CALM thy stars: a new spectral-based stellar variability modeling method for radial velocity searches

Exoplanet detection with precise radial velocity (RV) observations is currently limited by spurious RV signals introduced by stellar activity (i.e. faculae, starspots). Here we show that our modeling method, CCF Activity Linear Model (CALM), can significantly improve RV measurements by separating the activity signals from real center-of-mass RV shifts. We have tested our method on observations from both EXPRES and HARPS-N. These techniques can successfully predict and remove stellar activity and reduce the RMS by about 40% for the most active stars (Zhao, L.L. et al. 2022). We also successfully applied our methods to K2-167, a sub-Neptune sized planet which was first found using the transit method in 2015 and falls at the upper edge of the radius valley. Using CALM, we measured K2-167 b’s mass (de Beurs et al. submitted) and pave the way towards characterizing more of these types of systems that are critical to probing the formation physics (e.g. photoevaporation, core-powered mass loss) of the radius valley. These promising results inspire us to apply CALM to stars spanning the HR diagram, measure the masses of rocky exoplanets, and eventually help characterize habitable-zone Earth-mass exoplanets.