LITTLE green men on alien planets may be discovered even once they have died out, especially if their demise was sudden. So suggests a model of our world after the sun becomes a red giant.
Earth’s grim future emerged last year when Jack O’Malley-James of the University of St Andrews, UK, and colleagues modelled the sun’s expansion into a red giant. As Earth heats up in 1 billion years’ time animals and plants die, leaving microbes as the only survivors. As complex life is likely to exist for relatively short periods on alien planets too, this seemed to lower the odds of us finding them before they die out.
Now the team has modelled the gases produced by a die-off and microbial take-over, and it turns out that aliens could still be detected after mass extinction. Bizarrely, the most promising scenario is sudden extinction. In the team’s projections, a glut of corpses produces a spike in the smelly gas methanethiol. This only hangs around for 350 years but it converts to the more persistent ethane. So a planet high in ethane could be a sign of a post-apocalyptic world, once home to complex life (arxiv.org/abs/1310.4841).
A spike in ethane would suggest a post-apocalyptic alien world that once hosted complex life
In all likelihood, life will fade slowly, leaving no such spike, but simple life could still be detectable because the models show that surviving microbes would be sub-surface and produce methane.
Though geological processes also make methane, sometimes confusing things, these would have ground to a halt once a planet’s star reached the red giant stage. So methane could be taken as a stronger sign of microbes in these cases.