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

Abstract

VLT/RISTRETTO, ELT/ANDES and ELT/PCS will soon enable atmospheric characterization of non-transiting, small rocky exoplanets orbiting closer than the inner edge of the Habitable Zone around M dwarfs, combining high-contrast imaging with high-resolution spectroscopy in reflected light. A key parameter for reflected-light observability is wavelength-dependent reflectivity, shaped by climate, surface and atmospheric properties. This work refines spectral reflectivity predictions for this population. Using Ross 128 b as a prototype, we provide physically consistent geometric albedo estimates within the RISTRETTO and ANDES spectral ranges across diverse atmospheric scenarios. We run 3D global climate model simulations of Ross 128 b for varying atmospheric compositions, surface pressures, water inventories and spin-orbit resonances to explore its possible climate regimes. We then compute synthetic reflectance spectra with Pytmosph3R to assess spectral signatures per scenario and discuss detectability and instrument capacity for constraining climate. Results show hazeless rocky planets receiving irradiation similar to Ross 128 b exhibit low reflectivity, with geometric albedos of 0.07-0.2 in the RISTRETTO bandpass and 0-0.14 in the ANDES bandpass across all scenarios. This low reflectivity can stem 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 the parameter configurations. These features are characteristic of the climate moist bistability found in close-in, low-water-reservoir planets in previous studies. Our results suggest arbitrary albedo assumptions, such as the common 0.3 Earth-like value, can overestimate reflectivity for this population, highlighting the need for accurate climate models to improve reflectivity predictions and optimize preparation for next-generation spectrographs.