Free Astronomy Magazine September-October 2014

32 SEPTEMBER-OCTOBER 2014 ASTROBIOLOGY ecules with the same functions, or it could have became extinct (or heavily regressed) for unfathomable reasons. We do not know how long a civilization can last on a planet. We only have one example which, moreover, is rather per- turbing, but tens of thousands of years as a minimum seems a likely time frame, and hence the chances of finding traces of CFCs in some other parts of the Galaxy are mi- croscopic, but perhaps greater than zero. It will all depend on the extent, intensity and duration of the search. Since Lin, Abad and Loeb will be, in a way, guests of other observational programs, they decided to direct their attention toward a specific scenario, that on balance is the only one with the slightest chance of providing positive results through JWST, even if it does not seem particularly inspir- ing. The scenario is that of an Earth-sized planet in orbit around a white dwarf, in- side the habitable zone. In practice two spheres of 10,000-20,000 km in diameter, one roughly as heavy as the Earth, the other as the Sun, spaced 1-2 million km apart, with the lightest of the two (the plan- et) obviously orbit- ing around the more massive (the white dwarf). To simplify the data processing procedure, Lin and colleagues plan to only consider white dwarfs with surface tem- perature (and hence spectrum) compa- rable to that of the Sun; it thus follows that the habitable zone will be on av- erage centred at just 0.01 astronom- ical units from the white dwarf. The orbital motion of the poten- tially habitable planet will con- sequently be very rapid, and in such an arrangement it will be probably possible to see re- peated eclipses (usually par- tial) of the white dwarf. Every time the planet’s at- mosphere will happen to be interposed between the ob- server and the bright “sur- face” of the white dwarf, to the spectrum of the lat- ter will be added the absorp- tion lines typical of the atoms and molecules that make up the gas of the plan- etary atmosphere. Among them (hopefully) there will be the CFCs. A white dwarf shines on the glacial landscape of a planet located at the outer limits of the habitable zone. According to the au- thors of the work discussed in this article, a native civili- zation of such a cold planet could mitigate its climate by deliberately realising large amounts of CFCs into the atmosphere and thus create a considerable green- house effect. [Anonymous]