Saturn's Titan: A Strict Test for Life's Cosmic Ubiquity1

By Lunine, Jonathan I. | Proceedings of the American Philosophical Society, December 2009 | Go to article overview

Saturn's Titan: A Strict Test for Life's Cosmic Ubiquity1


Lunine, Jonathan I., Proceedings of the American Philosophical Society


IS LIFE A COMMON OUTCOME of physical and chemical processes in the universe? Within our own solar system, a successful search for even primitive life, were it to have an origin independent from life on Earth, would dramatically advance a positive answer (1). The most stringent test for a second independent origin of life would come from examination of either the most physically remote from Earth, or the most exotic type of, planetary environment in which one might plausibly imagine a form of life could exist. In this paper I argue that Saturn's moon Titan is the best such target in our solar system. Further, Titan might be a type example of a planetary environment abundant throughout the cosmos.

THE TRUNCATED DRAKE EQUATION

The Copernican revolution of the fifteenth through seventeenth centuries moved the Earth squarely away from the center of the universe and fostered a cosmology in which every aspect of the Earth and solar system was, or was predicted to be, a common facet of the cosmos (2). This "Copernican" assumption about the universe is confirmed in every step upward in spatial scale, from planetary systems to galaxies to clusters of galaxies to large-scale cosmic structures. Where it seems to fail at present is in comprehending the nature of the universe relative to other putative "universes"; modern cosmology has difficulty explaining the very peculiar properties of this universe that make life possible (3). While the spirit of Copernicanism was extended to biology in the nineteenth century through the gradually developing understanding that cellular processes work on the same physical and chemical principles as nonliving matter, the notion that life is ubiquitous throughout our universe remains an assumption with as yet no direct evidence (4).

The problem of our own unique versus common cosmic status as an intelligent and technological life form is symbolized in the Drake equation, first formulated almost fifty years ago (5). The equation expresses the number of observable extraterrestrial civilizations in our Milky Way Galaxy as equal to R X fp X ne X f, X fi X fc X L, where R is the rate of formation of suitable stars (it is sufficient to assume those similar in mass and composition to the Sun) in our galaxy, fp the fraction of stars with planets, ne the average number of such planetary systems with a habitable, or life-sustaining, environment, i\ the fraction of habitable planets on which life actually forms, fi the fraction of those lifebearing planets with intelligent life, fc the fraction of those intelligencebearing planets with a civilization technically capable of transmitting signals, and L the average lifetime of such a civilization. From the point of view of "astrobiological Copernicanism" - whether life itself is a common cosmic phenomenon - it is enough to consider a "truncated Drake equation" of the first four terms only, multiplied by the duration of time over which a planet remains habitable. This is the number of life-bearing planets in the Galaxy.

Much of twentieth-century astronomy not concerned with cosmology was devoted to establishing the first term in the Drake equation, while the other terms remained speculative. In 1996 it became possible to survey widely for, and detect, planets around other stars. It is now known that approximately 10% or more of stars that are similar to the Sun in their "spectral class" (chemical composition and mass) have planets, though up until 2009 planets the size of Earth could not be detected (6). As of this writing, NASA's Kepler mission has begun a survey that will fully address the second, and partly address the third, term of the Drake equation by surveying for planets the size of our own world, and slightly smaller, around stars of the same spectral class as the Sun. Once it is completed, should the statistical occurrence of Earth-sized "exoplanets" be high enough, the next step would be to build spaceborne devices sufficiently sensitive to study directly any Earth-sized planets that might be present around the stars nearest to our solar system, detect their atmospheres, and measure their masses (7). …

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