From: University of Washington
Posted: Tuesday, June 26, 2018
Researchers with the University of Washington-led Virtual Planetary Laboratory are central to a group of papers published by NASA researchers in the journal Astrobiology outlining the history -- and suggesting the future -- of the search for life on exoplanets, or those orbiting stars other than the sun.
The research effort is coordinated by NASA’s Nexus for Exoplanet Systems Science, or NExSS, a worldwide network dedicated to finding new ways to study the age-old question: “Are we alone?”
A theme through the research and the discussions behind it is the need to consider planets in an integrated way, involving multiple disciplines and perspectives.
“For life to be detectable on a distant world it needs to strongly modify its planet in a way that we can detect,” said UW astronomy professor Victoria Meadows, lead author of one of the papers and principle investigator of the Virtual Planetary Laboratory, or VPL for short. “But for us to correctly recognize life’s impact, we also need to understand the planet and star -- that environmental context is key.”
Work done by NExSS researchers will help identify the measurements and instruments needed to search for life using future NASA flagship missions. The detection of atmospheric signatures of a few potentially habitable planets may possibly come before 2030, although whether the planets are truly habitable or have life will require more in-depth study.
The papers result from two years of effort by some of the world’s leading researchers in astrobiology, planetary science, Earth science, heliophysics, astrophysics, chemistry and biology, including several from the UW and the Virtual Planetary Laboratory, or VPL. The coordinated work was born of online meetings and an in-person workshop held in Seattle in July of 2016.
The pace of exoplanet discoveries has been rapid, with over 3,700 detected since 1992. NASA formed the international NExSS network to focus a variety of disciplines on understanding how we can characterize and eventually search for signs of life, called biosignatures, on exoplanets.
The NExSS network has furthered the field of exoplanet biosignatures and “fostered communication between researchers searching for signs of life on solar system bodies with those searching for signs of life on exoplanets,” said Niki Parenteau, an astrobiologist and microbiologist at NASA’s Ames Research Center, Moffett Field, California, and a VPL team member. “This has allowed for sharing of ‘lessons learned’ by both communities.”
The first of the papers reviews types of signatures astrobiologists have proposed as ways to identify life on an exoplanet. Scientists plan to look for two major types of signals: One is in the form of gases that life produces, such as oxygen made by plants or photosynthetic microbes. The other could come from the light reflected by life itself, such as the color of leaves or pigments.
Such signatures can be seen on Earth from orbit, and astronomers are studying designs of telescope concepts that may be able to detect them on planets around nearby stars. Meadows is a co-author, and lead author is Edward Schwieterman, a VPL team member who earned his doctorate in astronomy and astrobiology from the UW and is now a post-doctoral researcher at the University of California, Riverside.
Meadows is lead author of the second review paper, which discusses recent research on “false positives” and “false negatives” for biosignatures, or ways nature could “trick” scientists into thinking a planet without life was alive, or vice versa.
In this paper, Meadows and co-authors review ways that a planet could make oxygen abiotically, or without the presence of life, and how planets with life may not have the signature of oxygen that is abundant on modern-day Earth.
The paper’s purpose, Meadows said, was to discuss these changes in our understanding of biosignatures and suggest “a more comprehensive” treatment. She said: “There are lots of things in the universe that could potentially put two oxygen atoms together, not just photosynthesis -- let’s try to figure out what they are. Under what conditions are they are more likely to happen, and how can we avoid getting fooled?”
Schwieterman is a co-author on this paper, as well as UW doctoral students Jacob Lustig-Yaeger, Russell Deitrick and Andrew Lincowski.
With such advance thinking, scientists are now better prepared to distinguish false positives from planets that truly do host life.
Two more papers show how scientists try to formalize the lessons we have learned from Earth, and expand them to the wide diversity of worlds we have yet to discover.
David Catling, UW professor of Earth and space sciences, is lead author on a paper that proposes a framework for assessing exoplanet biosignatures, considering such variables as the chemicals in the planet’s atmosphere, the presence of oceans and continents and the world’s overall climate. Doctoral student Joshua Krissansen-Totton is a co-author.
By combining all this information in systematic ways, scientists can analyze whether data from a planet can be better explained statistically by the presence of life, or its absence.
“If future data from an exoplanet perhaps suggest life, what approach can distinguish whether the existence of life is a near-certainty or whether the planet is really as dead as a doornail?” said Catling. “Basically, NASA asked us to work out how to assign a probability to the presence of exoplanet life, such as a 10, 50 or 90 percent chance. Our paper presents a general method to do this.”
The data that astronomers collect on exoplanets will be sparse. They will not have samples from these distant worlds, and in many cases will study the planet as a single point of light. By analyzing these fingerprints of atmospheric gases and surfaces embedded in that light, they will discern as much as possible about the properties of that exoplanet.
“Because life, planet, and parent star change with time together, a biosignature is no longer a single target but a suite of system traits,” said Nancy Kiang, a biometeorologist at NASA’s Goddard Institute for Space Studies in New York and a VPL team member. She said more biologists and geologists will be needed to interpret observations “where life processes will be adapted to the particular environmental context.”
The final article discusses the ground-based and space-based telescopes that astronomers will use to search for life beyond the solar system. This includes a variety of observatories, from those in operation today to ones that will be built decades in the future.
Taken together, this cluster of papers explains how the exoplanet community will evolve from their current assessments of the sizes and orbits of these faraway worlds, to thorough analysis of their chemical composition and eventually whether they harbor life.
“I’m excited to see how this research progresses over the coming decades,” said Shawn Domagal-Goldman, an astrobiologist at NASA’s Goddard Space Flight Center, Greenbelt, Maryland, and a VPL team member. He is also a co-author on four of the five papers.
“NExSS has created a diverse network of scientists. That network will allow the community to more rigorously assess planets for biosignatures than would have otherwise been possible.”
* “Exoplanet Biosignatures: Understanding Oxygen as a Biosignature in the Context of Its Environment,” Victoria S. Meadows et al., 2018 June, Astrobiology [https://doi.org/10.1089/ast.2017.1727].
* “Exoplanet Biosignatures: A Framework for Their Assessment,” David C. Catling et al., 2018 June, Astrobiology [https://doi.org/10.1089/ast.2017.1737; other papers in the series: https://www.liebertpub.com/toc/ast/18/6].
NExSS is an interdisciplinary, cross-divisional NASA research coordination network. This UW release is based on a NASA release [https://www.nasa.gov/feature/will-we-know-life-when-we-see-it-nasa-led-group-takes-stock-of-the-science].
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