Free Astronomy Magazine September-October 2014

ASTROBIOLOGY but also if within its biosphere there is a civi- lization as advanced as (or more than) ours. Obviously, discovering that aliens may have reached the point of releasing into the air they breathe a quantity of CFCs at least 10 times greater than that which we hu- mans breathe, would leave us somewhat perplexed about their intelligence, but as Lin and colleagues pointed out in an arti- cle published in late July in The Astrophys- ical Journal Letters , a native civilization of a planet located at the outer limits of the habitable zone of their star, and thus characterized by freezing temperatures, would find very useful to discharge large amounts of CFCs into the atmosphere to boost the greenhouse effect and hence global temperature. Inevitably the ques- tion arises of whether on such hostile -al- beit habitable- planet life can reach so high evolutionary levels as to be able to willingly modify its climate. It does seem more reasonable to assume that they are B elow, a spec- tacular depic- tion of a rocky planet almost completely formed around a white dwarf. The stabil- ity of those little degenerate stars can offer to a planet in their habitable zone a long period with- in which develop forms of life. [Anonymous] the inhabitants of a very hot planet that (unwisely) abuse of CFCs to keep cool. In any case, the use or misuse of CFCs can only represent a very brief period in the evolu- tion of a civilization, given the heavy envi- ronmental impact of those substances. Thinking of catching the aliens “red-hand- ed” by detecting their presence through that narrow time frame in which the CFCs are released, is decidedly limiting. But some of those molecules can persist in the atmosphere for long periods, as for example is the case for CF 4 , which has a half-life of 50,000 years, while others last considerably less, even just 10 years. This offers a great opportunity, namely, that of being able to detect traces of an alien civi- lization even tens of thousands of years after it has ceased to release that kind of pollutants into the atmosphere. The reasons why it may have stopped should be basically two: it could have dis- covered more environmentally friendly mol-