. P. Barabashov, V. G. Fesenkov, and V. V. Sharonov. Measurements of the polarization of a planet’s light are used in interpreting such observations. The presence in an atmosphere of solid and liquid particles, called aerosols, greatly increases light scattering and leads to exaggerated data on the gaseous component of the planet’s atmosphere; for example, until the mid-1960’s the thickness of the Martian atmosphere was exaggerated by a factor of 10–20. Measurement of the albedo, color, and light polarization of individual planetary surface features unfortunately does not provide a simple answer to the question of the nature of the features.
The thickness of a planet’s atmosphere is estimated from the pressure of the gases at the planet’s surface, that is, from the value that an aneroid barometer would show. Expressed in millibars (mb), this quantity does not coincide with the actual atmospheric pressure on the planet’s surface, which is proportionally dependent on the acceleration of gravity on the planet. However, knowledge of this quantity makes it possible to compare directly the atmosphere of a planet with that of the earth and to calculate the total mass of the planet’s gaseous envelope. The thickness of the atmosphere, or of some gas in the atmosphere, can be characterized by a special quantity, expressed in meter-atmospheres (m-atm) or centimeter-atmospheres (cm-atm), equivalent to the altitude (in meters or centimeters) to which the atmosphere would extend if it had a uniform density corresponding to a pressure of 1 atm ~ 1.013 mb and a temperature of 0°C. On earth this quantity is about 8,000 m-atm, and on Mercury 1–3 cm-atm. On Mars the atmospheric pressure at the surface is 5–8 mb (aneroid), and on Venus about 100 atm. The giant planets have very thick atmospheres.
The chemical composition of a planet’s atmosphere is determined from observations of the intensity of the molecular absorption bands that arise in the spectrum of solar radiation after that radiation has twice passed through the planet’s atmosphere —before and after reflection from the planet’s surface. The complexity
| Table 2b. Physical characteristics of the planets (1973 data) | ||||||
|---|---|---|---|---|---|---|
| Planet | Spherica albedo visual) | Equilibrium temperature (°C) | A verage measured temerature (°C) | Coordinates of northern end of planet’s axis of rotation (1,950.0) | Number of satellites | |
| right ascension | declination | |||||
| 1 Point on the planet for which the sun is at zenith 2Surface temperature 3 Much higher according to radio band measurements | ||||||
| Mercury | 0.07 | 4-230 | + 3401 | 254° | +70° | 0 |
| Venus | 0.76 | -44 | + 4802 | 273.0 | + 66.0 | 0 |
| Earth | 0.39 | -23 | +12 | — | +90 | 1 |
| Mars | 0.16 | -57 | -53 | 317.32 | + 52.68 | 2 |
| Jupiter | 0.67 | -160 | -1453 | 268.00 | + 64.52 | 13 |
| Saturn | 0.69 | -190 | -1703 | 38.50 | + 83.31 | 10 |
| Uranus | 0.93 | -210 | -2103 | 76.76 | + 14.92 | 5 |
| Neptune | 0.84 | -220 | -160 | 294.91 | + 40.53 | 2 |
| Pluto | 0.1 | -230 | — | ? | ? | ? |
of this method is connected with the fact that on a spectrogram obtained at the earth’s surface these bands are difficult to separate from bands arising during the transmission of the light through the earth’s atmosphere. These complications are partially eliminated by balloon observations. Using this method, it is comparatively easy to detect gases in a planet’s atmosphere that are either absent or present only in small quantities in the earth’s atmosphere. These include carbon dioxide (CO2), methane (CH4), ammonia (NH3), and hydrogen (H2). It is more difficult to detect water vapor (H2O) and oxygen (O2), and it is almost impossible to detect helium (He), nitrogen (N2), argon (Ar), and a number of other gases that produce absorption bands in the far ultraviolet region of the spectrum. By the time of the space age, the primary component of the atmospheres of Venus and Mars had already been established as CO2, and the primary components of the atmospheres of the superior planets as molecular hydrogen H2 (about 85 km-atm above the cloud layer of Jupiter), Ch4, and NH3. By analogy with the composition of the sun’s atmosphere, the presence of a large quantity of helium is assumed.
The space age has brought new methods of investigating planetary atmospheres. By measuring the attenuation of radio signals from space probes traveling behind a planet, it is possible, because of space absorption, to derive the scale height of the atmosphere and thus to determine the ratio of its temperature T to the average molecular weight μ. However, this method is applicable ony to rarefied atmospheres or to the upper layers of thicker atmospheres. Far more effective is direct contact by landing modules of space probes. Such an experiment was carried out in the 1960’s with the landing on Venus of the Venera probes (USSR). Measurements of the intensity of a molecular band in the spectrum of a specific area of the planet, made during flyby, also makes it possible to determine the distance to the planet’s surface at that point, that is, the planet’s relief below the trajectory of the spacecraft. Similar valuable results were obtained by the Mars 3 and Mars 5 (USSR) and Mariner 9 (United States) probes. As a result of the rotation of a planet, different parts of its surface pass below the probe’s orbit. By virtue of this fact, a large part of the relief of Mars has been determined with an accuracy of a few hundred meters.
