One example she suggests in her paper is looking for an element that is in disequilibrium in a planet's atmosphere. (In other words, if scientists see a lot of silicon in an atmosphere that physics predicts would have little, perhaps that could show silicon-based life there.) The advantage is the aliens wouldn't necessarily have to be sending out signals for us to find, she added.
A recent blog post from the SETI Institute's Seth Shostak highlighted this fact for Earth, pointing out our oxygen signature has broadcast life for more than two billion years.
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Indeed, much of astrobiology itself is built on assumptions. Scientists commonly speak of a "habitable zone" for planets, based mostly on where Earth is located in our own solar system. Exoplanet biology is also compared with our own, such as when instruments for Mars are tested here based on their abilities to find Earth microbes.
Radio and optical searches, she added, are still the best tool we know of for searching given our known physics of the universe. But she does point out in the paper that the SETI Institute has been focused on the endpoint of the famous Drake equation -- how many communicating civilizations there are -- rather than "trying to understand or quantify the processes taking place within each of the factors's [for life] that lead to the next."
Even Frank Drake himself, she added, said his equation was not aimed to look for microbes or non-communicating species.
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Cabrol said the goal of her paper -- which she emphasized is just a single data point, open to discussion and dispute from other astronomers -- is not to necessarily say these ideas are wrong. Instead, her hope is that after the SETI Institute organizes a workshop next year to explore these issues, that astronomers may have a better idea about where to point their telescopes.