In an earlier campaign to characterize the mass of the transiting temperate super-Earth K2-18b with HARPS , a second , non-transiting planet was posited to exist in the system at \sim 9 days .
Further radial velocity follow-up with the CARMENES spectrograph visible channel revealed a much weaker signal at 9 days which also appeared to vary chromatically and temporally leading to the conclusion that the origin of the 9 day signal was more likely to be related to stellar activity than to being planetary .
Here we conduct a detailed re-analysis of all available RV time-series—including a set of 31 previously unpublished HARPS measurements—to investigate the effects of time-sampling and of simultaneous modelling of planetary + activity signals on the existence and origin of the curious 9 day signal .
We conclude that the 9 day signal is real and was initially seen to be suppressed in the CARMENES data due to a small number of anomalous measurements although the exact cause of these anomalies remains unknown .
Investigation of the signal ’ s evolution in time , with wavelength , and detailed model comparison reveals that the 9 day signal is most likely planetary in nature .
By this analysis we reconcile the conflicting HARPS and CARMENES results and measure precise and self-consistent planet masses of m _ { p,b } = 8.63 \pm 1.35 and m _ { p,c } \sin { i _ { c } } = 5.62 \pm 0.84 Earth masses .
This work—along with the previously published RV papers on the K2-18 planetary system—highlight the importance of understanding one ’ s time-sampling and of simultaneous planet + stochastic activity modelling , particularly when searching for sub-Neptune-sized planets with radial velocities .