Efforts to discover and understand alien life are underway in many fields, but a question has been niggling at scientists. Will we be able to recognize the hallmarks of life from species extremely different from us? The discovery of thousands of exoplanets and improvements in our telescopes make it likely we will soon find the first indirect signals of life on a planet.
The signals are called biosignatures, and their potential range has now been published in a series of five papers in the journal Astrobiology. The work comes from NASA’s Nexus for Exoplanet System Science (NExSS), where a group of leading scientists in astronomy, biology, and geology discuss what alien life might be like and how we might look for it.
"What does a living planet look like?” co-author Mary Parenteau, an astrobiologist and microbiologist at NASA’s Ames Research Center, said in a statement. “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. That’s why we are considering a broad range of biosignatures.”
The presence of oxygen is considered the most promising biosignature as many lifeforms use it and photosynthetic organisms produce it, but it’s not a certain bet. Oxygen is also released in several chemical processes that don’t need life to happen. And there are simple and even not-so-simple lifeforms that can thrive without oxygen.
“On early Earth, we wouldn’t be able to see oxygen, despite abundant life,” added Victoria Meadows, an astronomer at the University of Washington in Seattle and lead author of one of the papers. “Oxygen teaches us that seeing, or not seeing, a single biosignature is insufficient evidence for or against life – overall context matters.”
The NExSS papers suggest that instead of focusing on a single characteristic, scientists should focus on several traits at the same time. The best estimation for life will come from a more global view, taking into consideration the characteristics of the planet and its parent star.
“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?” stated Martin Still, NASA exoplanet scientist at headquarters in Washington.
The NExSS next hope to suggest some requirements for a future flagship mission to detect life on distant exoplanets. It's possible the first atmospheric signatures from potentially habitable planets may come before 2030, according to the team.