At present, scientists have only one scientific source of information on the emergence of life in a planetary environment—our own planet Earth. They currently believe that all carbon-based terrestrial organisms have descended from a common, single occurrence of the origin of life in the primeval "chemical soup" ocean of an ancient Earth. How can exobiolo-gists project this singular fact to possibly billions of unvisited worlds in the galaxy? As credible scientists, they can only do so with great sense of technical caution. Exobiologists recognize full well that their models of extraterrestrial life-forms and their probabilistic estimates concerning the cosmic prevalence of life on suitable extrasolar planets can easily become prejudiced, or chauvinistic.
Chauvinism is defined as a strongly prejudiced belief in the superiority of one's group. Applied to speculations about extraterrestrial life, this word can take on several distinctive meanings, each heavily influencing any subsequent thought on the subject. Some of the more common forms
The spectral classification of stars allows astronomers to assign individual stars scientifically useful, quantitative designations. In the 1890s, astronomers at the Harvard College Observatory (HCO) introduced a system of letters that corresponded to different types of stars, based on the stellar spectral lines. The Harvard classification system corresponded roughly to the observable, surface temperatures of these various groups of stars. Astronomers still use this system and classify stars as O (hottest), B, A, F, G, K, or M (coolest). M stars are numerous, long-lived but very dim, while O and B stars are very bright but short-lived and rare.
The Harvard system involves a sequence established in order of decreasing surface temperature, ranging from about 35,000 kelvins for O-type stars to less than 3,500 kelvins for M-type stars. The corresponding colors of the stars in this system are: O (very hot, large blue stars), B (large blue stars), A (blue-white stars), F (white stars), G (yellow stars), K (orange-red stars), and M (red stars).
In 1943, the astronomers William Wilson Morgan (1906-94) and Philip Child Keenan (1908-2000) refined the Harvard classification system by subdividing each lettered spectral classification into 10 subdivisions, denoted by the numbers 0 to 9. By convention within modern astronomy, the hotter the star, the lower this number. Astronomers classify the Sun as a G2 star. This means the Sun is a bit hotter than a G3 star and a bit cooler than a G1 star. For comparison, Betelgeuse is an M2 star and Vega is an A0 star.
Astronomers have also found it helpful to categorize stars by luminosity class. The standard stellar luminosity classes are: la for bright supergiants, lb for supergiants, II for bright giants, Ill for giants, IV for subgiants, V for main sequence stars (also called dwarfs), VI for subdwarfs, and VII for white dwarfs. This classification scheme is based upon a single spectral property—the width of a star's spectral lines. From astrophysics, scientists know that spectral-line width is sensitive to the density conditions in a star's photosphere. In turn, astronomers can correlate a star's atmospheric density to its luminosity. The use of luminosity classes allows astronomers to distinguish giants from main-sequence stars, supergiants from giants, and so forth. As a main-sequence (dwarf) star, astronomers assign the Sun the stellar luminosity class V.
With a surface temperature of 5,800 kelvins, the full spectral classification of the familiar yellow star that humans call the Sun is G2V. G-star (or solar-system) chauvinism implies, therefore, that life can only originate in a star system like our own—namely, a system containing a single G-spectral-class star.
of extraterrestrial life chauvinisms are: G-star chauvinism, planetary chauvinism, terrestrial chauvinism, chemical chauvinism, oxygen chauvinism, and carbon chauvinism. Although such heavily steeped thinking may not actually be wrong, it is important to realize that it also sets limits, intentionally or unintentionally, on contemporary speculations about life in the universe.
Planetary chauvinism assumes that extraterrestrial life has to develop independently on a particular type of planet, while terrestrial chauvinism stipulates that only "life as scientists know it on Earth" can originate elsewhere in the universe. Chemical chauvinism demands that extraterrestrial life be based on chemical processes, while oxygen chauvinism states that alien worlds must be considered uninhabitable if their atmospheres do not contain oxygen. Finally, carbon chauvinism asserts that extraterrestrial life-forms must be based on carbon chemistry.
These chauvinisms, singularly and collectively, impose tight restrictions on the type of planetary system that scientists suspect might support the rise of living systems elsewhere in the universe—possibly to the level of intelligence. If such restrictions are indeed correct, then the overall search for extraterrestrial intelligence is now being properly focused on Earth-like worlds around Sunlike stars.
If, on the other hand, life is actually quite prevalent and capable of arising in a variety of independent biological scenarios (for example, silicon-based or sulfur-based chemistry), then our contemporary efforts in modeling the cosmic prevalence of life and in trying to describe what "little green men" really look like is somewhat analogous to using the atomic theory of Democritus, the ancient Greek philosopher (ca. 460-ca. 370 b.c.e.), to help describe the inner workings of a modem nuclear-fission reactor.
As scientists continue to explore planetary bodies in this solar system—especially Mars and the interesting larger moons of Jupiter (such as Europa) and Saturn (such as Titan)—they will be able to assess more effectively how valid these extraterrestrial life chauvinisms really are. The discovery of extinct or existing alien life on any (or all) of these neighboring worlds would suggest that the universe is probably teeming with life in a wide variety of previously unimaginable forms.
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