© 1975-1979, 2008 Robert A. Freitas Jr. All Rights Reserved.

Robert A. Freitas Jr., Xenology: An Introduction to the Scientific Study of Extraterrestrial Life, Intelligence, and Civilization, First Edition, Xenology Research Institute, Sacramento, CA, 1979; http://www.xenology.info/Xeno.htm

Chapter 5.  General and Comparative Planetology

“I have chosen that part of Philosophy which is most likely to excite curiosity; for what can more concern us, than to know how this world which we in habit is made; and whether there be any other worlds like it, which are also inhabited as this is?”
          -- Bernard de Fontenelle, Conversations About the Plurality of Worlds (1686)

“We know the prodigality of Nature. How many acorns are scattered for one that grows to an oak? And need she be more careful of her stars than of her acorns?”
          -- Sir Arthur Stanley Eddington (1882-1944), in The Nature of the Physical World (1928)¹⁵⁴⁹

“Roll on, thou deep and dark blue Ocean -- roll!...
Dark-heaving -- boundless, endless, and sublime,
The image of eternity.”
          -- Lord Byron (1788-1824), Childe Harold

“Geologists believed that Mount Lookitthat was geologically recent. A few hundred of thousands of years ago, part of the planet’s skin had turned molten. Possibly a convection current in the interior had carried more than ordinarily hot magma up to melt the surface; possibly an asteroid had died a violent, fiery death. A slow extrusion had followed, with viscous magma rising and cooling and rising and cooling until a plateau with fluted sides and an approximately flat top stood forty miles above the surface.
“It had to be recent. Such a preposterous anomaly could not long resist the erosion of Mount Lookitthat’s atmosphere.”
          -- Larry Niven, in A Gift from Earth (1968)²³¹

Historically, scientists have been willing to populate the Moon, Mars, and even Sol with a great multitude of living beings. But they often were loath to extend this cosmic fecundity to regions outside our own solar system. The main hangup was that until only a few decades ago, the very idea of an abundance of planets circling other stars was scoffed at by most professional astronomers. Sol’s family of worlds was believed to be an extreme rarity, if not an absolutely unique event, in the Galaxy.

The cause of this pessimism regarding possible habitats for life in the universe was due in part to the currency of the so-called “catastrophic” theories of solar system formation. These held that the planets were born when a vagabond star passed too close to Sol, ripping away rather sizeable hunks of solar matter. The filaments of star-stuff then condensed into solid worlds, which fortuitously assumed nicely circular orbits around the sun.

The problem with this model is that stars are very far apart in the Disk of the Galaxy, so collisions of this sort must be quite improbable. The catastrophic theories lead to the inevitable conclusion that there are less than perhaps twenty solar systems in the entire Galaxy.²⁰ This, in turn, implies that few if any habitable worlds exist outside our own solar system.

In the 1930’s and early 1940’s a dramatic turnabout in attitude occurred.²⁰³⁸ Young stars in the process of formation were observed to be embedded in dense dust clouds lacked by older stars. Young stars were also seen to possess large amounts of angular momentum which older stars don’t have. Nearby suns were observed to wobble very slightly from side to side as they traveled through space, as if thrown off balance by the presence of a heavy, unseen companion. These and other observations were hailed as strong evidence that many, if not all stars, are accompanied by a planetary entourage.

Today, astronomers think of solar system formation, not as an exceedingly rare event, but as a normal and common adjunct to stellar evolution. With two hundred billion stars in our Milky Way Galaxy, and more than a billion galaxies in the universe at large, the number of possible habitats for life becomes truly staggering. If there are 10²⁰ planetary systems throughout the cosmos, then on the average more than a million of them are born every hour.²⁰

The central objective of the science of general planetology is fairly straightforward: To study the physical and chemical properties of all non-self-luminous material bodies, whether they occur in our own system or in orbit around some distant star.* A planet, consequently, is defined as any aggregate of matter possessing insufficient mass to sustain spontaneous thermonuclear reactions in its interior.²¹⁴

Xenology has two questions to ask of planetology. First, exactly how common are solar systems in the Galaxy? How many of them are there, under what conditions do they arise, and where are we most likely to find them? Questions of planetary evolution and distribution are of immense xenological importance, both in the practical sense of knowing where to search for extraterrestrial life and in the theoretical sense of being able to assess the uniqueness of life on Earth.

The second question posed by xenologists is whether or not our solar system (Table 5.1) and home planet (Table 5.2) are "typical" ones. This is basically a test of the Hypothesis of Mediocrity. Are conditions here roughly the same as on worlds circling other suns, or are things vastly different? What is the allowable range of planetary characteristics such as surface temperature, pressure, gravity, atmospheric composition, lithospheric structure, meteorology, seismology, and so forth (Figure 5.1)? Virtually anything we can learn about a planet enhances our understanding of the lifeforms indigenous thereto. It has been said that there is no property of a planet that is not of some xenological significance.⁶³⁰

Figure 5.1 Estimated Ranges of Some Interesting Properties for Terrestrial-type Planets

* The reader is strongly advised to peruse a copy of Stephen Dole’s Habitable Planets for Man,²¹⁴ which is an excellent introduction to general planetology with an eye to the specific problem of finding human-habitable worlds.

Last updated on 6 December 2008