PhD defense by Salomé Grouffal: Detection and characterisation of long-period exoplanets: toward Earth-like planets

Tristan GUILLOT (Observatoire de la Côte d’Azur) – Reviewer

Susana BARROS (Instituto de Astrofísica e Ciências do Espaço) – Examiner

Lucile MIGNON (Institut de Planétologie et d’Astrophysique de Grenoble) –  Examiner

Amaury TRIAUD (University of Birmingham) – Examiner

Stéphane UDRY (Observatoire de Genève) – Examiner

Vincent LE BRUN (Laboratoire d’Astrophysique de Marseille) – President 

Thirty years after the discovery of the first exoplanet around a main-sequence star, over 5800 exoplanets have been confirmed. These discoveries, made using a variety of techniques, have revealed a remarkable diversity of planetary systems and reshaped our understanding of planetary formation and evolution. Among them, transiting planets, those that pass in front of their host stars, offer unique opportunities to precisely measure both mass and radius, as well as probe their atmosphere via transmission spectroscopy. Increasingly detailed studies of multi-planet systems enable comparative planetology both across systems and within them. Despite these advances, no system resembling the Solar System has yet been detected. This is largely due to observational biases that favour short-period planets in current detection methods, such as radial velocity and transits. Addressing the question of the uniqueness of the Solar System, and the Earth in particular, requires detecting longer-period planets in the habitable zones of Sun-like stars. The PLATO mission (ESA) aims to achieve this goal. In the meantime, ground-based instruments are improving their precision to characterise low-mass, long-period planets. Together, these efforts will expand exoplanet characterisation and enable comparisons with Solar System planets. In this context, this thesis addresses the challenges of detecting and characterising long-period exoplanets through a comprehensive study of a multi-planet system using different techniques.

The first part focuses on the characterisation of the HIP 41378 system, a bright F-type star hosting five transiting planets with periods up to 542 days. This system is a perfect testbed for the targets of the upcoming PLATO mission. The orbital periods of planets observed with only one or two transits are first determined, before combining transit data, radial velocities, and transit timing variations to constrain the orbital parameters and masses of the system’s seven low-mass planets. Particular attention is given to HIP 41378 f, a cold giant on a 542-day orbit. A multi-instrument observational campaign yields a measurement of the projected spin-orbit angle via the Rossiter–McLaughlin effect, revealing a misalignment. In addition, the planet’s unexpectedly low density challenges standard models of internal structure and formation. This case study provides key insights for the design of future observations with PLATO as well as ground-based follow-up strategies for long-period planets.

The second part addresses exoplanet detection with the radial velocity method. A new approach is developed to correct instrumental variations in the SOPHIE spectrograph using environmental monitoring of pressure and temperature, enhancing sensitivity to low-mass planets. The work also includes a contribution to the KOBE survey, which targets habitable-zone planets around K-dwarf stars using the CARMENES spectrograph.