Out of the 5322 exoplanets discovered so far, only 665 have been well characterized by measuring precisely both their radius and mass. The combination of transit photometry and high-precision velocity measurements allows to constrain the mean bulk density of the exoplanets and obtain a first-order estimate of their composition and inner structure. Cool, low mass stars are important targets to characterize exoplanets as M dwarfs represent most of the stellar population in our solar neighborhood, and are known to frequently host multiple small exoplanets. Due to the intrinsic faintness of M dwarfs at visible wavelengths, the near-infrared spectral range is preferred to study these cool stars as they emit most of their light in this region of the spectrum. ExTrA, a new near-infrared photometer and NIRPS, a new near-infrared spectrograph, both located at La Silla Observatory in Chile represent a chance to improve our understanding of planetary systems surrounding M dwarfs by measuring with precision both the planetary radius and the planetary mass.
During my PhD, I collected all potential exoplanets orbiting M dwarfs observed by the TESS mission, an all-sky survey for transiting exoplanets orbiting bright and nearby stars. From these targets, I scheduled the follow-up observations using the ExTrA facility. Started from the raw data, I generated usable light curves. I then analyzed the light curves to iden- tify any planetary transits and modeled them to obtain the planetary parameters. While M dwarfs are good candidates for exoplanet detection, their stellar companions can cause false positives. This is why additional observations with ground-based instruments are required to confirm the companion’s planetary nature, and the combination of observations with TESS in the visible and with ExTrA in the near-infrared turned out to be very useful to validate the planetary candidate and obtain a precise radius measurement. From there, more observations are needed to fully characterize exoplanets. For the different systems I published during my thesis, I gathered and modeled observations from the HARPS and ESPRESSO spectrographs.
The NIRPS spectrograph will soon also be able to provide the mass of the exoplanets detected in transits, especially around M dwarfs. I proposed two programs as part of the NIRPS Guaranteed Time Observations. The first program will use radial velocity measurements to complement our ExTrA photometry in order to populate the mass-radius diagram for mid-to-late M dwarfs. The second program attempts to discover additional planets in known systems by calculating the probability of these undetected planets’ existence and the cost of detected them with current instruments like NIRPS.
The synergies between both near-infrared instruments will allow us to characterize exoplanets orbiting M dwarfs and offer a unique opportunity to study planetary formation and evolution processes around these low-mass, cool, long-lived stars.
