Simulating RISTRETTO: Proxima b detectability in reflected light

¹ Département d’Astronomie, Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland
² Laboratoire de Météorologie Dynamique, IPSL, CNRS, Sorbonne Université, 4 place Jussieu, 75252 Paris Cedex 05, France
³ Laboratoire d’astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France

^★ Corresponding author: maddalena.bugatti@unige.ch

Received: 14 July 2025
Accepted: 26 August 2025

Abstract

Context. The characterization of exoplanet atmospheres is one of the key topics in modern astrophysics. To date, transmission spectroscopy has been the primary method used, but upcoming instruments, such as those on the European Extremely Large Telescope (ELT), will lay the foundation for advancing reflected-light spectroscopy. The main challenge in this area of research is the high contrast ratio between the planet and its star, especially for Earth-like planets, making it extremely difficult to isolate the planetary signal from that of its host star. RISTRETTO, a high-resolution integral-field spectrograph designed for ESO’s VLT, aims to address these limitations through a combination of extreme AO, coronagraphy, and high-resolution spectroscopy.

Aims. The goal of this paper is to demonstrate the detectability of the temperate rocky planet Proxima b with RISTRETTO, using realistic end-to-end simulations and a specifically developed data analysis methodology.

Methods. We generated synthetic observations ensuring that the simulated spectra accurately reflect the complexities of real observational conditions. First, we created high-resolution star and planet spectra, selecting realistic observational epochs and conditions and incorporating the predicted performance of the AO and coronagraphic systems. Finally, we implemented noise and spectrograph effects through the Pyechelle spectrograph simulator. We then applied a state-of-the-art methodology to isolate the signal of the planet from the one of its host star and proceeded to fit several planetary models in order of increasing complexity. We also introduced a method to determine the sky orientation of the stellar spin axis, which constrains the orientation of the planetary orbit for aligned systems.

Results. Assuming an Earth-like atmosphere, our results show that RISTRETTO can detect Proxima b in reflected light in about 55 hours of observing time, offering the ability to characterize the planet orbital inclination, true mass, and broadband albedo. In addition, molecular absorption by O₂ and H₂O can be detected in about 85 hours of observations.

Conclusions. These findings highlight the potential of RISTRETTO to significantly advance the field of exoplanetary science, by enabling reflected-light spectroscopy of a sample of nearby exoplanets ranging from gas giants to temperate rocky planets. This work sets the stage for detailed atmospheric characterization of Earth-like planets with next-generation AO-fed high-resolution spectrographs on extremely large telescopes, such as ELT-ANDES and ELT-PCS.