- 09 October 2013 by Lisa Grossman
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Since the 1960s, strategies for hunting aliens rested on the assumption that they would use Earth-like chemistry – a huge assumption. Now, Sara Seager at the Massachusetts Institute of Technology and her colleagues are broadening the search. They propose a way of identifying the signatures of non-Earth-like life forms in alien atmospheres.
Seager (see “Rockstar planet hunter: Genius award will free my brain“) and MIT colleagues William Bains and Renyu Hu suggest looking for any gas that is out of equilibrium. If, for instance, astronomers detected high concentrations of a gas that degrades naturally, that would indicate something was replenishing supplies. On Earth, oxygen, ozone and methane eliminate each other rapidly and other gases are destroyed by ultraviolet radiation from the sun. Without life, those gases would not be here at all.
The team built a model that predicts how elements combine naturally in a given atmosphere. Unpredicted amounts of a molecule might indicate the presence of life. Geophysical factors like volcanoes can also keep an atmosphere stocked with gases. To tell the difference, the model calculates the mass of living things you would need on the surface to produce the outliers. If the estimate reflects a reasonable amount, then you might have just found aliens, says Seager. If it suggests a planet would have to carry more biomass than is physically plausible, the chemistry is probably not generated by alien life.
Take Saturn’s moon, Titan. Sunlight should trigger acetylene production in its atmosphere, but there is none. Some have said this could be a sign of life. Seager’s team finds that, were this the case, Titan would have to be covered in a 1.5-metre-thick layer of acetylene-eating life. What we know about its rocky, stream-covered surface suggests that is unlikely. The chemistry, the team concludes, is probably due to geological processes (Astrophysical Journal, doi.org/n5z).
“Sara’s study helps us find weird life where before we would have been flying blind,” says Shawn Domagal-Goldman of NASA’s Goddard Spaceflight Center in Greenbelt, Maryland.
On planets with atmospheres dominated by hydrogen, the model predicts that methyl chloride, dimethyl sulfide and nitrous oxide could indicate the presence of life: plausible amounts of alien life at the surface could produce them in quantities that are detectable (arxiv.org/abs/1309.6016). Domagal-Goldman says future work should focus on nailing down what other processes could also produce these gases to rule out false positives.
With some luck, Seager’s alien chemical signals will be visible to the James Webb Space Telescope, due to launch in 2018.
Domagal-Goldman likens Seager’s study to an opening door. “It opens the possibility that we might have a way to look for that life. That wasn’t true before. Everything we’ve found on these exoplanets has surprised us. When we start looking for life, we’re not going to be able to be surprised if we don’t know how to look for the weird stuff.”
This article appeared in print under the headline “Sniff out the alien molecules”
Listen up, NSA. If alien technology is monitoring us, we may be able to tap its communications.
In the 1980s, astronomers suggested that it might be possible to explore the galaxy with a fleet of self-replicating probes. At each new planetary system, they would mine asteroids for construction materials, build more probes and send them on to the next system, spreading across the galaxy within a few hundred million years. Some said the fact that we haven’t seen such spacecraft in our solar system means aliens have not explored the galaxy, and probably don’t exist. Michaël Gillon of the University of Liège in Belgium thinks that this conclusion is premature.
The alien probes would be too small to see easily, he says, but to be useful they would need to talk to each other. He proposes locations where probes could sit at the margins of our solar system to relay messages between them (Acta Astronautica, doi.org/n57).
We could be in the perfect position to intercept signals, he says. The Allen Telescope Array in northern California is dedicated to the search for extra-terrestrial intelligence, and could be used for this purpose.
“It wouldn’t be a message from another star, it would be from within our solar system, so easier to detect,” Gillon says. “It’s a bit science fiction, I know. But this doesn’t require Star Trek technology.