Observability of temperate exoplanets with Ariel

While the Ariel mission is primarily designed for the study of warm and hot objects, with an equilibrium temperature above 500 K, in this paper we want to explore a larger sample of possible colder targets. We thus investigate the detectability with Ariel of “temperate” exoplanets (with an equilibrium temperature of 400 K). We first consider the case of hydrogen-rich exoplanets (from Jupiters to sub-Neptunes) and we calculate their infrared transmission spectrum for several classes of stars. We consider the Tier 2 mode of Ariel, for which the resolving power (R = 50 for λ < 4 μm and R = 15 for λ > 4 μm) is sufficient to get information about the chemical composition of the objects. Results show that temperate Jupiters and sub-Neptunes around all types of stars from G2 to M8, with revolution periods of a few tens of days and transit durations of a few hours, could be observed with Ariel, up to distances of about 50 pc for Jupiters and 25 pc for sub-Neptunes. In the case of temperate super-Earths, we estimate that they will not be observable in the Ariel Tier 2 mode. In a study of currently available target candidates, we find one sub-Neptune (TOI-178 g) as possibly observable in Ariel’s Tier 2. This on-going study is a follow-up of “Transit spectroscopy of temperate Jupiters with ARIEL: A feasibility study” (Encrenaz et al., Exp. Astr. 46:31–44, 2018).