Abstract
Abstract Exoplanets with radii between those of Earth and Neptune have stronger surface gravity than Earth, and can retain a sizable hydrogen-dominated atmosphere. In contrast to gas giant planets, we call these planets gas dwarf planets. The James Webb Space Telescope (JWST) will offer unprecedented insight into these planets. Here, we investigate the detectability of ammonia (NH3, a potential biosignature) in the atmospheres of seven temperate gas dwarf planets using various JWST instruments. We use petitRadTRANS and PandExo to model planet atmospheres and simulate JWST observations under different scenarios by varying cloud conditions, mean molecular weights (MMWs), and NH3 mixing ratios. A metric is defined to quantify detection significance and provide a ranked list for JWST observations in search of biosignatures in gas dwarf planets. It is very challenging to search for the 10.3–10.8 μm NH3 feature using eclipse spectroscopy with the Mid-Infrared Instrument (MIRI) in the presence of photon and a systemic noise floor of 12.6 ppm for 10 eclipses. NIRISS, NIRSpec, and MIRI are feasible for transmission spectroscopy to detect NH3 features from 1.5–6.1 μm under optimal conditions such as a clear atmosphere and low MMWs for a number of gas dwarf planets. We provide examples of retrieval analyses to further support the detection metric that we use. Our study shows that searching for potential biosignatures such as NH3 is feasible with a reasonable investment of JWST time for gas dwarf planets given optimal atmospheric conditions.
Highlights
The Kepler Space Mission (Borucki et al 2010) has shown that super-Earths/mini-Neptunes are among the most abundant type of planet (Fressin et al 2013; Fulton et al 2017)
Based on our transmission spectroscopy detection metric (Equation (4)), we find that the Near-Infrared Spectrograph (NIRSpec)/G235M, NIRSpec/ G395M, and Near Infrared Imager and Slitless Spectrograph (NIRISS)/Single Object Slitless Spectroscopy (SOSS) modes are best suited to detect ammonia for our targets, with one exception—LP 791-18 c for Target LHS 1140 b LP 791-18 c K2-3 c K2-18 b TOI-270 c TOI-270 d GJ 143 b
We modeled seven promising gas dwarfs for the detection of the potential biosignature ammonia using the Mid-Infrared Instrument (MIRI), NIRSpec, and NIRISS instruments on the upcoming James Webb Space Telescope (JWST) mission: GJ 143 b, TOI-270 c, TOI-270 d, K2-18 b, K2-3 c, LHS 1140 b, and LP 791-18 c
Summary
The Kepler Space Mission (Borucki et al 2010) has shown that super-Earths/mini-Neptunes are among the most abundant type of planet (Fressin et al 2013; Fulton et al 2017). Their formation history, internal and atmospheric composition, and chemistry remain poorly understood due to their relatively small size and the presence of clouds (Benneke et al 2019a; Madhusudhan et al 2020). With a growing list of potentially habitable planets, the search for biosignatures is the logical step in exoplanet studies Biosignatures such as O2 and CH4 are familiar to Earth-like planets (Des Marais et al 2002). The observation is very challenging in terms of the signals (∼10 ppm) and the required telescope time (Lustig-Yaeger et al 2019; Suissa et al 2020)
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