A Revised Mass and Period for the Habitable Zone super-Earth GJ 3378 b: A Planet Straddling the Cosmic Shoreline

The nearby ( d = 7.7 pc) M4V star GJ 3378 is a target of our radial velocity (RV) exoplanet survey of fully convective stars in the solar neighborhood with the near-IR Habitable-zone Planet Finder (HPF) spectrometer on the Hobby–Eberly Telescope (HET) at McDonald Observatory. Recently, C. Moutou et al. announced the discovery of an m sin i = 5.2 6 − 0.97 + 0.94 M ⊕ planet, GJ 3378 b, with an orbital period of 24.73 ± 0.06 days, based on SPIRou RV data. Here, we present our HPF RVs for GJ 3378, as well as additional Doppler spectroscopy from the extreme-precision NEID spectrometer on the WIYN telescope at Kitt Peak National Observatory. We have analyzed the HPF + NEID RVs jointly with the published RVs from the CARMENES and SPIRou spectrometers. We present an orbital model for GJ 3378 b that differs significantly from the C. Moutou et al. solution. The joint RV model reduces the orbital period to P = 21.45 ± 0.01 days and the minimum mass to m sin i = 2.3 ± 0.4 M ⊕ . The shortened orbital distance remains within the conservative circumstellar liquid-water habitable zone (HZ), while the reduced mass increases the likelihood that the planet has a terrestrial composition. The revised planet properties place it near the “cosmic shoreline,” where planets in the HZs of M dwarfs may lose their atmospheres due to radiative stripping.