Principle of Mediocrity

The principle of mediocrity is a rather general assumption (or speculation) that is often used in discussions concerning the nature and probability of extraterrestrial life. It assumes that things are pretty much the same all over—that is, it assumes that there is nothing special about Earth or humans' solar system. By invoking this hypothesis, scientists are suggesting that other parts of the universe are pretty much as they are here. This philosophical position allows them to then take the things they know about Earth, the chemical evolution of life that occurred here, and the facts that they are learning about other planetary bodies in this solar system and extrapolate this knowledge to develop concepts of what may be occurring on alien worlds around distant suns.

The simple premise of the principle of mediocrity is very often employed as the fundamental starting point for contemporary speculations about the cosmic prevalence of life. If Earth is indeed nothing special, then perhaps a million worlds in Milky Way galaxy (which is one of billions of galaxies) not only are suitable for the origin of life but also have witnessed its chemical evolution in their primeval oceans and are now (or at least were) habitats for a myriad of interesting living creatures. Some of these living systems may also have evolved to a level of intelligence such that the alien creatures are at this very moment gazing up into the heavens of their own world and wondering if they, too, are alone.

If, on the other hand, Earth and its intricately interwoven biosphere really are something special, then life—especially intelligent life that is capable of comprehending its own existence and contemplating its role in the cosmic scheme of things—may be a rare, very precious jewel in a vast, lifeless cosmos. Should Earth be unique—or Earth-like planets elsewhere be very rare—then the principle of mediocrity would be most inappropriate for use in estimating the probability that extraterrestrial life exists elsewhere in the universe.

Today, scientists cannot pass final judgment on the validity of the principle of mediocrity. They must, at an absolute minimum, wait until advanced robotic spacecraft and possibly human explorers have made more detailed investigations of the interesting other worlds in this solar system. Other worlds of particular interest to exobiologists include the planet Mars and certain moons of the giant outer planets Jupiter (especially its moon Europa) and Saturn (especially its moon Titan). Once scientists through their robot surrogates have explored these alien worlds in depth, they will have a much more accurate technical basis for suggesting that Earth is either "something special" or else "nothing special"—as the principle of mediocrity implies.

extrasolar planetary systems. To ascertain whether any of these extrasolar planets may be life sustaining, scientists will need to use advanced space-based systems—such as NASA's Kepler mission and the Space Interferometry Mission (SIM) in the next 10 years and the planned Terrestrial Planet Finder (TPF) and Life Finder missions another decade or so later. Instruments that are carried onboard spacecraft for these new missions will assist in the direct detection of terrestrial (that is, small and rocky) planetary companions to other stars, as well as investigate the composition of their atmospheres. (See chapters 3 and 7.) Liquid water is a basic requirement for life as scientists presently know and understand it. Specifically, during the next several decades, scientists will use advanced space-based instruments to detect key biomarkers that may strongly suggest whether planets revolving around other stars may indeed be life sustaining.

When viewed from a distance, planet Earth has known, readily identifiable surface biosignatures, or signs of life from changing vegetation patterns. In astronomy and exobiology, a biosignature is a spectral, photometric, or temporal signal whose origin specifically requires a biological agent. Earth also has several distinctive atmospheric biosignatures. Atmospheric biosignatures include the characteristic spectra of life-related molecular compounds such as oxygen (O2), which is produced by photosynthetic plants and bacteria. (Chapter 3 provides more details about planetary biosignatures of importance in exobiology.)

This inspirational view of the "rising" Earth greeted the Apollo 8 astronauts (Frank Borman, James A. Lovell, Jr., and William Anders) as they came from behind the Moon after performing the lunar-orbit insertion burn (December 1968). Each astronaut was awestruck by the stark contrast between barren and desolate lunar surface below them and the living, blue marble floating majestically in space in the distance. (NASA)

This inspirational view of the "rising" Earth greeted the Apollo 8 astronauts (Frank Borman, James A. Lovell, Jr., and William Anders) as they came from behind the Moon after performing the lunar-orbit insertion burn (December 1968). Each astronaut was awestruck by the stark contrast between barren and desolate lunar surface below them and the living, blue marble floating majestically in space in the distance. (NASA)

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