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A Comprehensive Understanding of Planet Formation Is Required for Assessing Planetary Habitability and for the Search for Life

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A Comprehensive Understanding of Planet Formation Is Required for Assessing Planetary Habitability and for the Search for Life A comprehensive understanding of planet formation is required for assessing planetary habitability and for the search for life Dániel Apaia,b,c, Fred Cieslad, Gijs D. Muldersb,c, Ilaria Pascuccib,c, Richard Barrye, Klaus Pontoppidanf , Edwin Berging, Alex Bixela,c, Sean Brittainh, Shawn D. Domagal-Goldmane , Yasuhiro Hasegawai , Hannah Jang-Condellj , Renu Malhotrab,c, Michael R. Meyerg, Andrew Youdina, Johanna Teskek, and Neal Turneri aSteward Observatory, The University of Arizona, Tucson, AZ, USA; bLunar and Planetary Laboratory, The University of Arizona, Tucson, AZ, USA; cEOS Team, NASA Nexus for Exoplanet System Science; b,dUniversity of Chicago, Chicago, IL, USA; eNASA Goddard Space Flight Center, MD, USA; fSpace Telescope Science Institute, MD, USA; gUniversity of Michigan, Ann Arbor, MI, USA; hClemson University, Clemson, SC, USA; iJet Propulsion Laboratory, Pasadena, CA, USA; jUniversity of Wyoming; kHubble Postdoctoral Fellow, Department of Terrestrial Magnetism Carnegie Institution of Washington, Washington, DC, USA This manuscript was compiled on March 26, 2018 Dozens of habitable zone, approximately earth-sized exoplanets are known today. An emerging frontier of exoplanet studies is identify- ing which of these habitable zone, small planets are actually hab- itable (have all necessary conditions for life) and, of those, which are earth-like. Many parameters and processes influence habitabil- ity, ranging from the orbit through detailed composition including volatiles and organics, to the presence of geological activity and plate tectonics. While some properties will soon be directly observ- able, others cannot be probed by remote sensing for the foreseeable future. Thus, statistical understanding of planetary systems’ forma- tion and evolution is a key supplement to the direct measurements of planets’ properties. Probabilistically assessing parameters we can- not directly measure is essential to reliably assessing habitability, to prioritizing habitable-zone planets for follow-up, and for interpreting possible biosignatures. arXiv:1803.08682v1 [astro-ph.EP] 23 Mar 2018 Significance Statement Astronomical observations of individual exoplanets can only provide an incomplete picture of the properties and pro- cesses that determine whether a rocky planet is habitable and/or earth-like. Important statistical context can, however, be gained from understanding how planetary systems form and evolve. Efficient, quick-paced, and comprehensive multi- disciplinary research progress is essential to realize the po- tential of planet formation/evolution studies, in time to guide our next major steps in the exploration of potentially habitable exoplanets. .. 2To whom correspondence should be addressed. E-mail: apai arizona.edu http://eos-nexus.org/whitepapers/ NAS Committee on Exoplanet Science Strategy | March 26, 2018 | 1–6 Table 1. Planetary properties directly relevant to habitability. While Examples for the Importance of Formation and Evolu- some of these can be measured by remote sensing now or in the near future, several of the key parameters will be accessible only through tion our understanding of planetary systems’ formation and evolution. We bring two examples for the importance of understanding planet formation and evolution for establishing a planet’s hab- PropertiesandProcesses Remote Formation Constrained by Sensing Evolution itability and, through this process, for identifying the ideal target set for biosignature searches. Mass Y N Proxima Centaturi b: The indirectly discovered habitable Radius Y N Present-day Irradiation Y N zone planet Proxima Centauri b (2) is a good example for RotationPeriod Y N the kind of information that should be available for future Present-dayAtmosphericComposition Y N potentially habitable exoplanets and the evaluation process Present-day Atmospheric/Volatile Loss Y Y these and future detections will require. Detailed Bulk Composition (Si/Fe/Ni/O/C) N Y In short, little information is known about the planet itself: Organics/Volatile Inventory (H/C/N) N Y only its orbital period, equilibrium temperature, an m sin(i) Orbital Evolution N Y measurement, and a loose constraint on orbital eccentricity. Earth-like Geological Activity N Y Much more is known, however, about the host star Proxima Past Atmospheric / Volatile Loss N Y Centauri and about the population of close-in small planets around M dwarf stars. Many follow-up studies assumed a planet with the face value of the m sin(i) measurement with- out considering either the uncertainty of the measured value, Habitability and Planetary Properties the fact that it represents a lower