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Exoplanet Biosignatures Workshop-Without-Walls Review Papers - Discussion Forum

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19 Apr 2017

Welcome to the community discussion forum for the the NExSS Exoplanet Biosignatures Workshop-Without-Walls!

The workshop was held as a series of online participatory state-of-the-science review sessions during June 13-July 15, 2016, and an in-person meeting including video-conferencing participation in Seattle, WA, July 27-29, 2017.  The culmination of these meetings is a set of 5 review papers on the science, technology, and future of remote searches for signs of life on exoplanets.  To ensure broad interdisciplinary input, the international scientific community was widely solicited to participate in the writing of these manuscripts.  It is intended that they will serve as in-depth references to inspire students to senior researchers in research topics to further the search for life outside the Solar System.   These papers will be published as products of the Exoplanet Exploration Program Analysis Group (ExoPAG) Study Analysis Group 16 (SAG16).

Drafts of the papers were posted for a 3-week community comment period May 16 - June 9, 2017.   The final articles have now been published Open Access, links below.  You may view the discussions without login. To post comments, please login or register here.  

Conference contacts:
Shawn Domagal-Goldman, shawn.goldman *at*
Nancy Y. Kiang, nancy.y.kiang *at*
Niki Parenteau, mary.n.parenteau *at*

Review Papers for Discussion

0. Overview article:  Exoplanet Biosignatures: At the Dawn of a New Era of Planetary Observations

Astrobiology Special Collection on Exoplanet Biosignatures, Vol. 18, No. 6, 2018.

Contact:  Nancy Kiang, nancy.y.kiang *at*
Summary:  This introductory article provides a guide to the complementary scope and highlights salient features of the 5 major review papers on Exoplanet Biosignatures, below.  Strong themes that emerged from the workshop were that biosignatures must be interpreted in the context of their environment, and that frameworks must be developed to link diverse forms of scientific understanding of that context to quantify the likelihood that a biosignature has been observed. Models are needed to explore the parameter space where measurements will be widespread but sparse in detail. Given the technological prospects for large ground-based telescopes and space-based observatories, the detection of atmospheric signatures of a few potentially habitable planets may come before 2030.

1. Exoplanet Biosignatures: A Review of Remotely Detectable Signs of Life

Astrobiology Special Collection on Exoplanet Biosignatures, Vol. 18, No. 6, 2018.

Contact: Edward Schwieterman, edward.schwieterman *at*
Summary: This paper provides an in-depth review of current understanding of potential exoplanet biosignatures including gaseous, surface, and temporal biosignatures. We focus particularly on advances made since the review by Des Marais et al. (2002).  This paper does not propose new biosignatures strategies, but reviews currently existing literature to provide a foundation for a path forward. We survey some biogenic spectral features that are well-known in the specialist literature but not yet robustly vetted in the context of exoplanet biosignatures.  We also briefly review advances in assessing biosignature plausibility, including novel methods of determining chemical disequilibrium and the minimum biomass required for a given atmospheric signature. 

2. Exoplanet Biosignatures: Understanding Oxygen as a Biosignature in the Context of Its Environment

Astrobiology Special Collection on Exoplanet Biosignatures, Vol. 18, No. 6, 2018.

Contact: Victoria S. Meadow, vsm *at*
Summary: In this paper we provide an overview of the end-to-end development of O2 as a biosignature for extrasolar planetary observations.   We describe how, during the coevolution of life with the early Earth’s environment, the interplay of sources and sinks of O2 may have suppressed its accumulation in the atmosphere for several billion years, a false negative for biologically-generated O2.   Meanwhile, recent computer modeling research on potential mechanisms in exoplanet environments that may generate relatively high abundances of atmospheric O2 in the absence of a biosphere illustrate the concept of false positives.   We then describe current knowledge of specific photometric, spectroscopic and time-dependent observations of environmental context that could be made by future observatories to identify O2 as a biosignature, and discriminate it from potential false positives.   O2 was originally believed to be an unambiguous indicator for life, but the recent rich body of interdisciplinary research on the early Earth, and the predictive power of star-planet computer models, illustrate O2 as a model for the importance of environmental context in the being able to recognize and interpret biosignatures.

3. Exoplanet Biosignatures: A Framework for Their Assessment

Astrobiology Special Collection on Exoplanet Biosignatures, Vol. 18, No. 6, 2018.

Contact: David Catling, dcatling *at*
Summary: We present a general scheme for observing potential exoplanet biosignatures and gaining and expressing confidence levels for positive detection of signs of life. An appropriate framework uses models with data (in the form of exoplanetary system properties and spectral or photometric data) to find the Bayesian likelihoods of those data occurring if the exoplanet has or does not have life. The latter includes the case of false positives, i.e., where abiotic sources mimic biosignatures. Prior knowledge (including all factors that influence habitability and previous exoplanet observations) would be combined with the likelihoods to arrive at the probability of life existing on a given exoplanet given the observations.

4. Exoplanet Biosignatures: Future Directions

Astrobiology Special Collection on Exoplanet Biosignatures, Vol. 18, No. 6, 2018.

Contact: Sara I. Walker, sara.i.walker*at*
Summary: We summarize novel concepts about planetary biosignatures that are just emerging in the literature, addressing the importance of environmental context and biology that may be very different from Earth.  Topics include evaluating:  the evolutionary trajectory of coupled systems to identify high- vs. low-probability outcomes; classification of biosignatures from process-based, multi-disciplinary perspectives;   laboratory and theoretical validation outside of Earth-like conditions.  We summarize the debates over these novel ideas, proposals from the community for developing them further, and consider modeling of observational discriminatory power, and set the stage for future instrument development requirements.

5. Exoplanet Biosignatures: Observational Prospects

Astrobiology Special Collection on Exoplanet Biosignatures, Vol. 18, No. 6, 2018.

Contact: Yuka Fujii, yuka.fujii.ebihara *at*
Summary: While a variety of exoplanet biosignatures have been proposed, none of these has been detected or constrained. We provide an overview of the prospects for biosignature detection and general characterization of potentially habitable exoplanets, focusing on temperate Earth-size planets. We review the planned space-based and ground-based projects as well as the basic methods these missions will employ, and summarize what kind of properties maybe observable as the new facilities come on line. We distinguish reasonable expectations for the first constraints on spectroscopic features of atmospheres (and perhaps surfaces) of transiting and non-transiting planets obtainable before 2030, versus larger surveys to address statistical questions such as the occurrence rate of habitable environments, for future projects beyond 2030. The broad outlook which this paper presents is useful in considering new methodologies to characterize exoplanets of astrobiological interest, and in developing a framework to evaluate the possibility of biosphere based on the observables. 

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