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More than 3,700 exoplanets (planets around other stars) have been discovered in the last 30 years. In response to this, Nexus for NASA's Exoplanet Systems Science (NExSS) brought together researchers from around the world to help answer the question: "Are we alone?
Formed by NASA three years ago, NExSS is an international network of researchers that brings together various disciplines to determine how to characterize, and possibly search for, signs of life, called biosignatures, on exoplanets.
To contribute to this effort, NExSS has produced a comprehensive series of articles describing the past, present and future of research on looking for signs of life on exoplanets.
These articles are presented in a special edition of the scientific journal Astrobiology, published this month. The special edition is the result of years of work by some of the world's leading researchers in astrobiology, planetary science, earth sciences, heliophysics, astrophysics, chemistry and biology.
An interdisciplinary team of scientists from the School of Earth and Space Exploration at Arizona State University contributed to this special edition, including astrobiologist Sara Imari Walker, the oceanographer Hilairy Hartnett, astrophysicist Evgenya Shkolnik and geological science students Theresa Fisher and Harrison Smith.
"Astrobiology is now a mature science. For the first time in human history, we will no longer need to satisfy ourselves simply by speculating on whether we are alone – we are now able to testing it and assigning a significance level to the probability are not alone, "Walker said.
In their article "Exoplanet Biosignatures: An Examination of Detectable Life-Distance Signs", contributing authors Hartnett, Walker and Fisher look to life on Earth for analog signatures that can help search for life elsewhere. In particular, they suggest that the Earth's biosphere, both in the present and through geologic time, gives a glimpse of what might be possible on other planets.
"It's been 16 years since the last review of planetary biosignatures, and since that time we've discovered many new ways to detect life on other planets, these are things that we can see – green plants and purple algae – to measurements of gases that live organisms produce – oxygen, methane or other trace gases, "said Hartnett. "One of the things ASU researchers are working on now is how do you differentiate between living and non-living things when they produce similar signatures, because we want to be sure that we know what it's all about?" looks like life before saying on another planet. "
In their article "Exoglanet Biosignatures: Future Directions," contributing authors Walker, Shkolnik, and Smith suggest that large statistics of exoplanet samples orbit a wide variety of stars, combined with the ambiguity of our understanding of the universal properties of life, for biosignatures must extend beyond those of known life.
"By focusing on detectability instead of habitability, we avoid making assumptions about extraterrestrial life, and how its needs may differ from the life of the Earth," Smith said. "It's important because we discover many planets that do not seem habitable by our standards."
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