evolved as an aid in finding potentially habitable albeit exotic planets with

Evolved as an aid in finding potentially habitable

This preview shows page 36 - 37 out of 48 pages.

evolved as an aid in finding potentially habitable (albeit exotic) planets with CO 2 - , CH 4 - , and even H 2 -rich atmospheres; potentially life-bearing worlds around white dwarfs; ocean worlds; desert worlds; planets around red giant stars; worlds around pre-main-sequence stars; planets orbiting A-stars; and even worlds orbiting binary stars. The HZ can be utilized to rank potentially habitable planets in the habitable zone and is even capable of filtering out HZ worlds that are least likely to host life. While we are not aware of another example of life besides our own, we should not pretend that we already know how life must be on other planets. We are not even sure how life originated on this one. Likewise, the greenhouse gas combinations and pressures that are most favorable to the emergence of life (both terrestrial and extraterrestrial) are also unknown. In other words, just because our habitable planet exhibits certain characteristics, it does not follow that other habitable planets should also share those same characteristics. The principle of mediocracy is irrelevant because we have no basis to determine how common our version of life is on a universal scale (also see Appendix B2 in Heller and Armstrong [ 265 ]). We simply do not have a second example of life to compare ours with. For these reasons, it is illogical to employ any one particular version of the HZ over all other ones. Instead, we should try to find as many types of potentially habitable planets as possible, using as holistic and flexible an approach as is feasible, as has been advocated throughout this review (Section 15 , in particular). Only then can we maximize our chances of success. In contrast, an approach that is too geocentric or focused on finding “Earth-like life” may reduce such chances of success. For example, extraterrestrial observers would have deemed our own Earth uninhabitable if they simply relied on measuring our planet’s oxygen levels, which were low for most of its history. Let’s try to minimize such oversights by searching for life that may be unlike that which has been on Earth since the Cambrian explosion or at the Great Oxidation Event. However, this may require more capable telescopes and improved observational techniques in addition to more creative habitable zone definitions. After all, we do not know if life is rare or common in the universe. So, if we want to maximize our chances of finding life elsewhere, we should assume that life may be rare and design such missions accordingly. To be clear, I am not advocating to minimize the search for “Earth-like life”, but we should challenge ourselves with creative assessments of habitable scenarios that are unfamiliar to us. In the absence of visiting alien worlds firsthand, or a major breakthrough in direct imaging, even beyond what may be coming with next-generation missions, scientific speculation is the best path forward.
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