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We present a comprehensive analysis of 10 years of HARPS radial velocities ofthe K2V dwarf star HD 13808, which has previously been reported to host twounconfirmed planet candidates. We use the state-of-the-art nested samplingalgorithm PolyChord to compare a wide variety of stellar activity models,including simple models exploiting linear correlations between RVs and stellaractivity indicators, harmonic models for the activity signals, and a moresophisticated Gaussian process regression model. We show that the use ofoverly-simplistic stellar activity models that are not well-motivatedphysically can lead to spurious `detections' of planetary signals that arealmost certainly not real. We also reveal some difficulties inherent inparameter and model inference in cases where multiple planetary signals may bepresent. Our study thus underlines the importance both of exploring a varietyof competing models and of understanding the limitations and precision settingsof one's sampling algorithm. We also show that at least in the case of HD13808, we always arrive at consistent conclusions about two particular signalspresent in the RV, regardless of the stellar activity model we adopt; these twosignals correspond to the previously-reported though unconfirmed planetcandidate signals. Given the robustness and precision with which we cancharacterize these two signals, we deem them secure planet detections. Inparticular, we find two planets orbiting HD 13808 at distances of 0.11, 0.26 AUwith periods of 14.2, 53.8 d, and minimum masses of 11, 10 Earth masses.

monthly notices of the royal astronomical society

The HARPS search for southern extra-solar planets XLV. Two Neptune mass  planets orbiting HD 13808: a study of stellar activity modelling's impact on  planet detection