Confirmation of the radial velocity super-Earth K2-18c with HARPS and CARMENES^★,★★

¹ Department of Astronomy & Astrophysics, University of Toronto, 50 St. George Street, M5S 3H4, Toronto, ON, Canada
e-mail: cloutier@astro.utoronto.ca
² Centre for Planetary Sciences, Department of Physical & Environmental Sciences, University of Toronto Scarborough, 1265 Military Trail, M1C 1A4, Toronto, ON, Canada
³ Institut de recherche sur les exoplanètes, Département de physique, Université de Montréal, 2900 boul. Édouard-Montpetit, Montréal, Quebec, H3T 1J4, Canada
⁴ Departamento de Astronomía, Universidad de Concepción, Casilla 160-C, Concepción, Chile
⁵ CNRS, IPAG, Université Grenoble Alpes, 38000 Grenoble, France
⁶ Observatoire Astronomique de lÚniversité de Genève, 51 chemin des Maillettes, 1290 Versoix, Switzerland
⁷ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
⁸ Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal

Received: 31 July 2018
Accepted: 9 October 2018

Abstract

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 ~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 related to stellar activity than to a planetary presence. Here we conduct a detailed reanalysis of all available RV time-series – including a set of 31 previously unpublished HARPS measurements – to investigate the effects of time-sampling and of simultaneous modelling of planetary plus activity signals on the existence and origin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. Using this analysis, we reconcile the conflicting HARPS and CARMENES results and measure precise and self-consistent planet masses of m_p,b = 8.63 ± 1.35 and m_p,c sini_c = 5.62 ± 0.84 Earth masses. This work, along with the previously published RV papers on the K2-18 planetary system, highlights the importance of understanding the time-sampling and of modelling the simultaneous planet plus stochastic activity, particularly when searching for sub-Neptune-sized planets with radial velocities.