Constraining the atmospheric properties of the transiting hot Neptune HAT-P-26b through multi-band photometry.

Over the past few decades, the discovery of exoplanets has transformed our understanding of the universe, challenging the existing theories about the formation and evolution of planetary systems. Studying the atmospheres of exoplanets is a critical component of the ongoing efforts to better understand the composition, formation and evolution of planetary systems, as well as the potential habitability of other worlds. HAT-P-26b is a Neptune-mass transiting exoplanet that orbits around a K-type star. Its discovery was announced in 2011 by Hartman and collaborators. Planets in this mass range are particularly interesting, as their composition can vary widely between ice/volatile dominated planets and rock-dominated interiors with substantial H/He envelopes. To distinguish between these possibilities, atmospheric characterization is crucial. The main purpose of this thesis is to constrain the atmospheric properties of HAT-P-26b through multi-band photometry. To achieve this, I analyzed several data sets obtained from observations carried out by various ground-based telescopes (including TNG, Copernico Telescope, WHT, STELLA) and the TESS space telescope, spanning from UV to near infrared spectral range. Additionally, this thesis aims to conduct a follow-up study to investigate the possible presence of Transit-Timing Variations (TTVs). In my analysis, I found a value of the planetary radius of Rp = 0.780 +0.063−0.064 Rjup, for the light curve acquired in the Sloan u′ band. For the light curves collected in the visible and near infrared spectral range, I found values of the planetary radius between 0.6267 and 0.6711 Rjup, in accordance with previous studies performed in the same spectral range. These results suggest that the planet radius in the near UV spectral range is 16%-24% larger than in the visible and infrared spectral range. The obtained result can be attributed to the presence of a strong Rayleigh scattering in the atmosphere of HAT-P-26b, which is likely caused by a significant presence of hydrogen and helium molecules in the atmosphere of the planet. The presence of a strong UV slope should be confirmed by additional observations, preferably with at a higher spectral resolution. No clear TTV periodicity was identified, possibly because the mid-transit times I fitted, which include those from literature, were not derived through a homogeneous analysis. I estimated a TTV upper limit of 1.18 ± 0.14 minutes, that is consistent with a TTV induced by stellar activity.