Except for the occasional theory that life might exist on other planets in the Solar System, the idea that extraterrestrial life might have evolved on other planets circling other stars was relegated to the realm of science fiction until the late 1980s and early 1990s when the first confirmed extra-solar planets were found orbiting other stars. Prior to those announcements, all sorts of fantastical creatures were envisioned on any number of worlds in thousands of stories, many of which dealt with aliens from world's such as the Earth. When it came to astrobiology and the search for alien life forms, the parameters of the searches tended to do the same -- look for alien life on planets found to be consistent with conditions that exist on Earth. A new study from a team of scientists from Washington State University, NASA, the Search For Extraterrestrial Intelligence (SETI), the German Aerospace Agency, and four other universities suggest that future parameters for the search for extraterrestrial life be expanded to include not just worlds where conditions for life are like those on Earth but also planets where conditions could harbor extreme forms of life inconsistent with human existence.
In the paper, "A Two-Tiered Approach to Assessing the Habitability of Exoplanets," which appeared recently in the online December edition of Astrobiology magazine, lead author and astrobiologist Dirk Schulze-Makuch of Washington State University and his colleagues, set out to broaden the indices of what might constitute habitable planets. He and his nine co-authors decided that a two-part approach should be taken when assessing the habitability of extra-solar (or exo-) planets.
"The first question is whether Earth-like conditions can be found on other worlds, since we know empirically that those conditions could harbor life," Schulze-Makuch noted in the paper. "The second question is whether conditions exist on exoplanets that suggest the possibility of other forms of life, whether known to us or not."
The team arrived at two indices for planet categorization: an Earth Similarity Index (ESI) for planet's that have more Earth-like features and a Planetary Habitability Index (PHI) for categorizing worlds with a various chemical and physical parameters that are accepted as theoretically conducive to life in more extreme, not so Earth-like conditions.
"Habitability in a wider sense is not necessarily restricted to water as a solvent or to a planet circling a star," the authors write of their conclusions. "For example, the hydrocarbon lakes on Titan could host a different form of life. Analog studies in hydrocarbon environments on Earth, in fact, clearly indicate that these environments are habitable in principle. Orphan planets wandering free of any central star could likewise conceivably feature conditions suitable for some form of life."
The idea that alien life could exist in such extreme conditions has been given impetus from a variety of sources, foremost being the discovery of life on Earth in extreme environments such as living organisms existing at crushing depths in sulfurous deep-sea volcanic areas, far underground inside miles-deep mines, and the recent discovery of living organisms adapting to arsenic as part of their chemical make-up at Mono Lake in California. Known as extremophiles, these organisms lend themselves to the possibility that conditions for life could exist in even the most unusual and restrictive places.
As for the search for extraterrestrial life -- and intelligent alien life -- the number of planets orbiting other stars has been growing steadily for the past two decades. It was announced in February that NASA's Kepler mission had spotted another 1,200 possible planets. Those extra-solar planets, of course, were still to be confirmed. As of Nov. 18, the officially confirmed (according to the Extrasolar Planets Encyclopedia) number of planets was 702. NASA's PlanetQuest is more conservative but it numbers the confirmed exoplanets at over 680.
And as the number of confirmed planets increases, the possibility that life may be detected on one or a number of them will increase as well. With various methodologies, including spectrographic analysis of planetary atmospheres, many of the planets will be studied to ascertain whether or not they could harbor life. If the two-part indices proposed by Schulze-Makuch, et. al., are adopted by astrobiologists, the future of the search for extraterrestrial life will allow for the possibility of not only life on Earth-like worlds but also on other planets where conditions for alternate forms of life might exist.
Although definitive proof of life on other worlds might be some time coming, erasing the anthropomorphic and Earth-centric constraints on those searching for its existence in the universe might allow said existence to be found just a bit sooner. Until then, there are plenty of aliens, intelligent and otherwise, populating the pages of science fiction.
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