Life Beyond the Solar System: Remotely Detectable Shawn Domagal-Goldman 1,NancyY.Kiang2,NikiParenteau3,DavidC.Catling4,Shiladitya DasSarma 5,YukaFujii6,ChesterE.Harman7,AdrianLenardic8,EnricPall´e9,ChristopherT. Reinhard 10,EdwardW.Schwieterman11,JeanSchneider12,HarrisonB.Smith13,Motohide Tamura 14,DanielAngerhausen15,GiadaArney1 ,VladimirS.Airapetian16,NatalieM. Batalha 3 ,CharlesS.Cockell17,LeroyCronin18,RussellDeitrick19,AnthonyDelGenio2 , Theresa Fisher 13 ,DawnM.Gelino20,J.LeeGrenfell21,HilairyE.Hartnett13 ,Siddharth Hegde 22,YasunoriHori23,Bet¨ulKa¸car24,JoshuaKrissansen-Totten4 ,TimothyLyons11 , William B. Moore 25,NorioNarita26,StephanieL.Olson11 Heike Rauer 27,TylerD.Robinson 28,SarahRugheimer29,NickSiegler30,EvgenyaL.Shkolnik13 ,KarlR.Stapelfeldt30 ,Sara Walker 31 1NASA Goddard Space Flight Center,2NASA Goddard Institute for Space Studies,3NASA ,4Dept. and Space Sciences / Program, University of Washington,5Institute of Marine and Environmental Technology, University of Maryland School of Medicine, Baltimore, Maryland,6Earth-Life Science Institute, Tokyo Institute of Technology and NASA Goddard Institute for Space Studies,7Columbia University and NASA Goddard Institute for Space Studies,8Rice Univer- sity ,9Instituto de Astrof´ısica de Canaria, Spain,10School of Earth and Atmospheric Sciences, Georgia Institute of Technology,11Dept. Earth Sciences, University of California, Riverside, California,12Paris Observatory,13School of Earth and Space Exploration, State University,14University of Tokyo / Astrobiology Center of NINS,15Center for Space and Habitability, Bern University, Switzerland,16NASA Goddard Space Flight Center and American University,17UK Centre for Astrobiology, School of Physics and Astronomy, University of Edinburgh,18School of Chemistry, University of Glasgow, UK,19Dept. As- tronomy, University of Washington,20NASA Science Institute,21Dept. Extrasolar Planets and Atmospheres, German Aerospace Centre,22Carl Sagan Institute, and Cornell Center for and , Cornell University,23 Astrobiology Center and National Astronomical Observatoryof Japan,24Depts. of Molecular and Cellular Biology and Astronomy, ,25Hampton Uni- versity and National Institute of Aerospace,26Dept. of Astronomy, The University of Tokyo,27German Aerospace Centre (DLR) Institute of Planetary Research ,28Dept. Physics and Astronomy, Northern Arizona University,29Centre for , School of Earth and Environmental Sciences, University of St. Andrews, UK,30Jet Propulsion Laboratory, California Institute of Technology, and 31School of Earth and Space Exploration and Beyond Center for Fundamental Concepts in Science, Arizona State University This white paper summarizes the products from the Exoplanet Biosignatures Workshop Without Walls (EBWWW). The following people also contributed to the EBWW. Science Organizing Committee (SOC): Daniel Apai, Shawn Domagal-Goldman, Yuka Fujii, Lee Grenfell, Nancy Y. Kiang, Adrian Lenardic, Nikole Lewis, Timothy Lyons, Hilairy Hartnett, Bill Moore, Enric Pall´e, Niki Parenteau, Heike Rauer, Karl Stapelfeldt, Sara Walker. Online and In-Person Workshop Participants: SOC named above as well as Giada Arney, William Bains, Robert Blankenship, David Catling, Charles Cockell, David Crisp, Sebastian Danielache, Shiladitya DasSarma, Russell Deitrick, Anthony Del Genio, Drake Deming, Steve Desch, David Des Marais, Theresa Fisher, Sonny Harman, Erika Harnett, Siddharth Hegde, Yasunori Hori, Renyu Hu, Bet¨ul Ka¸car, Jeremy Leconte, Andrew Lincowski, Rodrigo Luger, Victoria Meadows, Adam Monroe, Norio Narita, Christopher Reinhard, Sarah Rugheimer, Andrew Rushby, Edward Schwieterman, Nick Siegler, Evgenya Skolnick,HarrisonSmith,MotohideTamura, Mike Tollion, Margaret Turnbull, and Mary Voytek. exoplanets | habitability | biosignatures | astrobiology

nexss.info/groups/ebwww/ NAS Astrobiology 1–6 Introduction For the first time in human history, we will soon be able to apply to the scientific method to the question ”Are We Alone?” The rapid ad- vance of exoplanet discovery, planetary systems science, and telescope technology will soon al- low scientists to search for life beyond our So- lar System through direct observation of extra- solar planets. This endeavor will occur alongside searches for habitable environments and signs of life within our Solar System. While these searches are thematically related and will inform each other, they will require separate observa- Fig. 1. An overview of the past, present, and future of tional techniques. The search for life on exoplan- theory research. Research historically has fo- ets holds potential through the great diversity of cused on cataloguing lists of substances or physical features worlds to be explored beyond our Solar System. that yield spectral signatures as indicators of potential life on exoplanets. Recent progress has led to understanding However, there are also unique challenges related of how non-living