New interferometric measurements to understand the formation of β Pictoris b

Published in Astronomy & Astrophysics, two new studies present an in-depth analysis of the atmosphere of the planet β Pictoris b using interferometry with the Very Large Telescope Interferometer (VLTI). These observations extend, for the first time, both the spectral resolution and wavelength coverage of exoplanet spectra obtained through ground-based interferometry. The analysis of its atmospheric carbon-to-oxygen ratio provides key clues about its formation process. The results highlight the powerful potential of the GRAVITY and MATISSE instruments, developed by international consortia involving several scientists from the Institut de planétologie et d’astrophysique de Grenoble (IPAG-OSUG, CNRS/UGA), for characterizing planets inaccessible to space observatories.

Exoplanet spectroscopy allows scientists to determine the chemical composition and thermal structure of planetary atmospheres. This field has seen rapid progress with the advent of the James Webb Space Telescope (JWST). Yet, ground-based observations still hold a major advantage: through interferometry, it is possible to combine the light of multiple telescopes, achieving the resolving power of a giant virtual telescope, capable of detecting and characterizing planets orbiting close to their host stars and beyond the reach of JWST.

Observing from the ground: the power of interferometry

Among such targets, β Pictoris b stands out. This gas giant belongs to a very young planetary system (180 times younger than our Solar System). It orbits within belts of ice and dust similar to the Kuiper Belt. Studying this planet offers new insight into the conditions under which planetary systems form and evolve.

However, its proximity to its star (nine times the Earth–Sun distance) makes it difficult to characterize with the JWST. The two new studies show that interferometry overcomes this challenge. They present infrared spectra of β Pictoris b with unprecedented precision, revealing its molecular atmospheric content and measuring its carbon-to-oxygen ratio, an indicator of the planet’s formation history.

Two VLTI instruments probing the planet’s atmosphere

This work relies on the second-generation VLTI instruments GRAVITY and MATISSE, installed at the Very Large Telescope in Chile. Developed by international consortia including several CNRS laboratories [1], these instruments provide both high spectral resolution and access to the mid-infrared. This approach enables precise analysis of molecular absorption features and detailed modeling of atmospheric abundances, allowing direct comparison with predictions from planetary formation models.

New prospects for exoplanet research

The results confirm that the carbon-to-oxygen ratio of β Pictoris b strongly constrains its formation scenario among competing hypotheses.

Two formation pathways can be distinguished: gravitational instability, where a planet forms through fragmentation and collapse of the primordial disk; and core accretion, where rock and ice build up into progressively larger bodies, eventually accumulating surrounding gas.

These observations mark the first demonstration of MATISSE’s ability to observe exoplanets. In the coming years, the fourth data release of the European Gaia mission—expected in late 2026—should reveal a diverse population of young giant exoplanets, many of which may be prime targets for VLTI interferometry.

Reference

Houllé, M., Millour, F., Berio, P., et al. (2025). The mid-infrared spectrum of β Pictoris b. First VLTI/MATISSE interferometric observations of an exoplanet. Astronomy & Astrophysics. DOI : 10.1051/0004-6361/202453323

Ravet, M., Bonnefoy, M., Chauvin, G., et al. (2025). Multi-modal atmospheric characterization of β Pictoris b: Adding high-resolution continuum spectra from GRAVITY. Astronomy & Astrophysics. DOI : 10.1051/0004-6361/202553885

This actuality was initially published by CNRS INSU.

Updated on 10 December 2025