Temperature. Direct measurements of the total heat flux or radiation of a planet in certain regions of the infrared spectrum, made by such instruments as bolometers, make it possible to determine the planet’s overall temperature or the temperature of individual areas. The same task can be carried out by measuring a planet’s heat fluxes by radio methods in the centimeter, decimeter, and meter regions of the spectrum. Minimum temperatures are obtained from such measurements on the assumption that a planet behaves as a blackbody. There are grounds to assume that the true temperatures are only slightly higher than the temperatures obtained by this method. Moreover, radio measurements make it possible to determine the temperature at different levels in a planet’s atmosphere and even at various depths below its surface, within a few meters, since radiation of different frequencies undergoes varying absorption in the atmosphere and in the solid crust of the planet. It was by the radio-measurement technique that the true surface temperature of Venus—about + 500°C—was determined. Bolometric measurements, on the other hand, gave only the temperature of the upper atmosphere, at the cloud level (about -40°C).
Comparison of the theoretical equilibrium temperature—that is, the temperature a planet would have if solar radiation were its only heat source—with the measured temperature makes it possible to determine whether a planet has internal sources of heat, which then escapes to the surface. This process depends considerably on the thermal conductivity of the planet’s crust and atmosphere. The atmosphere may give rise to a strong greenhouse effect, which means that the atmosphere transmits optical radiation from the sun but to a large extent traps long-wave (thermal) radiation of the planet itself. A planet lacking an atmosphere therefore is colder and has a higher daily temperature range than a planet with an atmosphere. It is precisely for this reason that on Venus the temperature is 550°C higher below the thick atmosphere than at the cloud level, and the daytime and nighttime temperatures are essentially the same. For Jupiter, which has an equilibrium temperature of 110°K, measurements in the infrared region have given a temperature of 123°K, and in millimeter and centimeter waves 150°K. It is still higher in the decimeter band, but this is due to the nonthermal radiation of the planet, to which the concept of temperature is inapplicable. For the other giant planets, the degree to which the measured temperatures exceed equilibrium temperatures is even greater, but the measurements are less accurate. Only thermal measurements made by large telescopes in the infrared region of the spectrum are useful in determining the temperature of individual details of a planet’s surface. It has been established in this manner, for example, that the daytime summer temperatures in the equatorial region of Mars may be appreciably greater than 0°C, while the nighttime temperatures may be about — 60°C; it also has been established that the dark maria are warmer than the bright “continents.”
Investigations of both the temperature and the chemical composition of a planet’s atmosphere (the presence of oxygen and water) make it possible to conclude whether the existence of life on a planet is possible. Thus, it may be concluded from what is known of Mars that life may exist in elementary forms on the planet. The possibility of life on other planets of the solar system, even in elementary forms, is doubtful.
Internal structure. Observations of variations in the orbit of a planetary satellite, in particular the rotation of the orbital plane and the precession of the orbit in this plane, make it possible to determine mathematically the shape of a planet and its degree of oblateness. The speed of this rotation is greater the greater the difference I between the oblateness e and one-half the ratio x, which is the ratio of the centrifugal force at the planet’s equator to the force of gravity. The quantity I can be determined from the results of lengthy observations of the satellite, and x can be calculated from the known dimensions and mass of the planet and from its rate of rotation. The degree of dynamical flattening is then determined from the equation ∈ = T + x/2- It follows from theory that e depends on the mass distribution within the planet; specifically, ∈ varies from a value of x/2 for a planet all of whose mass is concentrated at the center to 5x/4 for planets that are homogeneous from center to periphery. If the mean density of a planet is known, the regularities indicated above make it possible, by estimating the possible values of the pressure within the planet and taking into account the planet’s chemical composition, to assess the nature of the matter in the deep interior of the planet and its state of aggregation. Additional information on the mass distribution within a planet can be obtained by determining the rate of precession of the axis of rotation; but for this purpose, long-term observations of the rotation (lasting a few centuries) are needed.
As can be seen from Table lb, the mean density of terrestrial planets greatly exceeds that of the giant planets, whose mean density is close to that of the sun (1.4 g/cm3). Moreover, the giant planets have incomparably greater masses, because of which the pressure in their interiors is much higher. Thus, it ir:ay be assumed with high probability that Mercury, which has a high density compared with the other planets, has a dense iron core containing about 60 percent of the planet’s mass. Venus, which is similar to the earth in mass and density, has at its center a core richer in iron than that of the earth, and the silicate density in its mantle is several times greater than that of the earth’s mantle. The earth has a complex structural shell (mantle) that extends to a depth of 2,900 km. Below the mantle is the core, which is apparently metallic (iron); the core is fluid at the boundary with the mantle and solid at the center. If the core of Mars, which has a comparatively low density, is iron, it must be small—no more than 30 percent of the radius, and more likely 15–20 percent. But the density of the silicate rocks in Mars’ mantle is somewhat higher than that of the earth.
