Temperate Exoplanet Study with JWST and ARIEL

The subjects of our research are temperate exoplanets, which is a class of exoplanets with an equilibrium temperature between 300K and 500K, and our goal is to expand the atmospheric characterization and the investigation of haze and cloud formation focusing on these exoplanets. The study of this family of exoplanets will close a knowledge gap on the temperate conditions that are intermediate to those seen in our solar system, and they will be among the next observation targets for both JWST (James Webb Space Telescope) and the upcoming ARIEL (Atmospheric Remote-sensing Infrared Exoplanet Large-survey) (Tinetti et al., 2018; Pascale et al., 2018). This research emerged from Encrenaz et al. (2018, 2022) and motivated studies that employed the improved constraints from the JWST observations for hot-Jupiter atmospheres, such as WASP-39b (Arfaux and Lavvas, 2023), and sub-Neptune atmospheres, such as GJ 1214b (Lavvas et al., 2023).
In our research, we will be using a 1D self-consistent model including haze/cloud microphysics, disequilibrium chemistry, and radiative transfer interactions to simulate the atmospheric structure of temperate exoplanets from the deep (10³ bar) to the upper thermosphere ( 10⁻¹⁰ bar). This model was used in exoplanet studies, like in Arfaux and Lavvas (2022), offering a more detailed correspondance of the atmospheric composition and the transit abservations. Utilizing this tool we look into the TOI-270 system, located at 22.45 pc away, which was detected by the Transiting Exoplanet Survey Satellite (TESS). This exoplanetary system consists of a M3 type host-star and three transiting planets, super-Earth TOI-270b and two sub-Neptunes TOI-270c,d (Günther et al., 2019). Studying an exoplanetary system will provide insight on how a host-star shapes the conditions of the planets atmospheres differently and how this is influenced by the distance to the star. Apart from this system, we also study planetary super-Neptune sized TOI-3884b, transiting a M4 type host-star at 43.34 pc, which is also a target for the CYCLE 3 GO program.
Our work is continued on K2-18b, transiting an M2-3 host star 38 pc away, that has gained attention due to the discovery of planetary transits by the extended Kepler Mission—K2 (Foreman-Mackey et al., 2015; Montet et al., 2015) and its potential biosignature features (Madhusudhan et al., 2023, 2025). It was a target within the CYCLE 1 GO program, and according to the observations, carbon-bearing molecules appear to dominate, but this is not enough for theoretical research to fully comprehend the physical and chemical conditions of the deeper atmosphere.
All of our findings will be assisted and cross-checked by observations sensitive enough on the infrared wavelength range, where the spectrum of these temperate exoplanets provides the most information about their atmosphere (signatures of water vapor, carbon dioxide and methane), and which is also the field of expertise for the JWST and ARIEL, allowing us to broaden our knowledge of the physical processes that govern this unknown yet territory of exoplanets.

References

Arfaux, A. and Lavvas, P. (2022). A large range of haziness conditions in hot-jupiter atmospheres. Monthly Notices of the Royal Astronomical Society, 515(4):4753–4779.
Arfaux, A. and Lavvas, P. (2023). A physically derived eddy parametrization for giant planet atmospheres with application on hot-jupiters. Monthly Notices of the Royal Astronomical Society, 522(2):2525–2542.
Encrenaz, T., Coustenis, A., Gilli, G., Marcq, E., Molaverdikhani, K., Mugnai, L. V., Ollivier, M., and Tinetti, G. (2022). Observability of temperate exoplanets with Ariel. Experimental Astronomy, 53(2):375–390.
Encrenaz, T., Tinetti, G., and Coustenis, A. (2018). Transit spectroscopy of temperate Jupiters with ARIEL: a feasibility study. Experimental Astronomy, 46(1):31–44.
Foreman-Mackey, D., Montet, B. T., Hogg, D. W., Morton, T. D., Wang, D., and Sch¨olkopf, B. (2015). A systematic search for transiting planets in the k2 data. The Astrophysical Journal, 806(2):215.
Günther, M. N., Pozuelos, F. J., Dittmann, J. A., et al. (2019). A super-earth and two sub-neptunes transiting the nearby and quiet Mdwarf TOI-270. Nature Astronomy, 3:1099–1108.
Lavvas, P., Paraskevaidou, S., and Arfaux, A. (2023). Photochemical hazes clouds in the atmosphere of gj 1214 b in view of recent JWST observations. 55th Annual Meeting of the Division for Planetary Sciences, id. 223.08. Bulletin of the American Astronomical Society e-id 2023n8i223p08, 55(8).
Madhusudhan, N., Constantinou, S., Holmberg, M., Sarkar, S., Piette, A. A. A., and Moses, J. I. (2025). New constraints on DMS and DMDS in the atmosphere of K2-18 b from JWST miri. The Astrophysical Journal Letters,983(2):L40.
Madhusudhan, N., Sarkar, S., Constantinou, S., Holmberg, M., Piette, A. A. A., and Moses, J. I. (2023). Carbon-bearing molecules in a possible hycean atmosphere. The Astrophysical Journal Letters, 956(1):L13.
Montet, B. T., Morton, T. D., Foreman-Mackey, D., Johnson, J. A., Hogg, D. W., Bowler, B. P., Latham, D. W., Bieryla, A., and Mann, A. W. (2015). Stellar and planetary properties of k2 campaign 1 candidates and validation of 17 planets, including a planet receiving earth-like insolation. The Astrophysical Journal, 809(1):25.
Pascale, E., Bezawada, N., Barstow, J., et al. (2018). The ARIEL space mission. In Lystrup, M., MacEwen,H. A., Fazio, G. G., Batalha, N., Siegler, N., and Tong, E. C., editors, Space Telescopes and Instrumentation 2018: Optical, Infrared, and Millimeter Wave, volume 10698 of Society of Photo-Optical Instrumentation Engineers (SPIE) Conference Series, page 106980H.
Tinetti, G., Drossart, P., Eccleston, P., et al. (2018). A chemical survey of exoplanets with ARIEL. Experimental Astronomy, 46(1):135–209.