V1298 Tau - Extracting planetary signals buried in extreme stellar activity

Current theories predict that very young giant planets have large radii and very low densities before they slowly contract to reach their final size after several hundred million years. Open stellar clusters and young moving groups of stars offer a unique opportunity to study exoplanets at their infant stages because of the well-constrained ages. The extreme stellar activity of young stars makes measuring the masses of their planets a very challenging task. Planet-induced radial velocity signals are typically smaller in amplitude and at lower frequencies than stellar-induced signals. These situations place very demanding requirements for RV observation campaigns and puts us in a worst-case scenario for the standard methods of stellar activity modelling. With an estimated age of 20 million years, V1298 Tau is one of the youngest solar-type stars known to host transiting planets; it harbours a system composed of four planets, two Neptune-sized, one Saturn-sized and one Jupiter-sized. Using more than 200 radial velocity measurements, condensed into a 4-month period, we were able to measure the radial velocity amplitude induced by the planet V1298 Tau b, and to pick up a second signal that was suspected to be linked to V1298 Tau e. The analysis required the combination of radial velocities, ground-based photometry contemporary to the RV campaign, and K2 photometry. The result obtained indicated that V1298 Tau might have contracted much faster than predicted by theoretical models, highlighting the importance of attempting these difficult cases