planets could produce similar signatures. to the relatively limited data this search will ob- In the future, the field should strive to utilize what are in- tain on any individual world. herently limited data to deliver quantitative assessments of This white paper reviews the scientific com- whether or not a given planet has life. (Credit: Aaron Gron- munity’s ability to use data from future tele- stal) scopes to search for life on exoplanets. This material summarizes products from the Exo- review). These manuscripts were written by an planet Biosignatures Workshop Without Walls interdisciplinary and international community of (EBWWW). The EBWWW was constituted by scientists, incorporating input from both an open aseriesofonlineandin-personactivities,with public comment period and an anonymous jour- participation from the international exoplanet nal peer review process. As such, they represent and astrobiology communities, to assess state of the community-wide scientific consensus on the the science and future research needs for the re- state of the field, and on the research priorities mote detection of life on planets outside our So- to further the search for life on exoplanets. lar System. These activities culminated in five manuscripts, submitted for publication, which Progress Since 2015 Astrobiology Strategy respectively cover: 1) a review of known and Expanding the library of signs of life. Analyses proposed biosignatures (Schwieterman et al., in of a planet’s spectrum, even from a single spa- press), 2) a review of O2 as a biosignature as an tial element, can yield information on the pres- end-to-end example of the contextual knowledge ence or absence of chemicals that absorb spe- required to rigorously assess any claims of life cific wavelengths of light. It is this limited in- on exoplanets (Meadows et al., in press); 3) a formation upon which many of our proposed generalized statistical approach to place qualita- biosignatures, as well as other features of the tive understanding and available data in a formal planet’s environmental context, must be identi- quantitative framework according to current un- fied. Much of the history of remote detection derstanding (Catling et al., in press); 4) identifi- of biosignatures focused on spectral features of cation of needs to advance that statistical frame- specific biological byproducts or global phenom- work, and to develop or incorporate other con- ena resulting from life. A review of exoplanet ceptual frameworks for biosignature assessment biosignatures is presented in Schwieterman et (Walker et al., in review), and 5) a review of al. (in press), updating a prior review by Des the upcoming observatories - both planned and Marais et al. (2002), which was considered in possible - that could provide the data needed to the writing of the Astrobiology Strategy 2015 search for exoplanet biosignatures (Fujii et al., in document. There have been three major devel- opments in exoplanet biosignature science since

Remote Biosignatures NAS Astrobiology 1 2015: the generation of a broader list of potential nature: (1) reliability; (2) survivability; and (3) biosignatures, more comprehensively simulations detectability. However, a number of potential of these signatures in the context of planetary en- ”false positives” for O2/O3 biosignatures exist, vironments, and consideration of abiotic means rendering additional environmental context crit- through which these signatures could be gener- ical for interpreting oxygen-based biosignatures. ated on both living and non-living worlds. For example, information about the host star Novel candidate biosignatures. There has (spectral type, age, activity level), major planet been a large expansion in the proposed biosig- characteristics (size, orbit, mass), and accessory natures for the community to consider. For atmospheric species (H2O, CO2,CO,CH4,N4) photosynthetic pigments, organisms that extend can all help to diagnose pathological high-O2/O3 the wavelengths of light that can drive oxy- cases. Similarly, Earth’s atmospheric evolution genic photosynthesis have been discovered (Ho demonstrates that biogenic gases may remain at et al. 2016; Li et al., 2015), increasing the undetectable levels despite their production by a types of star-planet combinations that can sus- surface biosphere (Rugheimer and Kaltenegger, tain this metabolism (Takizawa et al., 2017). in press). Surface pigments other than those used for oxy- Planetary characteristics that may enhance genic photosynthesis have also been proposed, the likelihood of such ”false negatives” should including bacteriorhodopsin and other pigments be considered when selecting targets for biosig- (e.g., Schwieterman et al., 2015a, Hegde et al., nature searches. Careful selection of targets 2015). For atmospheric biosignatures, several can help mitigate against the likelihood of false thousand volatile gases have been identified