Detecting Biosignatures in the Atmospheres of Gas Dwarf Planets with the James Webb Space Telescope

Phillips, Caprice L.; Ji, Wang; Kendrew, Sarah; Greene, Thomas P.; Hu, Renyu; Valenti, Jeff A.; Panero, W. R.; Schulze, J.

doi:10.3847/1538-4357/ac29be

Cited by 14 publications

(14 citation statements)

References 73 publications

(103 reference statements)

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“…We studied NH 3 as a case study for photochemical runaway and its implications for the detectability of trace biosignature gases. To simulate NH 3 runaway, we choose an H 2 -N 2 -dominated atmospheric scenario, corresponding to the Cold Haber World scenario (Seager et al 2013a(Seager et al , 2013bPhillips et al 2021), in which life would have a metabolic incentive for the high production of NH 3 . We model the photochemical runaway of NH 3 in this planetary scenario (Section 2.1), the implications of runaway for the detectability of NH 3 (Section 2.2), and its climate feedback (Section 2.3).…”

Section: Methodsmentioning

confidence: 99%

Photochemical Runaway in Exoplanet Atmospheres: Implications for Biosignatures

Ranjan

Seager

Zhan

et al. 2022

ApJ

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About 2.5 billion years ago, microbes learned to harness plentiful solar energy to reduce CO2 with H2O, extracting energy and producing O2 as waste. O2 production from this metabolic process was so vigorous that it saturated its photochemical sinks, permitting it to reach “runaway” conditions and rapidly accumulate in the atmosphere despite its reactivity. Here we argue that O2 may not be unique: diverse gases produced by life may experience a “runaway” effect similar to O2. This runaway occurs because the ability of an atmosphere to photochemically cleanse itself of trace gases is generally finite. If produced at rates exceeding this finite limit, even reactive gases can rapidly accumulate to high concentrations and become potentially detectable. Planets orbiting smaller, cooler stars, such as the M dwarfs that are the prime targets for the James Webb Space Telescope (JWST), are especially favorable for runaway, due to their lower UV emission compared to higher-mass stars. As an illustrative case study, we show that on a habitable exoplanet with an H2–N2 atmosphere and net surface production of NH3 orbiting an M dwarf (the “Cold Haber World” scenario), the reactive biogenic gas NH3 can enter runaway, whereupon an increase in the surface production flux of one order of magnitude can increase NH3 concentrations by three orders of magnitude and render it detectable by JWST in just two transits. Our work on this and other gases suggests that diverse signs of life on exoplanets may be readily detectable at biochemically plausible production rates.

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Section: Methodsmentioning

confidence: 99%

Photochemical Runaway in Exoplanet Atmospheres: Implications for Biosignatures

Ranjan

Seager

Zhan

et al. 2022

ApJ

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“…It is worth emphasizing that NH 3 or N 2 O alone would not necessarily be technosignatures, as either of these species could be false positives for life (e.g., Harman & Domagal-Goldman 2018) or could arise from nontechnological life (e.g., Roberson et al 2011;Seager et al 2013aSeager et al , 2013bSneed 2020;Phillips et al 2021;Huang et al 2022;). Rather, it is the combination of NH 3 and N 2 O that would indicate disruption of a planetary nitrogen cycle from an ExoFarm, which may also show elevated abundances of NO x gases as well as CH 4 .…”

Section: Agriculture and Nitrogenmentioning

confidence: 99%

“…Past photochemical modeling studies that have included NH 3 considered anoxic early Earth scenarios where the focus was determining the plausible greenhouse impact of NH 3 to revolve the faint young Sun paradox (Kasting 1982;Pavlov et al 2001). More recent studies have considered NH 3 biosignatures in H 2 -dominated super-Earth atmospheres, which would greatly favor the spectral detectability of the gas relative to high molecular weight O 2 -rich atmospheres (Seager et al 2013a(Seager et al , 2013bSneed 2020;Phillips et al 2021). On H 2 planets with surfaces saturated with NH 3 , deposition is inefficient, and sufficient biological fluxes can overwhelm photochemical sinks and can allow large NH 3 mixing ratios to be maintained (Huang et al 2022;.…”

Section: Next Stepsmentioning

confidence: 99%

Disruption of a Planetary Nitrogen Cycle as Evidence of Extraterrestrial Agriculture

