A team of leading astronomers, biologists and geologists have come together under the Nexus for Exoplanet System Science (NExSS), a NASA research coordination network dedicated to the study of planetary habitability, to make an inventory of the most promising signs of life, called biosignatures.
Artist’s conception of what life could look like on the surface of an exoplanet. Image credit: NASA.
NExSS considered how to interpret the presence of biosignatures, should we detect them on extrasolar planets.
“We’re moving from theorizing about life elsewhere in our Milky Way Galaxy to a robust science that will eventually give us the answer we seek to that profound question: Are we alone?” said Dr. Martin Still, an exoplanet scientist at NASA Headquarters.
The assessment comes as a new generation of space and ground-based telescopes are in development.
The NASA/ESA/CSA James Webb Space Telescope will characterize the atmospheres of some of the first small, rocky planets.
Other observatories — such as the Giant Magellan Telescope and the Extremely Large Telescope — are planning to carry sophisticated instruments capable of detecting the first exoplanet biosignatures.
Through their work, NExSS scientists aim to identify the instruments needed to detect potential life for future NASA 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.
Since humans won’t be able to visit distant planets and collect samples anytime soon, the light that a telescope observes will be all we have in the search for extraterrestrial life.
Telescopes can examine the light reflecting off a distant world to show us the kinds of gases in the atmosphere and their ‘seasonal’ variations, as well as colors like green that could indicate life.
These kinds of biosignatures can all be seen on our fertile Earth from space, but the new worlds we examine will differ significantly.
For example, many of the promising planets astronomers have found are around cooler stars, which emit infrared light, rather than our Sun’s high emissions of visible-light.
“What does a living planet look like? 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,” explained Dr. Mary Parenteau, an astrobiologist and microbiologist at NASA’s Ames Research Center.
The NExSS scientists assert that oxygen — the gas produced by photosynthetic organisms on Earth — remains the most promising biosignature of life elsewhere, but it is not foolproof. Abiotic processes on a planet could also generate oxygen.
Conversely, a planet lacking detectable levels of oxygen could still be alive — which was exactly the case of Earth before the global accumulation of oxygen in the atmosphere.
“On early Earth, we wouldn’t be able to see oxygen, despite abundant life,” said Dr. Victoria Meadows, an astronomer at the University of Washington in Seattle.
“Oxygen teaches us that seeing, or not seeing, a single biosignature is insufficient evidence for or against life — overall context matters.”
Rather than measuring a single characteristic, the researchers argue that we should be looking at a suite of traits.
A planet must show itself capable of supporting life through its features, and those of its parent star.
The NExSS team will create a framework that can quantify how likely it is that a planet has life, based on all the available evidence.
With the observation of many planets, astronomers may begin to more broadly classify the ‘living worlds’ that show common characteristics of life, versus the ‘non-living worlds.’
“We won’t have a ‘yes’ or ‘no’ answer to finding life elsewhere. What we will have is a high level of confidence that a planet appears alive for reasons that can only be explained by the presence of life,” said Dr. Shawn Domagal-Goldman, an astrobiologist at NASA’s Goddard Space Flight Center.
The NExSS researchers reported their results and analysis in the June 2018 issue of the journal Astrobiology.
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Nancy Y. Kiang et al. 2018. Exoplanet Biosignatures: At the Dawn of a New Era of Planetary Observations. Astrobiology 18 (6): 619-629; doi: 10.1089/ast.2018.1862
Victoria S. Meadows et al. 2018. Exoplanet Biosignatures: Understanding Oxygen as a Biosignature in the Context of Its Environment. Astrobiology 18 (6): 630-662; doi: 10.1089/ast.2017.1727
Edward W. Schwieterman et al. 2018. Exoplanet Biosignatures: A Review of Remotely Detectable Signs of Life. Astrobiology 18 (6): 663-708; doi: 10.1089/ast.2017.1729
David C. Catling et al. 2018. Exoplanet Biosignatures: A Framework for Their Assessment. Astrobiology 18 (6): 709-738; doi: 10.1089/ast.2017.1737
Yuka Fujii et al. 2018. Exoplanet Biosignatures: Observational Prospects. Astrobiology 18 (6): 739-778; doi: 10.1089/ast.2017.1733
Sara I. Walker et al. 2018. Exoplanet Biosignatures: Future Directions. Astrobiology 18 (6): 779-824; doi: 10.1089/ast.2017.1738
This article is based on text provided by the National Aeronautics and Space Administration.