as positive O2/O3 signals. For example, selec- worthy of further consideration (Seager et al., tion of older F, G, K or early M dwarf tar- 2016). On planets lacking oxygen, atmospheric gets (M0-M3) would help guard against false features such as organic hazes have also been positive O2/O3 signals associated with water identified as possible signs of life (Arney et al., loss, while potentially increasing the probabil- 2016). Sustained efforts at formal cataloguing of ity that biogenic O2/O3 will have accumulated the new wealth of biosignature features are crit- to detectable levels. We suggest an integrated ically needed. observation strategy for fingerprinting oxygenic photosynthetic biospheres on terrestrial planets 3D simulation of living worlds. Modeling tools with the following major steps: (1) planet detec- have become critical in simulating biosignatures tion and preliminary characterization; (2) search on a global scale. These include photochemi- for O2/O3 spectral features with high-resolution cal and climate models that can self-consistently spectroscopy; (3) further characterization and simulate these biosignatures within their plan- elimination of potential false positives; (4) de- etary context. A significant advance in this tailed characterization and the search for sec- area since 2015 is the utilization of 3-dimensional ondary biosignatures. The identification of a (3D) spectral models (e.g., Robinson et al., 2011; pigment spectral feature would be a particu- Schwieterman et al., 2015b). 3D general circu- larly complementary biosignature O2/O3 detec- lation models (GCMs) are emerging as impor- tion, because it would be consistent with the tant theoretical tools to explore the dynamics hypothesis that the O2 was generated by oxy- of planetary climates and to expand conceptu- genic photosynthesis. To further improve con- alization of the habitable zone (e.g., Turbet et fidence in identifying surface signs of photosyn- al.2016; Way et al., 2017). Further development thesis, the reflection spectra of the mineral back- of these modeling capabilities will be needed to ground must also be characterized. Newly devel- apply coupled biosphere-atmosphere processes to oped measurements such as the linear and cir- simulate biosignatures in a planetary systems sci- cular polarization spectra of chiral biomolecules ence context. can potentially help rule out such false positives. The importance of environmental context. In addition, models that address the surface cov- Oxygen-based biosignatures (O2 and/or O3)are erage of a planet are needed to better understand extremely promising, as they fulfill the three ma- the detectability of these signals. jor requirements of a robust atmospheric biosig-

2 nexss.info/groups/ebwww/ Remote Biosignatures Scientific Progress in the Next 20 Years Cross-disciplinary quantitative frameworks. Much of the top-level theory of biosignatures is described in qualitative terms, and the associ- ated advice to mission/instrument design teams is similarly qualitative. For example, we know that the confirmation of biosignatures requires acomprehensiveclassificationoftheplanetary environment, which in turn leads to a sugges- tion to obtain observations with as broad of a wavelength range as possible. But evaluation of detailed trade-offs for specific instruments, and Fig. 2. A Bayesian framework, applied to the detection of eventually the interpretation of data from biosig- life on extrasolar planets. Equation from Catling, et al., in nature searches, will be best enabled by a more press. Adapted from Walker et al., in review. quantitative framework. Amajorchallengeinsuchquantificationis =context),i.e.,P(data|context and life) and that assessing the presence or absence of life on P(data|context and no life), respectively. The aplanetisaninherentlycomplexproblem,re- conditional probabilities here account for the in- quiring comprehensive analyses of the planetary tertwining of life with its environment, such that context. And a planet will have multiple sys- they cannot be independent. P(life|context) is a tems that interact with each other, often in non- quantitative expression of likelihood of life given linear ways. Accounting for this in a quantified the context of the exoplanet, such as amenabil- manner – and doing so in a way that is flexible ity to habitability. This is distinct from P(life) enough to handle alien worlds with potentially (the probability of life occurring at all in the alien climates and potentially alien life - requires universe). The latter might be estimated from an encompassing framework. At the EBWWW, how quickly life emerged on Earth, but is truly avarietyofapproacheswerediscussed,including: quantifiable only with large statistics, after more process-based planet systems models; quantifica- examples of life have been already discovered, tion of thermodynamic and/or kinetic disequilib- which Walker et