Observability of temperate exoplanets with Ariel
Résumé
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 l < 4 mm and R=15
for l > 4 mm) 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 study, a follow-up of “Transit spectroscopy of temperate Jupiters
with ARIEL: A feasibility study” (Encrenaz et al., Exp. Astr. 46:31-44, 2018), is
submitted to the ARIEL special issue of Experimental Astronomy (manuscript under
revision)