Long-period TTV planets observed with ASTEP: analysis and modelling with Cesam2k20 in preparation for PLATO

Long-period transiting planets provide important constraints on planet formation and evolution, but remain difficult to characterize because their transits are rare and require extended photometric follow-up. Thanks to its unique Antarctic location, ASTEP (Antarctic Search for Transiting ExoPlanets) is particularly well suited to the monitoring of such systems, enabling continuous observations over long time baselines and contributing to the characterization of planets on orbits of several tens to hundreds of days. For long-period planets in multi-planet systems, mutual gravitational perturbations can generate detectable transit timing variations, providing a valuable route to measuring planetary masses and characterizing the architecture of systems that are otherwise difficult to constrain.

Several of these long-period TTV planets appear to have unusually low densities and inflated radii. The origin of these properties remains unclear, especially in systems where stellar irradiation alone cannot easily account for the observed inflation. These planets therefore represent important benchmarks for testing our understanding of formation pathways, interior composition, atmospheric evolution, and cooling histories. In this contribution, I will present the analysis of long-period TTV systems observed with ASTEP, with a focus on their photometric follow-up and physical characterization. I will also discuss preliminary interior and evolution models computed with the stellar evolution code Cesam2k20, recently extended to model exoplanets through updated equations of state, opacity tables, and planetary atmosphere prescriptions. Finally, I will discuss how these results fit into the perspective of PLATO, which is expected to uncover new long-period transiting planets for which dedicated ground-based follow-up will be essential.