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

TL;DR

This study refines the orbital and mass parameters of the habitable zone super-Earth GJ 3378b using combined RV data, revealing it is likely terrestrial and near the cosmic shoreline where atmospheric loss occurs.

Contribution

It provides a revised orbital period and mass for GJ 3378b based on joint RV analysis, differing from previous estimates and highlighting its potential terrestrial nature.

Findings

Revised orbital period of 21.45 days.

Reduced minimum mass to 2.3 Earth masses.

Planet remains within the habitable zone and near the cosmic shoreline.

Abstract

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-infrared spectrograph HPF on the Hobby-Eberly Telescope (HET) at McDonald Observatory. Recently, Moutou et al.~(2024) announced the discovery of an $m\sin i = 5.26^{+0.94}_{-0.97} M_\oplus$ planet, GJ 3378b, with an orbital period of $24.73 \pm 0.06$ 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 3378b that differs significantly from the Moutou et al.~solution. The joint RV model reduces the orbital period to $P = 21.45 \pm 0.01$d and the minimum mass to $m \sin i = 2.3 \pm 0.4 M_\oplus$. 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.