Few questions are more intriguing than the possibility of life elsewhere in the universe. But since aliens are not visiting our planet, and we are not going to their faraway homes any time soon, indirect evidence for the existence of biology on distant worlds is our best bet for answers.
The problem is that planets and moons are not just much tinier but also much dimmer than their host stars, making their direct observation extremely challenging. Fortunately, creative astronomers have devised observational methods that detect planets orbiting distant stars on our cosmic shore and, remarkably, for obtaining the approximate chemical composition of their atmospheres. That’s where life comes in: if life exists at a global scale on a planet, it can leave signals in the atmosphere. Like fingerprints, different kinds of biological activity will leave specific atmospheric imprints. We see this, for instance, with the abundance of oxygen in our atmosphere produced by photosynthesis. The challenge for us is to decipher the message life leaves on alien atmospheres.
That’s going to take powerful telescopes but also new ways of thinking about how to decode the information hidden in the light they capture from alien worlds. We propose that the information theory we now rely on to sift signal from noise in modern communications offers tools astronomers can use to detect signs of biological activity in other worlds. The approach comes in two steps: after capturing the light from the exoplanet, we use information theory to search for chemicals associated with the presence of life. What in communications are letters of an alphabet making up a sentence, in astrobiology will be specific chemicals that exist in a distant world’s atmosphere.
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