High Accuracy Radial Velocity Planet Searcher
Later studies on K2-18b using the High Accuracy Radial Velocity Planet Searcher (HARPS) and the Calar Alto high-Resolution search for M dwarfs with Exoearths with Near-infrared and optical Echelle Spectrographs (CARMENES) instruments also identified a likely second exoplanet, K2-18c, with an estimated mass of in a tighter, 9-day orbit, but this additional planet has not yet been confirmed, and may instead be due to stellar activity.