Structural composition and atmospheric analysis of two planetary systems : TOI-270 and TOI-776

Abstract Investigating the bulk interior and atmosphere of exoplanets has become a crucial component in understanding the birth and formation history of planetary systems. The presence of different chemical species in the core and atmosphere of the planets hint at different birth location of exoplanets in the same planetary system. In this work, we investigate the bulk structure of two planetary systems : TOI-270 & TOI-776 and explore their atmospheric properties. We provide a detailed spectroscopic and photometric estimates of the fundamental properties of the two host stars using optical spectra from HARPS, ESPRESSO and photometric data from Gaia, Pan-STARRS1, 2MASS and WISE. We employ machine learning models to estimate the radius and mass fractions of the different interior layers of the exoplanets. The results highlight that the outer planets of TOI-270 system- TOI-270 c & d - are likely formed beyond the ice line, accreting sufficient volatiles, while the inner planet - TOI-270 b - exhibits a dry and rocky composition. The interior modeling of TOI-776 system suggests a predominantly rocky composition for both its planets, consistent with their location in the radius valley. The massloss models also indicate that the four planets - TOI-270 c & d and TOI-776 b & c - retain a modest gaseous envelope contributing to their total mass. Finally, we performed atmospheric retrieval of the HST-WFC3 transit spectra of TOI-270 c which resulted in tentative detection of H2O in its atmosphere along with partial cloud coverage, consistent with the limited SNR of the data. Some amount of metal hydrides are also detected in the retrieval, but their consistency needs to be verified by future observations.