RIVERSIDE, Calif. (http://www. ucr. edu ) -- If you're looking for a manual on the hunt for alien life, you're in luck.

Some of the leading experts in the field, including a UC Riverside team of researchers, have written a major series of review papers on the past, present, and future of the search for life on other planets. Published in Astrobiology, the papers represent two years of work by the Nexus for Exoplanet Systems Science (NExSS), a NASA-coordinated research network dedicated to the study of planetary habitability, and by NASA's Astrobiology Institute.

Scientists have identified more than 3,500 planets around other stars (called exoplanets) and many more will be discovered in the coming decades. Some of these are rocky, Earth-sized planets that are in the habitable zones of their stars, meaning it's neither too hot nor too cold for liquid water -- and possibly life -- to exist.

The five papers will serve as a reference for scientists searching for signs of life, called biosignatures, in the data they collect from future telescope observations.

"In less than 30 years, we've gone from not knowing whether planets existed outside our solar system to being able to pinpoint potentially habitable planets and collect data that will enable us to look for the signatures of life," said Edward Schwieterman, a postdoctoral researcher in UCR's Department of Earth Sciences and lead author on the first paper in the series. "These advances offer unprecedented opportunities to answer the age-old question, 'are we alone?', but at the same time demand that we move forward with great care by developing robust models that allow us to seek and identify life with a high degree of certainty."

Schwieterman's paper reviews three types of biosignatures that astrobiologists have previously proposed as markers for life on other planets, all of which must be remotely detected since exoplanets orbit distant stars that we cannot reach in person. The markers include:

Gaseous biosignatures byproducts of life that can be detected in the atmosphere, such as oxygen produced by photosynthesis, as on Earth. Surface biosignatures life-induced changes in the absorption and reflection of light on the surface of a planet, such as the red-edge caused when plants absorb red light during photosynthesis but reflect infrared light that is not used. Temporal biosignatures time-dependent fluctuations in gaseous or surface biosignatures, such as biologically modulated changes in the Earth's atmosphere that occur during different seasons.

Schwieterman is part of UCR's NASA-funded Alternative Earths Astrobiology Center, an interdisciplinary group that is developing a "search engine" for life on other worlds by delving into our own planet's dynamic, 4.5-billion-year history. Though dramatically different in terms of atmospheric composition and climate, the different chapters of Earth's history have one thing in common: oceans teeming with a remarkable diversity of simple and complex life.

"We are using Earth to guide our search for life on other planets because it is the only known example we have," said Timothy Lyons, a distinguished professor of biogeochemistry and director of the Alternative Earths Astrobiology Center. "But Earth actually offers us a great diversity of possibilities. Rather than being constrained to a study of present-day life, we use geological and geochemical analyses to examine the billions of years that life survived, evolved, and thrived on Earth under conditions that are very different than today's, hence the concept of 'alternative Earths.'"

Schwieterman's review outlines the complexities of searching for life on planets that are too far away to visit, including phenomena called false positives and false negatives. "The search for life using biosignatures is not as simple as looking for a single molecule or compound. Atmospheric oxygen, for example, could be a sign of life, but there are many nonbiological ways that oxygen gas could be produced on an exoplanet. Conversely, it is possible that life could exist in the absence of oxygen gas, similar to early life on Earth or portions of the oceans today," Schwieterman said. "This is one reason temporal biosignatures, which are based on dynamic phenomena such as atmospheric seasonality, might be more robust biosignatures in some circumstances."

More research on the ways nature can fool scientists into thinking a lifeless planet is alive or vice versa is described in the second paper in the series. The third and fourth papers propose novel investigations that would expand our conception of biosignatures to myriad habitable planets that are radically different from past or present Earth. The final article discusses how the search for life through biosignatures is incorporated into telescope and mission design.

In addition to Schwieterman and Lyons, Stephanie Olson, a graduate student in Earth Sciences, contributed to this research. The team, together with Christopher Reinhard, an assistant professor at Georgia Institute of Technology and a member of UCR-led Alternative Earths Astrobiology team, contributed to several other papers in the series.

"Together, these papers highlight UCR's contributions to the understanding of exoplanet biosignatures and the implications for instrument design going forward," Schwieterman said.

"These articles will provide an entry point for people from disparate fields interested in how they too might contribute to the search for life outside our solar system."

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UCR's Alternative Earths team is funded by the NASA Astrobiology Institute.