al. (in review) expand upon. rium in a planet’s atmosphere (after Krissanssen- Bayes’ Theorem also provides a means to incor- Totten et al., 2016); assessment of the complex- porate uncertainty in data (Parviainen, 2017), ity of atmospheric photochemical networks (af- additional types and novel concepts of life, such ter Holme et al. 2011); and utilization of Bayes’ as exotic adaptations, network theory, alterna- Theorem to assess the data from a single planet tive chemistry, or statistics from ensemble in- or a series of planets. Bayes’ Theorem, in par- vestigations, and in general new data and ideas ticular, was identified as having the potential to as they develop (e.g. Deeg et al., 2017). The advance our field’s ability to synthesize sparse Bayesian approach thus affords the synthesis of data, and as a framework for combining under- diverse areas of knowledge into a quantitative standing from diverse scientific disciplines. framework. It also is highly useful for iden- According to Bayes’ Theorem, one can calcu- tifying the terms most challenging to quantify. late the conditional probability that something Given the highly interdisciplinary nature of the is true, such as the likelihood of a system having search for exoplanet biosignatures, adoption of a given property based on available data. An aBayesianconceptisencouragedtohelpscien- example mathematical formalism for exoplanet tists work across disciplines, identify the signifi- biosignatures is shown in Figure 2, from Catling cance of critical unknowns, and provide quanti- et al. (in press). This derivation specifically tative assessments of confidence in scientific con- dissects what might be observed (D = data) clusions. given either the presence or absence of life within The community is beginning to build com- aparticularexoplanetenvironmentcontext(C prehensive modeling tools, and the future re- search directions required to quantify our as-

Remote Biosignatures NAS Astrobiology 3 sessments are reviewed in the EBWWW paper cerns should not dissuade us from these obser- by Walker et al. (in review). The tools for vations, but they do make target selection and simulating data that could come from inhab- precursor observations of stellar host properties ited/uninhabited worlds are already under de- critical. The characterization of Earth-like HZ velopment with both flexible 1-dimensional at- planets around Solar-type stars will require more mospheric models that can be coupled to subsur- sensitive observations. The PLATO (2026-) mis- face and escape models, and comprehensive but sion is specifically targeted at transiting plan- less flexible 3-dimensional global climate models. ets in a wider parameter space, including small Current work - by large interdisciplinary teams HZ planets around Solar-type stars. The spec- -isincreasingthecomprehensivenessofthefor- troscopic characterization of potentially Earth- mer models as well as the flexibility of the latter like worlds around Sun-like stars demands space- ones. This development of models must continue based high-contrast observations. These obser- -andthecommunityinvolvementintheirde- vations are not feasible with current and planned velopment must be expanded. We also require facilities, but are among the driving science goals advancements in chemistry and biology research for HabEx and LUVOIR. on life’s origins on Earth, and the environments in which life might originate elsewhere, to help Existing and Needed Partnerships with our assessments of P(life). Finally, we must The EBWWW revealed that the search for ex- advance our grasp of the likelihood of certain bi- oplanet life is still largely driven by astronomers ological innovations, and better understand the and planetary scientists, and that this field re- full range of metabolisms life can utilize for ob- quires more input from origins of life researchers taining energy, beyond those found on modern- and biologists to advance a process-based under- day Earth. standing for planetary biosignatures. This in- cludes assessing the prior that a planet may have Telescopes in Planning or Development life, or a life process evolved for a given planet’s The most critical step in our search for ex- environment. These advances will require fun- trasolar life is to detect spectroscopic proper- damental research into the origins and processes ties of potentially habitable planets. A hand- of life, in particular for environments that vary ful of Earth-sized planets in the HZs of late- from modern Earth’s. Thus, collaboration be- type stars have already been identified (Anglada- tween origins of life researchers, biologists, and Escud´eet al., 2016; Dittmann et al., 2017; Gillon planetary scientists is critical to defining research et al., 2017), including a few that are close questions around environmental context. Private enough for follow-up observation. Soon, discov- partnerships - mostly limited to building