Investigating the atmospheric composition of the TOI-270 system: Haze and cloud formation

Temperate exoplanets (Teq between 300 and 500 K) occupy a key but still underexplored atmospheric regime between Solar System planets and hotter exoplanets. Investigating their atmospheres is essential for interpreting future observations from the James Webb Space Telescope (JWST) and the ARIEL (Atmospheric Remote-sensing Infrared Exoplanet Large-survey) mission. Here we study the TOI-270 system, discovered by TESS and located 22.45 pc away [7]. The system consists of an M3 host star and three transiting planets: the super-Earth TOI-270 b, Teq ~ 600 K, and the temperate sub-Neptunes TOI-270 c and TOI-270 d, Teq ~ 489 and 383 K respectively, with TOI-270 d possessing the most constraining transmission spectrum in the system. Assuming retained atmospheres, we explore a range of metallicities, haze fluxes, and cloud-species scenarios. Atmospheric structures are simulated using a 1D, self-consistent forward model coupling stellar energy deposition, disequilibrium chemistry, and haze/cloud microphysics from the deep atmosphere, 10³ bar, to the upper thermosphere, 10^-¹⁰ bar [1,2,9]. This system allows us to examine how irradiation and orbital distance may shape atmospheric composition, cloud formation, and possible formation pathways. Studying an exoplanetary system also helps separate effects caused by orbital distance from those caused by composition, making TOI-270 a useful study case for comparative atmospheric characterization and evolutionary processes.

By exploring a range of atmospheric metallicities from 50x to 400x solar, we evaluate haze formation and the vertical distribution of clouds. For TOI-270 d, the light-haze cases produce thermal structures and compositions that result in a deep cloud deck formation, ≥1 bar, composed of KCl, NaCl, and ZnS, providing a good match to most of the JWST transmission spectrum. The favored metallicity depends on the observational dataset, but the spectra generally favor 200–300x solar, in agreement with previous studies [3,5], with the main molecular signatures of H₂O, CH₄, CO₂, COS, and NH₃ (Fig. 1). As noted in previous studies, the loss of NH₃ improves the comparison with observations. Since condensation is unlikely due to the elevated temperature, we tested subsolar nitrogen compositions. Lowering [N/H] was more effective than lowering [N/S] in suppressing NH₃, but neither case changed the abundance of CS₂ near 4.5 microns. Despite many attempts to enhance the signature of CS₂ [3], such as including photoelectron chemistry [10] and modified stellar UV flux, none appears to produce the required amount around 10^-³ bar. Currently, the CS₂ mole fraction is around 10^-⁷, meaning that an increase of about 10⁴ would be required for detectability. Such an enhancement is unlikely because CS₂ is limited by the available sulfur from H₂S. Even at 400x metallicity, H₂S reaches only ~ 10^-2, and only part of this sulfur budget can form CS₂. Therefore, CS₂ cannot realistically exceed this upper limit.

In contrast, atmospheric constraints for TOI-270 b and TOI-270 c remain limited due to the quality and wavelength coverage of current observations, which include JWST dual-transit extractions [4,6] and HST/WFC3 data [11], respectively. For TOI-270 b, both a clear and light hazy atmosphere at ~10x solar metallicity with [C/O] < 0.55 reproduce the observations, though shorter-wavelength observations are required to assess the necessity of hazes. Nonetheless, the light haze’s simulated thermal structure and composition again favor cloud formation, with KCl, NaCl, and ZnS clouds forming near ~10^-¹ bar. The detection of H₂O in TOI-270 b is consistent with the interpretation of Coulombe et al. [4], who found that the planet can plausibly host a volatile-rich atmosphere and argued that stellar TLS (Transit Light-Source) contamination is unlikely. We therefore assume that TLS effects do not significantly affect the available spectra. For TOI-270 c, the low signal-to-noise ratio prevents meaningful discrimination between atmospheric scenarios, although the thermal structure permits cloud condensation future observations are needed to test whether hazes or clouds are required.

Concerning a possible formation pathway, the presence of H₂O in TOI-270 b and TOI-270 d suggests that water-bearing material may have been incorporated during formation, consistent with accretion near or beyond the H₂O ice line followed by inward migration [10,13]. For TOI-270 b, the oxygen-rich composition favored by our simulations may indicate enrichment by water- or oxide-rich solids. In contrast, the molecular inventory of TOI-270 d, including CO₂ and CH₄ together with the reduced NH₃ signature, may point to formation farther out in the disk, where ice-line chemistry and subsequent atmospheric reprocessing under irradiation shaped the present-day composition [12]. Overall, the TOI-270 planets may have been shaped by disk ice-line chemistry and inward migration, while their volatile-rich compositions and the low activity of the host star are more consistent with long-term atmospheric retention than with catastrophic atmospheric loss, since fully stripped planets would be expected to resemble compact rocky cores rather than volatile-bearing sub-Neptunes.

Figure 1: Simulated transmission spectrum of TOI-270 d compared with JWST observations. The upper two panels show the modeled 200x solar metallicity hazy atmosphere and two subsolar-composition variants overplotted with observational datasets. The third panel shows the wavelength-dependent contribution of individual opacity sources, computed as the difference in transit depth relative to the nominal model.

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