A broad exploration of climate and observability of close-in rocky exoplanets: Applications to Ross 128 b

The upcoming instruments, VLT/RISTRETTO, ELT/ANDES, and ELT/PCS, will soon enable atmospheric characterization of the growing population of non-transiting, small, rocky exoplanets orbiting closer than the inner edge of the habitable zone around M dwarf stars. This advancement will be possible thanks to the combination of high-contrast imaging and high-resolution spectroscopy in reflected light. A critical parameter for reflected-light observability is wavelength-dependent reflectivity, which is strongly influenced by climate, surface, and atmospheric properties. The objective of this work is to refine the predictions of spectral reflectivity for this population of planets. Using Ross 128 b as a prototype, we aim to provide physically consistent geometric albedo estimates within the RISTRETTO and ANDES spectral ranges across diverse atmospheric scenarios. Using the Generic Planetary Climate Model, we performed 3D climate simulations of Ross 128 b under varying atmospheric compositions, surface pressures, water inventories, and spin-orbit resonances to investigate the potential climate regimes of the planet. Then, we computed synthetic reflectance spectra with the radiative transfer code Pytmosph3R to assess spectral signatures for each scenario and discuss the planet detectability and instrument capacity for constraining the climate. The results show that hazeless rocky planets receiving stellar irradiation similar to Ross 128 b exhibit rather low reflectivity, with geometric albedos ranging from 0.07 to 0.2 in the RISTRETTO bandpass and from 0 to 0.14 in the ANDES bandpass across all simulated scenarios. This low reflectivity can result either from the lack of clouds or surface ice deposits on the dayside or from strong atmospheric absorption due to high water vapor concentrations, depending on parameter configurations. These climate features are characteristic of climate moist bistability in close-in planets with low water reservoirs, as investigated in previous studies. Our results suggest that arbitrary albedo assumptions, such as the commonly adopted 0.3 Earth-like value, can overestimate the reflectivity for this specific planet population when assessing detection limits. This highlights the importance of using more accurate climate models, as done in this study, to improve reflectivity predictions, better assess planet detectability, and optimize preparations for future observations with next-generation high-resolution spectrographs.