space- eries and astrophysical characterization of simi- flight hardware in the past - must expand to im- lar targets will be accelerated by TESS (2018-), prove our computational and modeling capabili- CHEOPS (2018-), and ongoing/future ground- ties. These collaborations could include the de- based RV surveys. Follow-up observations of velopment of generic research tools, as well as such targets could be conducted by the James specific collaborations to improve or re-write sci- Webb Space Telescope JWST (2019-), and the entific code. This latter area has tremendous next generation ground-based telescopes (GMT, potential for new public-private partnerships, as TMT, ELT: 2020s-) and next-generation flagship the codes required to quantify our certainty of a space telescopes (OST, LUVOIR, HabEx) armed biological detection will be complex, and codes with high-resolution and/or high-contrast instru- with such complexity should be crafted in part- ments. The detectability of the specific features nership with professional programmers. depends on the system properties of the tar- gets as well as the noise floor. And we note Realizing NASA’s astrobiology goals that the false positive concerns noted above (as To realize our goals, and to enable probabilis- well as concerns about habitability) are great- tic assessments of whether or not a planet has est for the stellar targets whose planets we will life, we require the following developments: be able to see with this technique. Such con-

4 nexss.info/groups/ebwww/ Remote Biosignatures • Amorecompleteincorporationofbiological Arney G., et al. (2016) Astrobiology 16:873. understanding into the field Catling D.C., et al. (in press) Astrobiology. • Models of fundamental abiotic processes under arXiv:1705.06381 planetary conditions different than our own Deeg H. J., et al. (2017) Handbook of Exo- • Evaluation of the wealth of potential new planets, Springer. biosignatures, both surface and gaseous, and Des Marais D.J., et al. (2002) Astrobiology consideration of their false positives 2:153. • An improved capability to predict the ex- Dittmann, J.A., et al. (2017) Nature 544:333. pression of photosynthesis in different stellar- Fujii Y., et al. (in review) Astrobiology. planetary environments arXiv:1705.07098 • Sustained institutional support to characterize Gillon M., et al. (2017) Nature 542:456. the physical and chemical properties of bio- Hegde S., et al. (2015) PNAS 112:3886. genic small volatile gases Ho M.Y., et al. (2016) Science 353:9178. • Development and infrastructure support for 3- Holme P., et al. (2011) PLoS ONE 6:19759. Dgeneralcirculationmodels(GCMs)forexo- Krissanssen-Totton J., et al. (2016) Astrobi- planets, to simulate biosignatures in 3-D • ology, 16:39. Expansion of coupling of 1D planetary mod- Li Y.Q., et al. (2015) Funct. Plant Biol. els for mantle, atmospheric chemistry, climate, 42:493. ocean, biology, and atmospheric escape pro- Luger R., et al. (2015) Astrobiology 15:119. cesses, with different stellar inputs, to simulate Meadows V.S., et al. (in press) Astrobiology. biosignatures in a planet systems context arXiv:1705.07560 • More accounting of model uncertainties • Finally, a Bayesian framework to foster in- Parviainen H. (2017) In: Handbook of Exo- tegration of diverse scientific disciplines and planets, Springer pp 1-24. to accommodate new data and novel concepts Robinson, T.D., et al. (2011) Astrobiology is advocated for further development in the 11:393. classroom and in collaborative research Rugheimer and Keltenegger (in press) Astro- phys. Journal. arXiv:1712.10027 That last goal is critical, as a quantitative ap- Schwieterman E.W., et al. (2015a) Astrobiol- proach will advance our field in multiple ways. ogy 15:341. For exoplanet astrobiologists, it will be a pow- Schwieterman, E.W., et al. (2015b) Astro- erful way to consider future mission/instrument phys. Journal 810:57. trade-offs, or to inform future target selection. Schwieterman E.W., et al. (in press) Astrobi- For our astrobiology peers searching for life on ology. arXiv:1705.05791 planets around other stars, it will provide a com- Seager S., et al. (2016) Astrobiology 16:465. parative tool with different proposed biosigna- Takizawa K., et al. (2017) Nature Sci. Re- tures for other targets. For our scientific col- ports 7:id.7561 leagues beyond astrobiology, it will provide a rig- Turbet M., et al. (2016) Astron. Astrophys. orous test of our conclusions. And for the gen- 596. eral public and to stakeholders, it will lead to the Walker S.I., et al. (in review) Astrobiology. ability to clearly and consistently communicate arXiv:1705.08071 our level of confidence that we are not alone. Way M.J., et al. (2017) Astrophys. Journal Suppl. Series 231:12. References Wordsworth R., et al. (2014) Astrophys. Anglada-Escud´e, G., et al. (2016) Nature Journal Letters, 785. 536:437.

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