Tuesday, November 26, 2013

How to detect life on a dying planet

In 1961, Frank Drake presented a way of calculating the probability of intelligent life outside of Earth. To summarize, he described the probability as a function of the total number of planets where life could form; the probability of life emerging; several steps involving the emergence of an intelligent civilization (essentially capable of radio astronomy); and lastly, the expected lifespan of intelligent civilizations, which Drake designated as L. This last one takes a kind of grim resonance when we think of the challenges that the products of human civilization pose to Earth's habitability in the distant future.
Mars as a model for a planet at the end of supporting life.
Credit: ESA/DLR/FU Berlin (G. Neukum)
But another limit on habitability comes from the lifespan of stars themselves. In about 3.5 billion years, our own sun will be much hotter, and Earth will be much less friendly to life, perhaps entirely lacking in plants  and therefore animals. However, a recent study suggests that surviving microbes on an Earth-like planet would still change the atmosphere in a way detectable by astronomy. Surviving methanogen bacteria and cloud-seeding microbial life could create spectral signatures visible even at stellar distances.
In a sense, searches along these lines would increase Francis Drake's Length factor in both directions, because microbes would detectably modify the atmosphere before and after intelligent life would be around.

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