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Margaret Turnbull of the Carnegie Institution of Washington. recently published a list of the top 100 stars most likely to harbor life. The stars were chosen based on a number of criteria, including size, composition, age and color, that would make them similar to the sun and enable planets resembling Earth to orbit them. But brown dwarfs have even a longer time available to evolve life, right? Could you comment on why brown dwarfs are apparently not listed as likely to harbor planets with life? I thought they were. Dick H. (via email)
The Doctor Responds: One reason that brown dwarfs do not appear in Turnbull's list of SETI candidate stars is that we have catalogued so few of them. Their very existence was first postulated only in the 1970s, and since the first confirmed brown dwarf detection in 1995, we have detected only a handful of these elusive neighbors. Thus, given the hundreds of billions of stars in our Galaxy, it is hardly surprising that no brown dwarfs have made the "top 100" list. Still, speculation abounds as to whether brown dwarfs in general (as opposed to any specific brown dwarf in particular) might warm a companion planet long enough to nurture life. See, for example: Andreyeschchev, A. and Scalo, J. "Duration and Habitability of Brown Dwarf Planets." Bioastronomy 2002: Great Barrier Reef Conference Proceedings (2002). It is my understanding that, unlike most stars, brown dwarfs are scarcely warmer than hot planets, with surface temperatures on the order of 1000 K. It is unclear whether such low temperatures can warm a planet sufficiently for the development of life. Further, although the lifetime of a brown dwarf may indeed be greater than that of a typical sun, the period of its radiant output may be very short indeed. It has been suggested that once they use up their meager supply of deuterium (perhaps in as little as 10 million years), they fade to black. So, with respect to sustaining life, their useful period may be far shorter than their functional lifetimes. Of course, Turnbull's candidate list (and others like it) are a useful tool for Targeted SETI, searches that concentrate on known stars. But, since there are far more stars than there are known candidates, SETI science embraces a second strategy, the All Sky Survey, in parallel with the targeted search. A sky survey could indeed turn up life in the neighborhood of a brown dwarf, the existence of which is not even known to us! Whether or not brown dwarfs warm good life-sites, they are important to SETI science for an entirely different reason: their very existence was first postulated (and their name coined) by a young grad student at the University of California, Berkeley, as part of her doctoral research. That student, Jill Tarter, went on to become one of the world's most visible, passionate, and respected SETI proponents. So, for Jill's sake, I hope your hypothesis has merit!
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