If scientists don't know what alien life will look like, how do they know what to look for?

As part of NASA's Nexus for Exoplanet System Science, an international team of astronomers, biologists and geologists have compiled the best strategies for anticipating and locating life beyond our solar system.

In a series of scientific papers published in the latest issue of the journal Astrobiology, NExSS researchers laid out their prescriptions for advancing the science of the search for alien life.

"We're moving from theorizing about life elsewhere in our galaxy to a robust science that will eventually give us the answer we seek to that profound question: are we alone?" Martin Still, NASA exoplanet scientist, said in an agency update.

The newly published papers address two main questions: what biochemical signals should scientists searching for alien life look for, and what instruments should they use to look for those signals?

The research also details the challenges complicating each of those questions.

"Detecting life is such a daunting challenge," Russell Deitrick, an astrophysicist at the University of Bern in Switzerland, said in a news release. "Just about every day I go from hopeful to cynical and back again."

As detailed by Deitrick and his colleagues, the in-depth study of alien atmospheres will offer scientists the best chance for honing in on the signatures of alien life.

But even as telescopes and their instruments become more powerful and advanced, allowing for the analysis of the chemical composition of faraway atmospheres, the first biochemical signals are unlikely to be sure things.

"Anything we detect will be ambiguous, so I see biosignature detection as really just the beginning of the search for life. After that we still have the whole process of vetting and understanding, which is why we are making so much effort to do this now."

One of the keys to identifying potential biochemical signatures is predicting what biological molecules will look like when illuminated by the faint emissions of stars cooler than our sun. Scientists must also anticipate life-supporting environs that look different than Earth's.

"We have to be open to the possibility that life may arise in many contexts in a galaxy with so many diverse worlds -- perhaps with purple-colored life instead of the familiar green-dominated life forms on Earth, for example," said Mary Parenteau, an astrobiologist and microbiologist at NASA's Ames Research Center. "That's why we are considering a broad range of biosignatures."

While an abundance of oxygen remains a promising biosignature, the gas can also be produced by non-biological processes. As well, life can exist without generating large amounts of oxygen.

"On early Earth, we wouldn't be able to see oxygen, despite abundant life," said Victoria Meadows, an astronomer at the University of Washington in Seattle. "Oxygen teaches us that seeing, or not seeing, a single biosignature is insufficient evidence for or against life -- overall context matters."

Moving forward, NExSS scientists suggest researchers should look for a combination of biochemical characteristics, not a single biosignature.

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