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Exoplanets with radii between those of Earth and Neptune have strongersurface gravity than Earth, and can retain a sizable hydrogen-dominatedatmosphere. In contrast to gas giant planets, we call these planets gas dwarfplanets. The James Webb Space Telescope (JWST) will offer unprecedented insightinto these planets. Here, we investigate the detectability of ammonia(NH$_{3}$, a potential biosignature) in the atmospheres of seven temperate gasdwarf planets using various JWST instruments. We use petitRadTRANS and PandExoto model planet atmospheres and simulate JWST observations under differentscenarios by varying cloud conditions, mean molecular weights (MMWs), andNH$_{3}$ mixing ratios. A metric is defined to quantify detection significanceand provide a ranked list for JWST observations in search of biosignatures ingas dwarf planets. It is very challenging to search for the 10.3--10.8 $\mu$mNH$_{3}$ feature using eclipse spectroscopy with MIRI in the presence of photonand a systemic noise floor of 12.6 ppm for 10 eclipses. NIRISS, NIRSpec, andMIRI are feasible for transmission spectroscopy to detect NH$_{3}$ featuresfrom 1.5 $\mu$m to 6.1 $\mu$m under optimal conditions such as a clearatmosphere and low MMWs for a number of gas dwarf planets. We provide examplesof retrieval analyses to further support the detection metric that we use. Ourstudy shows that searching for potential biosignatures such as NH$_{3}$ isfeasible with a reasonable investment of JWST time for gas dwarf planets givenoptimal atmospheric conditions.

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