Temperate exoplanets observable with Ariel : an update with new targets from TESS

In 2018 and 2022, we have published an analysis of the observability of temperate planets (with an equilibrium temperature of about 300-550 K) with ARIEL [1, 2]. This presentation is an update of this analysis which uses new targets identified in particular from the TESS database [3] and investigates their observability with ARIEL. Using the parameters of these new targets, and an atmospheric mean molecular weight of 2.3 g/mol (corresponding to a hydrogen-rich atmosphere), and using the ArielRad code [4], we give an estimate of the number of transits (T2) needed for these objects to be observed in the Tier 2 mode of the space mission, i. e. with a S/N @ 7 . The Tier 2 spectroscopic mode (R @10 for 1.1 mm < l < 1.95 mm, R = 50 for 1.95 mm < l < 3.9 mm, R @ 15 for  3.9 mm < l <7.8 mm) will allow us to infer constrains about the atmospheric composition of these targets. 

Preliminary results of this study are given below:

1. Using the candidate list of Edwards and Tinetti (2022) [3], we find 22 targets with T2 < 35, observable during the lifetime of ARIEL.
2. 7 among these targets, with a mass above 5 terrestrial masses, are presumably hydrogen-rich planets and should be observable with Ariel in the Tier 2 mode. In particular, the big Neptune TOI-3884 b, with T2 = 1, is expected to be observable also in the Tier 3 mode.
3. 8 other targets, with a mass between 1.5 and 5 terrestrial masses, will be observable in the Tier 2 mode if they have a hydrogen-rich atmosphere. The Ariel observation will thus allow us to determine whether they are sub-Neptunes or super-Earths. A promising case is LC 98-59 c and d for which the value of T2 is low (T2 = 7 and 6 respectively).
4. The 7 remaining candidates are exo-Earths, mostly from the TRAPPIST-1 system. Because their atmospheric mean molecular weight is expected to be as high as 18 g/mol (in the case of a H₂O-rich atmosphere) or even more (in case of a CO/N₂/CO₂-rich atmosphere), they are not expected to be observable with Ariel in the Tier 2 mode.

In the future, we plan to re-evaluate the observability of the candidates using different values of the atmospheric mean molecular weight. For the most favorable cases, we plan to calculate their synthetic spectra for different atmospheric compositions, considering also the possible effect of clouds.

References :

[1] Encrenaz et al., Exp. Astr. 46, 31 (2018)

[2] Encrenaz et al., Exp. Astr. 53, 375 (2022).

[3] Edwards, B. and Tinetti, G. Astron. J., 164, id.15, 25 pp. (2022)

[4] Mugnai, L. V. et al. 2021b, Astron. J, 161, 284 (2021)