A favorable sub-Neptune target for JWST and Ariel observations: TOI 1759 b

Following the extensive exploration of hot and warm exoplanets over the past two decades, recent improvements in instrumental techniques, from ground and space, have allowed the detection of “temperate” exoplanets, with equilibrium temperatures ranging between 300 and 500 K. Opening this new research field will not only enlarge our comprehension of the various physical properties of exoplanets, but also reduce the comprehension gap between these objects and the planets of our solar system, and provide a key step towards the study of habitable exoplanets. Over the past few years, we have started a program [1, 2] for identifying temperate exoplanets, which would be observable with the Ariel mission in the spectroscopic mode (Tier 2 mode). Using the TESS database and analyzing the observability of the candidates with Ariel, we have selected a list of 15 targets (a gas giant, a few big Neptunes and several super-Earths/sub-Neptunes) for which spectroscopic observations with Ariel would allow a characterization of their atmosphere and possibly an identification of the main atmospheric absorbers [3]. Among this list, the sub-Neptune TOI-1759 b appears as a favorable candidate. With a radius of about 3 earth radii and a mass of about 10 earth masses, TOI-1759 b is most likely a hydrogen-rich planet orbiting a M0.0 star located at 40 pc with a revolution period of 19 days. We have calculated the thermal structure, dis-equilibrium composition and size distribution of cloud and haze particles in its atmosphere using a model that couples self-consistently the involved physical and chemical processes [4]. Cloud nucleation rates reach significant values near a pressure level of 0.35 bar, where the condensing gas species (KCl, NaCl and Zn) approach their saturation limit. We have calculated the infrared synthetic spectrum of TOI-1759 b from the visible up to 20 μm for different metallicities and different haze and cloud conditions. Feasibility studies [5] suggest that information could be retrieved about the target’s atmosphere with 16 primary transit observations with Ariel (which could be achieved over a time range of less than a year), or a single primary transit observation with the JWST.   [1] Encrenaz, T.  et al., Exp. Astr. 46, 31 (2018)  [2] Encrenaz, T. et al.,  Exp. Astr. 53, 375 (2022). [3] Encrenaz, T. et al. Poster presented at the EGU General Assembly, Vienna, April 2023. [4] Arfaux & Lavvas, MNRAS, 515, 4753 (2022). [5] Tinetti et al., Exp.Astr. 46, 135 (2018).