Description du sujet de thèse

The thesis focuses on the development of innovative techniques for the search for exoplanets in binary systems close to the Sun, using very high precision astrometry from Gaia DR4 and GRAVITY+.
The thesis, which is observational in nature, falls within the general field of exoplanets. The proposed work lies at the interface between astrometry (space and ground-based) and optical interferometry.
The availability of Gaia DR4 astrometry at the end of 2026 opens up exceptional prospects for the detection of exoplanets in nearby stellar systems. At the same time, the GRAVITY+ instrument is in its final testing phase at Paranal, and a request for observing time has been submitted in early 2025 to prepare the thesis work.
- Space astrometry with Gaia DR4 : The publication at the end of 2026 of the 4th catalog of the European Gaia mission (Gaia DR4) will open up extraordinary possibilities for the study of a very large number of binary stars. For the first time, the instrument's individual astrometric measurements (on the order of a hundred per star) will be available. This will make it possible to carry out a very precise analysis of the anomalies present in the apparent motion of the stars. The orbital motion of two stars can be predicted very precisely using Kepler's laws. If an exoplanet orbits one of the two stars, it will induce an additional periodic displacement of the host star, which it will be possible to detect using Gaia data as an anomaly in relation to the two-body orbital model. This exoplanet search method is also applicable to single stars, for which the presence of a planet translates into an “oscillation” of its motion on the sky. In binary systems whose two components are measured separately by Gaia, such as GJ65AB, GJ725AB, 61 Cyg AB or GJ338AB (Kervella et al. 2022, A&A, 657, A7), it is possible to go even further in terms of sensitivity. By exploiting differential astrometry between the two stars, we can eliminate a significant proportion of systematic errors. It is therefore possible to achieve very high sensitivity in the detection of low-mass exoplanets.
- Ground-based interferometric astrometry with GRAVITY+ : From the ground up, long-baseline optical interferometric astrometry with the VLTI/GRAVITY instrument has produced a number of outstanding results in the field of exoplanets. In particular, observations of the nearby binary star GJ65AB have led to the discovery of a 36±7 Earth-mass planet orbiting one of its two components (GRAVITY Collab. 2024, A&A, 685, L9). This is the first detection of an exoplanet by astrometry from the ground. However, due to the differential nature of
the GRAVITY measurements, it remains unclear which of the two components of GJ65 is the planet's host. The Gaia DR4 measurements will soon enable us to identify the host component and specify the planet's mass.
- Thesis objective : This project aims to take advantage of the synergy between Gaia DR4 and the VLTI/GRAVITY+ interferometric instrument to search for exoplanets in a sample of binary stellar systems in the solar neighborhood. The first year of the thesis will be devoted to observational work with GRAVITY+ and to preparing the analysis of Gaia data. As soon as DR4 is published at the end of 2026, the Gaia+GRAVITY combination will be carried out for systems that allow it. In particular, GJ65 will be studied initially, as we have high-quality GRAVITY data. This will enable the student to carry out an astrometric search for low-mass planetary companions at high sensitivity (sub-Neptune for nearby stars) using an original and innovative method.

Contexte de travail

The student will be enrolled at Paris Observatory (Université Paris Sciences et Lettres) and recruited by the LIRA

This project is also part of the CNRS IRL FCLA international laboratory in Santiago, Chile, where Pierre Kervella is assigned between March 2025 and February 2028, and where the student will carry out a significant part of the thesis research.
The thesis must start between October 2025 and January 2026. The selected student must hold a Master's degree in astronomy or physics, and must register with the AAIF doctoral school through the Université Paris Sciences et Lettres (PSL).
We are looking for a student motivated by data analysis and interpretation. Experience in programming (Python language) is desirable. Knowledge of astrometry or optical interferometry would be appreciated, but is not essential.