Direct characterization of and search for gas giant exoplanets with near-infrared long-baseline interferometry

Understanding the formation of gas giant planets is a major goal of contemporary exoplanet science since these planets are believed to dominate the architecture and evolution of planetary systems. A detailed characterization of an extrasolar planet requires the combination of multiple techniques exposing its physical and orbital parameters, its atmospheric composition, and its dynamical interaction with the circumstellar environment. Near-infrared long-baseline interferometry with VLTI/GRAVITY has proven its ability to directly image already known gas giant exoplanets and contribute micro-arcsecond astrometry and medium-resolution K-band spectra toward the comprehensive characterization of these objects. Furthermore, GRAVITY demonstrated the first direct detection of exoplanets whose existence was predicted from radial velocity and Gaia measurements, heralding a new era for direct imaging of exoplanets where planet searches are no longer blind, but target systems with radial velocity or astrometric trends to maximize the survey efficiency. We present our study of the HD 206893 debris disk system, a clone of the archetypical beta Pictoris system, hosting the reddest known sub-stellar companion HD 206893 B. Using GRAVITY spectra and astrometry, we derive a super-solar C/O ratio and a mass close to the boundary between exoplanets and brown dwarfs. By combining our data with spectra from the literature, we find that its unusually red color is likely caused by a layer of sub-micron sized dust particles in its upper atmosphere, and future JWST observations (PI Kammerer) are required to constrain the size distribution of these particles. Finally, we illustrate our search for an additional planet in the HD 206893 system whose existence has been predicted from indirect techniques and highlight how GRAVITY observations of multi-planet systems can yield unprecedented constraints on planet masses, ages, and therefore formation scenarios.