limit, or the fact that the It is foreseen that remote sensing surveys for life beyond the nature of the planet (rocky, icy, gaseous) is undetermined. solar system will likely be limited to signatures originating Similarly, the formation and evolution of the planet is not from surface or near-surface life, for the lack of efficient ways understood. to probe sub-surface and deep ocean habitats. For a planetary A different approach – and one that may offer a template surface to be habitable it must not only allow for liquid water for future habitable planet interpretation – was offered by (3). to exist but the planet and planetary system must provide all In this study probability distributions representing observa- conditions necessary for life. Therefore, surface habitability tional constraints (both specific to the individual system as requires the availability of chemical ingredients necessary for well as derived from population statistics of close-in M dwarf life, the presence of an atmosphere, and a relatively stable exoplanets) were combined. Indeed, because several of the un- planetary climate. derlying probability distributions are asymmetric (and some are very broad) the nature of the planet is not straightforward to determine. In fact, that study found a broad probability Relevance of Not Directly Observable Parameters distribution (with 10-15% likelihood for Proxima Centauri b Multiple key parameters with direct impact on planetary hab- being a sub-neptune planet) and an expectation value for its itability do not lend themselves to remote-sensing measure- mass that is significantly higher than the measured msin(i) ments (see Table 1). Constraining or determining these pa- value and with a very asymmetric uncertainty. rameters can often be best achieved by understanding the Future studies of habitable exoplanets will most likely have formation and evolution of the planet, and its interactions to interpret the nature of individual planets by combining spe- with the host star, and the evolution of its host planetary sys- cific information (on the planet itself, the host star, other plan- tem. The most obvious such parameters are the detailed bulk ets in the system) with prior distributions of planet properties composition (Si, Fe, Mg, C, H, O, N) of the planet, including gained from exoplanet population studies (distributions of or- the planetary volatile and organics budgets. (Although bulk bital elements; mass distribution) and with predicted outcomes density constraints may allow the identification of clearly non- from planet formation models (volatile content, possible range earth-like planets, the degeneracies inherent to the equations of atmospheric loss, migration history, etc.). of state for different possible compositions do not allow iden- TRAPPIST-1 planets: The recently discovered habitable tifying habitable planets in general). Another example is geo- zone, roughly earth-sized TRAPPIST-1 planets (4) offer other logical activity, important for planetary habitability through examples for the challenges posed by the limited information providing a very large buffer for the atmosphere (acting as available on such worlds. Up to three of the planets may be a powerful stabilizer against atmospheric losses and climate in the present-day habitable zone; however, due to telescope fluctuations) and for its role in generating a magnetic field time limitations it is likely that only one or two of them can (which provides some protection against atmospheric solar be followed up spectroscopically by the James Webb Space wind stripping). Geological activity is extremely difficult to Telescope. But which one should be targeted? The observed assess via remote-sensing for planets that are broadly earth- properties of the planets (mass, density) provide important, like (although see (1) for some pathways possible in the dis- but limited insights (5). However, considering the properties tant future). However, the presence or absence of earth-like of the exoplanet population and possible formation/evolution composition and processes can be predicated probabilistically histories of the system is very likely to unveil major differ- if the formation history and bulk composition of the planet ences between the otherwise similar planets (e.g., (6)) and are reasonably well established. Table 1 provides an overview help identify one as the better target to invest JWST time of factors influencing planetary habitability and whether they in. Thus, TRAPPIST-1 is another system where the informa- are best characterized via remote sensing or via constraining tion coming directly from the planets must be complemented the formation and evolution of the given planet. by the much greater but more general body of information 2 | http://eos-nexus.org/whitepapers/ Apai et al. (context) emerging from planet formation and exoplanet pop- meteorites, where materials from very disparate disk environ-
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