K2-18 HARPS time-series

We also find strong evidence for a second, warm super-Earth K2-18c (m_p,c_sin(i_c_)=7.5+/-1.3M_{sun}_) at approximately nine days with a semi-major axis ~2.4 times smaller than the transiting K2-18b. After re-analyzing the available light curves of K2-18 we conclude that K2-18c is not detected in transit and therefore likely has an orbit that is non-coplanar with the orbit of K2-18b although only a small mutual inclination is required for K2-18c to miss a transiting configuration; |{Delta}i|~1-2{deg}.

A suite of dynamical integrations are performed to numerically confirm the system's dynamical stability. By varying the simulated orbital eccentricities of the two planets, dynamical stability constraints are used as an additional prior on each planet's eccentricity posterior from which we constrain e_b_<0.43 and e_c_<0.47 at the level of 99% confidence. The discovery of the inner planet K2-18c further emphasizes the prevalence of multi-planet systems around M dwarfs.

The characterization of the density of K2-18b reveals that the planet likely has a thick gaseous envelope which, along with its proximity to the solar system, makes the K2-18 planetary system an interesting target for the atmospheric study of an exoplanet receiving Earth-like insolation.