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Will we feel life when we see it?

This image is an artist's perception of how life can look on a distant planet. Credit: NASA

Over the last decade, we have discovered thousands of planets outside our solar system and learned that rocky tempered worlds are many in our galaxy. The next step will mean asking even bigger questions. Can any of these planets host life? And if so, can we recognize life elsewhere if we see it?

A group of leading researchers in astronomy, biology and geology have been gathered under NASA's Nexus for Exoplanet System Science or NExSS to find out our knowledge in the quest for life on remote planets and to lay the foundations for moving related science forward.

"We move from the theory of life elsewhere in our galaxy to a robust science that eventually gives us the answer we are looking for the deep question: Are we alone?" said Martin Still, an exoplanet researcher at NASA's headquarters in Washington.

In a set of five review papers published last week in the scientific journal Astrobiology NExSS researchers took a list of the most promising characters of life, called biosignatures. The authors contain four scientists from NASA's Jet Propulsion Laboratory in Pasadena, California. They considered how to interpret the presence of biosignatures, if we were to discover them in distant worlds. A primary concern is to ensure that science is strong enough to distinguish a living world from a bare planet masked as one.

Life can leave fingerprints of its presence in the atmosphere and on the planet's surface. These potential signs of life or biosignatures can be detected by telescope. Credit: NASA / Aaron Gronstal

The assessment comes as a new generation of space and ground-based telescopes are in development. NASA's James Web space telescope will characterize the atmosphere of some of the first small rocky planets. There are plans for other observers – such as the Giant Magellan Telescope and Extremely Large Telescope, both in Chile – to have sophisticated instruments that can detect the first biosignatures in distant worlds.

Through its work with NExSS, researchers are striving to identify the instruments needed to detect potential lives for future NASA flagship missions. The detection of atmospheric signatures of some potentially habitable planets may come before 2030, although determining whether the planets are really habitable or have lives will require more in-depth studies.

Since we will not be able to visit remote planets and collect samples at any time soon, the light a telescope observes will be all we have in the search for life beyond our solar system. Telescopes can examine the light that reflects a distant world to show us different types of gases in the atmosphere and their "seasonal" variations, as well as colors like green that can indicate life.

These types of biosignatures can all be seen on our fertile soil from space, but the new worlds we are investigating will differ significantly. For example, many of the promising planets we found are around cooler stars that emit light in the infrared spectrum, unlike our sun's high emission of visible light.

"How does a living planet look?" said Mary Parenteau, an astrobiologist and microbiologist at NASA's Ames Research Center in Silicon Valley and a co-author. "We must be open to the possibility that life can occur in many contexts in a galaxy with so many different worlds – perhaps with purple colored life instead of the familiar green-minded life forms on earth. That is why we are considering a wide range of biosignatures. "

Scientists argue that the oxygen gas produced by photosynthetic organisms on Earth remains the most promising biosignature in life elsewhere, but it is not foolproof. Abiotic processes on a planet can also generate oxygen. Conversely, a planet lacking detectable oxygen levels would still be able to live – which was exactly the case of the Earth before the global accumulation of oxygen in the atmosphere.

"On early earth, we could not see oxygen despite abundant life," said Victoria Meadows, an astronomer at the University of Washington in Seattle, and leading author of one of paper. "Acid teaches us to see or not see, a single biosignature is insufficient evidence for or against life-wide contextual questions."

Instead of measuring a single feature, NExSS researchers claim that we should look at a series of features. A planet must prove to be able to support life through its functions and its parenting star.

Since the data we collect from planets will be limited, researchers will quantify how likely a planet has life based on all available evidence. Follow-up observations are required for confirmation. Credit: NASA / Aaron

NExSS researchers will create a framework that can quantify the probability of a planet having life based on all available evidence. With observation of many planets, researchers can begin to classify the "living worlds" that show common characteristics of life, compared to the "non-living worlds".

"We will not have a" yes "or" no answer to finding life elsewhere, "said Shawn Domagal-Goldman, an astrobiologist at NASA's Goddard Space Flight Center in Greenbelt, Maryland, and a co-author. "What we are getting is a high degree of confidence that a planet appears alive for reasons that can only be explained by the presence of life."

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Jet Propulsion Laboratory

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