The picture is quite different for the giant planets. Their very low mean density and the specific chemical composition of their atmospheres indicate that they consist of substances similar to the sun, that is, primarily hydrogen and helium. The considerable heat flux emanating from Jupiter indicates a high temperature in the planet’s interior—perhaps up to 20,000°C. Such a heat flux attests to the existence of convective heat transfer in the interiors of Jupiter and Saturn. The interior of Jupiter is under tremendous pressure, greatly exceeding 2.5 million bars, at which molecular hydrogen undergoes a transition to the metallic phase and in every way resembles the alkali metals. It is difficult to say whether the hydrogen is in a liquid or gaseous state, since the temperature is not known sufficiently accurately. It must be assumed, however, that the metallic core of Jupiter is liquid, otherwise it would be difficult to explain Jupiter’s powerful magnetic field, which is much stronger than the earth’s field. Saturn’s structure resembles Jupiter’s. Uranus and Neptune, which are more dense, apparently contain much more helium. Their temperatures are lower, which signifies that these planets may possess near their centers cores consisting of a mixture of ice and compounds containing hydrogen, oxygen, carbon, nitrogen, sulfur, and other elements. Nothing is known of Pluto’s structure.
For a complete description of the planets of the solar system, it must be added that the terrestrial planets have few satellites —the earth has one and Mars two—while the giant planets have many—Jupiter has 13, Saturn ten, Uranus five, and only Neptune as few as two. Pluto apparently has no satellites.
Evolution and origin. Over their billions of years of existence, the planets of the solar system have undergone major changes. The low-mass planets, such as Mercury and in part Mars, could not retain the light gases in which the velocity of the thermal motion of the molecules may exceed or approach escape velocity. This is especially true of hydrogen and helium. By contrast, nitrogen, oxygen, carbon dioxide, and to a lesser degree water vapor are retained comparatively strongly by most of the planets. The gas generated during the slow evolution of the interior and absorbed there supplements the atmosphere, but the process of volatilization predominates on the smaller planets. The breaking down of the complex molecules of a gas, or water, by shortwave solar radiation that occurs in the upper layers of the atmosphere also facilitates the escape of the lighter components. Living organisms may play some role in the change in atmospheric composition. Thus, it is believed that on the earth the atmosphere was originally rich in H2O, CO2, and CH4 and heavier hydrocarbons, but as a result of the vital activity of simple microorganisms and vegetation under the influence of the sun’s energy, the carbon dioxide was broken down into carbon and oxygen. Although the oxygen was consumed intensively for the oxidation of rocks, much of it was preserved.
Thus, the terrestrial planets, which have small masses, have lost the volatile gases H2, He, and CH4; Mercury and to some extent Mars have also lost the heavier gases (O2 and CO2). The only exception is H2 bonded to O in water, which exists primarily in the liquid or solid phase on most planets. By contrast, the giant planets have retained all their gases, as a result of which the chemical composition of their atmospheres, and interiors, is the same as that of the sun.
From the foregoing it may be concluded that the composition of the sun and planets is similar and that they have a common origin. The chemical composition of meteroites and comets also resembles that of the sun. However, the attempt to base theories of the mechanism of formation of the planets on these similarities of composition has encountered a difficulty, namely, that the planets, whose total mass is 1/700 of the total mass of the entire solar system, account for 98 percent of the angular momentum, while the sun acounts for only 2 percent. Some cosmogonic hypotheses have attempted to attribute this large moment to the influence of a passing star; however, this has proved groundless, since the hypotheses do not explain why the specific angular momentum (per unit mass) increases greatly as a planet’s distance from the sun increases. In the mid-20th century, largely under the influence of the works of O. Iu. Shmidt and his students, the consensus began to form that, no matter what the mechanism of the process, the planetary system formed as a result of the differentiation of matter in a huge cloud of gas and dust. This cloud was originally cold; otherwise the hot gas would have dispersed quickly without attaching itself to the dust substrate during its condensation into the planets. During the process, some amount of heat was evolved because of the reduction of potential energy. Each planet was thus heated, a process that continued because of radioactive decay within the planet. The planet’s contents gradually entered the plastic or even the liquid state, at which time differentiation of the matter became possible; the heavier fractions, such as iron and nickel, moved to the center, while the light fractions surfaced, forming the planet’s mantle and crust. The gas located in the original cloud near the incipient sun heated and dispersed, a process that did not occur in clouds distant from the sun.
The solar system undoubtedly is not unique in our galaxy, much less in the universe. But as yet there is no direct proof that other such systems exist. Only the negligible periodic motions observed near some of the stars closest to us give a faint, indirect indication of this.
D. IA. MARTYNOV