About 50 years ago, it was recognized by E. Hertzsprung (1911) and H. N. Russell (1914) that the stars may be grouped into families by spectral type (temperature) and luminosity (absolute magnitude). Since the time of their discovery of some of the fundamental stellar properties, many general relationships between mass, luminosity, age, diameter (density), temperature, spectral type, composition, internal conditions, nuclear reactions, et cetera, have been deduced for stars. Ideas are continuing to be developed about the evolution of stars and the internal and external physical and chemical changes accompanying their aging processes. Concepts are being formed about the modes of formation of stars, the relationships between members of close binaries, the distribution of star types in galactic clusters, et cetera. It is not necessarily true that all these ideas are correct; many, in fact, are conflicting. The point is that the large, luminous bodies of matter in the universe called stars or suns are not individually unique and curious specimens, seemingly completely unrelated to one another; rather, they are now recognized to be members of a class with familial similarities. Although differing in mass and age, when their intrinsic dilferences are taken into account, the class of stars appears to form a continuum wherein all their other observable properties seem to be related to the primary accidents of mass, age, rate of rotation, propinquity to other massive bodies, and, possibly, primordial chemical composition.
The number of luminous bodies individually detectable in the sky runs into the millions, of which some 500,000 may be called well-observed stars. Consequently we have available a large population to study and compare, to analyze statistically for number and distribution, to analyze spectro-scopically, and so on.
Quite a different situation prevails when it comes to the bodies in the universe that are not self-luminous. For this class of bodies, the existence of which may be known only because of their ability to reflect light or be inferred from observed oscillations of nearby self-luminous bodies, the known members are primarily a few relatively small bodies within our solar system, that is, the planets, satellites, and asteroids.
Because the planets of the solar system are so few in number, they have usually been treated as unique objects and studied as such. Yet, conceptually, if some current ideas about the formation of stars are substantially correct, the number of stars in the universe may be exceeded by the number of bodies that are not self-luminous. It is to be expected that the class of bodies that are not self-luminous could be subdivided into families or classified in a number of ways according to physical or positional properties. From this point of view, then, the planets of the solar system might be regarded, not as unique specimens, but as members of a large family of such objects, the totality of which would form a continuum in which the relationships between the definable physical characteristics would be found to follow certain general laws of nature.
The word "planetology" has been used in the past to mean the "study and interpretation of surface markings of planets and satellites," where a planet is defined as "any body, except a comet or a meteor, that revolves about the sun of our solar system."* A more general term is needed to cover the physical properties of all non-self-luminous bodies, whether they are a part of our own solar system or are orbiting about some other star. To embrace this usage, the term "general planetology" is defined here as "a branch of astronomy that deals with the study and interpretation of the physical and chemical properties of planets." In this context, planets will then be defined as "massive aggregates of matter that are not large enough to sustain thermonuclear reactions in their interiors."
General planetology, then, is concerned primarily with deducing the interrelationships among the various properties of planet-like objects, roughly those objects with a mass less than about 1031 to 1032 grams (that is, less than about ¡¡^-o to 2a the mass of the Sun). A number of intrinsic, positional, and resultant (or secondary) properties of planetary objects are itemized below.
Intrinsic properties include mass, rate of rotation, and age.
Positional properties include mean distance from primary star (the star about which the planet orbits), inclination of equator to orbital plane, orbital eccentricity, relationships with other planetary bodies (satellites,
* Webster's New International Dictionary (2nd ed., unabridged). Springfield: G. & C. Merriam Co., 1951.
et cetera), and properties of the primary star (or star system) in which the planet exists.
Resultant properties include intensity and type of radiation received from primary (or primaries); surface temperature patterns; gravitational force at solid surface; atmospheric composition, density gradient, and temperature profile; internal structure and composition; atmospheric pressure at solid or liquid surface; atmospheric circulation patterns; tidal factors; radioactivity levels; existence of life forms; volcanic activity; and meteorite-infall rate.
If we look on the planetary bodies in the solar system now as members of a family and search for the familial resemblances, a number of interesting patterns emerge. Some of these will be discussed later in detail. Worthy of mention here are the relationships between mean density and mass; between rotational energy per unit mass and planetary mass; and among the possible atmospheric composition, the mass, and the intensity of the radiation field. Some other interesting relationships may be inferred from empirical extrapolations, or interpolations, or as perturbations from conditions known to exist on the Earth. Still others may be calculated from theoretical considerations.
At present it is obvious that our knowledge of the underlying laws of general planetology must be far from complete. For one thing, many of the data on the planets of the solar system are known only approximately (although they are often reported to three significant figures); hence certain data are not reliable enough to use in making correlations. The density of Mercury, for example, has been reported in the literature as being as low as 3.7 grams per cubic centimeter and as high as 6.2 grams per cubic centimeter (Firsoff, 1952); numerous intermediate values have also been given. This variation in reported density is a consequence of the extreme difficulty of measuring with precision either the mass or the diameter of Mercury. The physical dimensions and densities of Uranus and Neptune are also only very roughly known. In addition, our current knowledge concerning the properties and behavior of ordinary matter under extreme conditions of pressure (as must exist in the interiors of planets) is still quite rudimentary. It can not yet be said that we have anything but a fragmentary picture of the causes of mountain-building processes, earthquakes, and volcanoes, or of the structure of the Earth's crust, mantle, and interior.
Finally, there are many areas of study in which the complete elucidation of all the effects that could take place is so extraordinarily difficult and complex with the use of present techniques that it is often necessary to simplify the problems greatly in order to attack them at all. A case in point is the current state of Earth meteorology. Great strides are being made in the understanding of winds, storms, precipitation, air-circulation patterns, and other atmospheric phenomena; but many questions still remain unsolved (for example, the cause of the ice ages) and most weather prediction must still be done on a chiefly empirical basis. Yet, how much more difficult it would be to elucidate planetary meteorology in a completely general manner, taking into account every possible combination of atmospheric composition, mass, surface gravity, rate of rotation, land-sea ratio, tilt of axis, et cetera.
There are certain overriding or dominant astronomical factors, however, that permit estimations of the grosser aspects of planetary meteorology to be made. For example, the conditions necessary for the loss or retention of various gaseous atmospheric constituents can be estimated, and thus certain subclassifications of planets may be designated approximately. A knowledge of the relative universal abundances of the chemical elements, coupled with a knowledge of the chemical and physical properties of the most abundant elements and compounds, permits deductions concerning the atmospheric constituents that need to be considered.
The main objective of general planetology, in common with all science, is a fuller understanding of the universe in which we live. Some subsidiary objectives are to define the general characteristics of planetary systems; to obtain estimates of the number of planetary systems; to gain a clearer understanding of the characteristics of habitable planets and to obtain a more definitive estimate of the number of habitable planets in our Galaxy and in the universe; to indicate which of the stars in the neighborhood of the Sun would be most likely to possess habitable planets and what the probabilities might be; and finally, to obtain a better understanding of our own planet and an appreciation of the combination of factors that makes the Earth a comfortable place to live.
The following sections will deal specifically with the intrinsic and positional parameters of planets and some of the relationships that have been found or can be inferred. None of this material is absolute, above criticism, or secure from future modification or revision; but many interesting conclusions may be drawn, given even the present incomplete development of general planetology.
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