Exoplanets close to their host stars experience high amounts of irradiation, causing drastic atmospheric escape that can be measured as a gas outflow from the planet using certain chemical tracers. To date, exoplanet atmospheric escape has thus far only been probed using transmission spectroscopy to measure line absorption. While it is theoretically possible to measure outflows via emission spectroscopy, the observability of these signatures may limit the practical application of this method. In this work, we investigate different strategies of observing atmospheric outflow emission, finding that Hα and He∗ consistently give the highest signal-to-noise ratio (SNR) across all planets tested. We consider a variety of exoplanets with confirmed detections of the 10833Å metastable helium absorption line and other outflow tracers. We use the updated and improved PyTPCI (The-PLUTO-CLOUDY Interface) software and wrapper with enhanced stability and usability to run combined 1D photochemistry, spectrum synthesis, and hydrodynamics simulations of our chosen exoplanet systems. Using these results and information about the observational facilities that are most sensitive to each diagnostic, we calculate the resultant signal-to-noise ratio, eclipse depth, optical depth, bremsstrahlung flux, and theoretical mass loss rates for our target systems. Ultimately, we find that these signals will not be large enough to detect and distinguish during the secondary eclipse using existing facilities in less than 5 transits. We find a maximum predicted signal-to-noise ratio of 2.4 from the hot Jupiter HD 189733b at 10× solar metallicity in the He∗ line. For future observational campaigns, our work suggests focusing on bright, well-characterized systems like hot Jupiters HD 189733b and HD 209458b, so that any potential emission signals are maximized. Although these emissions are not currently detectable with Keck and HST, they may be within reach of the next generation of extremely large telescopes.
Riley Rosener (Mon,) studied this question.