Alternative Methylated Biosignatures I: Methyl Bromide, A Capstone Biosignature

Abstract

The first potential exoplanet biosignature detections are likely to be ambiguous due to the potential for false positives: abiotic planetary processes that produce observables similar to those anticipated from a global biosphere. Here we propose a class of methylated gases as corroborative `capstone' biosignatures. Capstone biosignatures are metabolic products that may be less immediately detectable, but have substantially lower false positive potential, and can thus serve as confirmation for a primary biosignature such as O${}_2$. CH${}_3$Cl has previously been established as a biosignature candidate, and other halomethane gases such as CH${}_3$Br and CH${}_3$I have similar potential. These gases absorb in the mid infrared at wavelengths that are likely to be captured while observing primary biosignatures such as O${}_3$ or CH${}_4$. We quantitatively explore CH${}_3$Br as a new capstone biosignature through photochemical and spectral modeling of Earth-like planets orbiting FGKM stellar hosts. We also re-examine the biosignature potential of CH${}_3$Cl over the same set of parameters using our updated model. We show that CH${}_3$Cl and CH${}_3$Br can build up to relatively high levels in M dwarf environments and analyze synthetic spectra of TRAPPIST-1e. Our results suggest that there is a co-additive spectral effect from multiple CH${}_3$X gases in an atmosphere, leading to increased signal-to-noise and greater ability to detect a methylated gas feature. These capstone biosignatures are plausibly detectable in exoplanetary atmospheres, have low false positive potential, and would provide strong evidence for life in conjunction with other well established biosignature candidates.

18 pages, 11 figures, appendix. Published in ApJ

References in corpus (21)

• Array Programming with NumPy
• Seven temperate terrestrial planets around the nearby ultracool dwarf star TRAPPIST-1
• How to Characterize Habitable Worlds and Signs of Life
• Observing the Atmospheres of Known Temperate Earth-sized Planets with JWST
• The Detectability and Characterization of the TRAPPIST-1 Exoplanet Atmospheres with JWST
• A Giant Planet Candidate Transiting a White Dwarf
• Habitability and Biosignatures of Hycean Worlds
• Impact of Clouds and Hazes on the Simulated JWST Transmission Spectra of Habitable Zone Planets in the TRAPPIST-1 System
• Detection of Ocean Glint and Ozone Absorption Using LCROSS Earth Observations
• Predicting the Extreme Ultraviolet Radiation Environment of Exoplanets Around Low-Mass Stars: the TRAPPIST-1 System
• Photochemistry of Anoxic Abiotic Habitable Planet Atmospheres: Impact of New H${}_2$O Cross-Sections
• A Jovian analogue orbiting a white dwarf star
• A Review of Exoplanetary Biosignatures
• Sensitive Probing of Exoplanetary Oxygen via Mid Infrared Collisional Absorption
• The White Dwarf Opportunity: Robust Detections of Molecules in Earth-like Exoplanet Atmospheres with the James Webb Space Telescope
• The K Dwarf Advantage for Biosignatures on Directly Imaged Exoplanets
• Rethinking CO Antibiosignatures in the Search for Life Beyond the Solar System
• Assessment of Isoprene as a Possible Biosignature Gas in Exoplanets with Anoxic Atmospheres
• Detectability of biosignatures on LHS 1140 b
• Photochemical Runaway in Exoplanet Atmospheres: Implications for Biosignatures
• H${}_2$-dominated Atmosphere as an Indicator of Second-generation Rocky White Dwarf Exoplanets