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Astronomy & Astrophysics, two new studies present an in-depth analysis of the planet Beta Pictoris b's atmosphere using interferometry with the Very Large Telescope Interferometer (VLTI). These observations have for the first time made it possible to obtain high-resolution infrared spectra of an exoplanet from the ground. The analysis of the carbon-to-oxygen ratio in its atmosphere provides clues about its formation process.
Exoplanet spectroscopy allows for the determination of the chemical composition and thermal structure of their atmospheres. This field is experiencing remarkable growth with the arrival of the James Webb Space Telescope (JWST). But ground-based observations retain a major advantage: thanks to interferometry, it is possible to combine light from several telescopes to achieve the equivalent of a giant telescope capable of detecting and characterizing planets close to their star that are inaccessible to the JWST.
Observing from the ground: the power of interferometry
Among these targets, Beta Pictoris b stands out: this gas giant is part of a very young system (180 times younger than our Solar System). It orbits within a set of ice and dust belts, analogous to the Kuiper Belt. Studying this planet helps us better understand the formation conditions and dynamical evolution of the Solar System.
However, its proximity to its star (nine times the Earth-Sun distance) makes its characterization difficult for the JWST. Two new studies demonstrate that interferometry overcomes this limitation. They present spectra of Beta Pictoris b in the infrared with unprecedented precision, highlighting the molecular content of this exoplanet's atmosphere and measuring its carbon-to-oxygen abundance ratio. This ratio allows us to trace the formation history of this planet.
Illustration generated via PlanetMaker/Gimp/Molview.
Planet color from https://iopscience.iop.org/article/10.3847/2515-5172/adec7d).
Spectrum from Houllé et al. 2025 (https://arxiv.org/pdf/2508.18366). Two VLT instruments to probe the planet's atmosphere
This work is based on the second-generation instruments GRAVITY and MATISSE, installed on the Very Large Telescope (VLT, Chile). These two interferometric instruments, developed by international consortia including several CNRS laboratories, have made it possible to achieve high spectral resolution and an extension into the mid-infrared.
This approach enables a fine analysis of molecular absorption lines and bands as well as detailed modeling of atmospheric molecular abundances and their comparison to those predicted by planetary formation models.
Unprecedented prospects for the study of exoplanets
The results confirm that the carbon-to-oxygen ratio of Beta Pictoris b strongly constrains its formation scenario among different hypotheses.
We can distinguish between gravitational collapse, where a planet forms through fragmentation and collapse of its primordial disk, and core accretion, where a planet forms by aggregating increasingly larger pieces of rock and ice, then gas from its primordial disk.
These observations constitute a first demonstration of MATISSE's capabilities for observing exoplanets. Soon, the fourth catalog from the European Gaia mission, expected by the end of 2026, should reveal a range of young giant exoplanets that can be characterized by interferometry with the VLT.