RISTRETTO spectrograph cleared for Proxima b atmospheric hunt

by Robert Schreiber
Geneva, Switzerland (SPX) Dec 09, 2025

The RISTRETTO project at the University of Geneva has reached a key stage, with several core elements of its high-precision spectrograph now prototyped and tested for observations of the nearby exoplanet Proxima b. The instrument is designed to study the faint reflected light of this Earth-sized planet, which orbits Proxima Centauri at a temperature compatible with liquid water but remains outshone by its host star by a factor of about 10 million. Two new studies in Astronomy and Astrophysics present both the instrumental developments and simulations that examine how RISTRETTO can detect Proxima b and search for signatures of oxygen or water in its atmosphere.

Large distances between stars make it difficult for astronomers to resolve exoplanets as distinct sources from their host stars. Only a small number of planetary systems can currently be separated by major observatories such as the European Southern Observatory's Very Large Telescope. The extreme brightness contrast between a star like Proxima Centauri and its planet further complicates the task, with the challenge described as comparable to identifying the glow of a firefly orbiting a lighthouse in New York as seen from the summit of the Matterhorn.

Researchers in the Department of Astronomy at the University of Geneva are addressing this problem through the RISTRETTO high-precision spectrograph, which will operate on the VLT using new instrumental techniques. To suppress the star's light, the team has developed a coronagraphic Integral Field Unit that covers the entrance to the instrument with an array of hexagonal lenses. Optical fibers route light from these microlenses into the spectrograph, where careful control of the light's propagation allows a partial extinction of the stellar signal and improves access to the planet's contribution.

"We have developed a fully functional prototype that we have just tested at the Geneva Observatory," comments Nicolas Blind, research engineer in the Department of Astronomy at the UNIGE Faculty of Science and lead author of the paper on this cutting-edge instrumental development. "The performance meets our expectations and confirms our initial decision to develop this technology for RISTRETTO."

This coronagraphic advance builds on earlier work on another core subsystem, the extreme adaptive optics. The XAO system corrects for distortions introduced by Earth's atmosphere, which would otherwise blur images and reduce the contrast needed to separate the planet from the star. Several XAO components were validated during tests at the Haute-Provence Observatory in early October, demonstrating that the system can deliver the image quality required for RISTRETTO.

Using these technical results, the team then constructed detailed computer models of RISTRETTO's performance on the VLT to assess its ability to detect Proxima b, which is similar to Earth in size and temperature. "We incorporated synthetic observations of the star Proxima and its planet into our RISTRETTO simulation," says Maddalena Bugatti, a doctoral student in the Department of Astronomy at the UNIGE Faculty of Science and lead author of the research article on these simulations. "With only 55 hours of observations with the VLT, we can detect the planet, and in 85 hours we can tell if there is oxygen or water in the planet's atmosphere."

The project recently received financial backing from Swiss watchmaker Swatch, funding that will support completion of the high-precision instrument ahead of its planned installation on the VLT in 2030. RISTRETTO is expected to act as a pathfinder for second-generation instruments on the Extremely Large Telescope, the 39-meter facility now under construction in Chile, by demonstrating techniques such as high-dispersion coronagraphy combined with extreme adaptive optics for the study of nearby rocky exoplanets.

"These initial successes and results are very encouraging," says Christophe Lovis, associate professor in the Department of Astronomy at the UNIGE Faculty of Science, head of the RISTRETTO project and co-author of the various studies. "The next steps include testing the spectrograph in a clean room and then in real observation conditions with the 152 cm telescope at the Haute-Provence Observatory, as well as the complete design of the extreme adaptive optics."

Research Report:Simulating RISTRETTO: Proxima b detectability in reflected light