Physical modelling of stellar activity effects to discover and measure exoearths

ongoing

Exoplanet research has experienced enormous growth and is now pushing instrumental limits to find temperate terrestrial planets and precisely characterise their atmospheres. Extreme-precision radial velocity (RV) instruments (e.g. ESPRESSO), offering 10 cm/s stability, and space telescopes (e.g. JWST, Ariel), attaining relative flux uncertainties of 10 ppm are becoming a reality. But there is a shadow of concern hovering over such fantastic performances. The intrinsic variability of the stellar hosts can overwhelm the instrument error to become the dominant uncertainty. Recent results with ESPRESSO and JWST data provide vivid illustrations of the daunting effects of such limiting factor. An ambitious, ground-breaking, and comprehensive effort to model and correct out stellar activity effects shall therefore be undertaken. Our pioneering results show that the unique combination of a physical model and contemporaneous multi-technique monitoring can disentangle the effects of stellar activity from RV time-series and transit spectroscopy. The SPOTLESS project will implement this methodology by building a realistic stellar activity simulator and develop correction strategies using, e.g., machine learning algorithms and direct inversion. This is more reliable than purely data-driven methods, which often suffer from overfitting due to the lack of a physical context. Four main objectives will be pursued: 1) A physical model of the active solar surface to explain Sun-as-a-star observables, 2) Generalise to other spectral types & parameters, 3) Physically-motivated RV activity corrections at 10 cm/s, and 4) Physically-motivated transit activity corrections at 10 ppm. From available and future observations, SPOTLESS will deliver new challenging exoplanet RV discoveries and unbiased transmission spectra, taking exoplanet research to the next level by enabling the detection and characterisation of exoearths, and the eventual search for signs of biological activity.