Confirmation of the Long-Period Planet Orbiting Gliese 411 and the Detection of a New Planet Candidate

Abstract

We perform a detailed characterization of the planetary system orbiting the bright, nearby M dwarf Gliese 411 using radial velocities gathered by APF, HIRES, SOPHIE, and CARMENES. We confirm the presence of a signal with a period near 2900 days that has been disputed as either a planet or a long-period stellar magnetic cycle. An analysis of activity metrics including the $\mathrm{H_\alpha}$ and $\mathrm{log'R_{HK}}$ indices supports the interpretation that the signal corresponds to a Neptune-like planet, GJ 411 c. An additional signal near 215 days was previously dismissed as an instrumental systematic, but we find that a planetary origin cannot be ruled out. With a semi-major axis of $0.5142\pm0.0042$ au, this candidate's orbit falls between those of its companions and is located beyond the outer edge of the system's habitable zone (determined using the moist greenhouse and maximum greenhouse limits in Kopparapu et al., 2013). It has a minimum mass of $4.1\pm0.6$ $M_\oplus$, giving a radial-velocity amplitude of $0.81\pm0.18$ $\mathrm{m\,s^{-1}}$. If confirmed, this would be one of the lowest-amplitude planet detections from any of these four instruments. Our analysis of the joint radial-velocity data set also provides tighter constraints on the orbital parameters for the previously known planets. Photometric data from TESS do not show any signs of a transit event. However, the outermost planet and candidate are prime targets for future direct imaging missions, and GJ 411 c may be detectable via astrometry.