Haqq-Misra¹,

Fauchez²,

Schwieterman³

et al. 2022

ApJL

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Agriculture is one of the oldest forms of technology on Earth. The cultivation of plants requires a terrestrial planet with active hydrological and carbon cycles and depends on the availability of nitrogen in soil. The technological innovation of agriculture is the active management of this nitrogen cycle by applying fertilizer to soil, at first through the production of manure excesses but later by the Haber–Bosch industrial process. The use of such fertilizers has increased the atmospheric abundance of nitrogen-containing species such as NH3 and N2O as agricultural productivity intensifies in many parts of the world. Both NH3 and N2O are effective greenhouse gases, and the combined presence of these gases in the atmosphere of a habitable planet could serve as a remotely detectable spectral signature of technology. Here we use a synthetic spectral generator to assess the detectability of NH3 and N2O that would arise from present-day and future global-scale agriculture. We show that present-day Earth abundances of NH3 and N2O would be difficult to detect, but hypothetical scenarios involving a planet with 30–100 billion people could show a change in transmittance of about 50%–70% compared to preagricultural Earth. These calculations suggest the possibility of considering the simultaneous detection of NH3 and N2O in an atmosphere that also contains H2O, O2, and CO2 as a technosignature for extraterrestrial agriculture. The technology of agriculture is one that could be sustainable across geologic timescales, so the spectral signature of such an “ExoFarm” is worth considering in the search for technosignatures.

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“…In fact, Phillips et al (2021) established several criteria that make exoplanet candidates attractive for NH 3 biosignature detections. First, exoplanets with radii >1.75 R ⊕ are best to ensure a gaseous envelope, but exoplanets with radii <3.4 R ⊕ are best to ensure the pressure is not great enough to produce abiotic NH 3 .…”

Section: Toi-2136mentioning

confidence: 99%

“…Our simulations do not include an NH 3 term, as TOI-2136b is not expected to produce abiotic NH 3 . Phillips et al (2021) simulated NH 3 features for a system similar to TOI-2136b: TOI-270 c (M * = 0.386; R p = 2.35 R ⊕ ; P = 5.66 days; Günther et al 2019). The TOI-270 c system ranks highest in their metric for biosignature detection, and its simulated NH 3 features are recoverable in a small number of transits.…”

Section: Toi-2136mentioning

confidence: 99%

TOI-1696 and TOI-2136: Constraining the Masses of Two Mini-Neptunes with the Habitable-Zone Planet Finder

Beard

Robertson

Kanodia

et al. 2022

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We present the validation of two planets orbiting M dwarfs, TOI-1696b and TOI-2136b. Both planets are mini-Neptunes orbiting nearby stars, making them promising prospects for atmospheric characterization with the James Webb Space Telescope (JWST). We validated the planetary nature of both candidates using high-contrast imaging, ground-based photometry, and near-infrared radial velocities. Adaptive optics images were taken using the ShARCS camera on the 3 m Shane Telescope. Speckle images were taken using the NN-Explore Exoplanet Stellar Speckle Imager on the WIYN 3.5 m telescope. Radii and orbital ephemerides were refined using a combination of the Transiting Exoplanet Survey Satellite, the diffuser-assisted Astrophysical Research Consortium (ARC) Telescope Imaging Camera (ARCTIC) imager on the 3.5 m ARC telescope at Apache Point Observatory, and the 0.6 m telescope at Red Buttes Observatory. We obtained radial velocities using the Habitable-Zone Planet Finder on the 10 m Hobby–Eberly Telescope, which enabled us to place upper limits on the masses of both transiting planets. TOI-1696b (P = 2.5 days; R p = 3.24 R ⊕; M p < 56.6 M ⊕) falls into a sparsely populated region of parameter space considering its host star’s temperature (T eff = 3168 K, M4.5), as planets of its size are quite rare around mid- to late-M dwarfs. On the other hand, TOI-2136b (P = 7.85 days; R p = 2.09 R ⊕; M p < 15.0 M ⊕) is an excellent candidate for atmospheric follow-up with the JWST.

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Detecting Biosignatures in the Atmospheres of Gas Dwarf Planets with the James Webb Space Telescope

Cited by 14 publications

References 73 publications

Photochemical Runaway in Exoplanet Atmospheres: Implications for Biosignatures

Photochemical Runaway in Exoplanet Atmospheres: Implications for Biosignatures

Disruption of a Planetary Nitrogen Cycle as Evidence of Extraterrestrial Agriculture

TOI-1696 and TOI-2136: Constraining the Masses of Two Mini-Neptunes with the Habitable-Zone Planet Finder

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