A Revised Mass and Period for the Habitable Zone super-Earth GJ 3378b: 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-infrared spectrograph HPF on the Hobby-Eberly Telescope (HET) at McDonald Observatory.

Recently, Moutou et al.~(2024) announced the discovery of an msini=5.26+0.94−0.97M_⊕ planet, GJ 3378b, 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 3378b that differs significantly from the Moutou et al.~solution. The joint RV model reduces the orbital period to P=21.45±0.01d and the minimum mass to msini=2.3±0.4M_⊕. 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.

Angular extents of the optimistic HZ (R. K. Kopparapu et al. 2013) for nearby stars hosting exoplanets. The inner and outer HZ limits are marked with circles colored by stellar effective temperature (in K), and known exoplanets are indicated as diamonds at their projected angular separations. The green shaded region indicates where planets might be accessible to imaging facilities on 30 m-class telescopes, limited by inner working angle (IWA) and 10−8 -level contrast. The HZ of GJ 3378 is partially accessible to future imagers, but imaging GJ 3378b will require them to achieve better IWAs than 2λ/D. — astro-ph.EP

Paul Robertson, Michael Endl, William D. Cochran, Gudmundur Stefánsson, Suvrath Mahadevan, Caleb I. Cañas, Gogod James, Roan Arendtsz, Ryan C. Terrien, Chad F. Bender, Scott A. Diddams, Mark R. Giovinazzi, Arvind F. Gupta, Samuel Halverson, Shubham Kanodia, Daniel M. Krolikowski, Sarah E. Logsdon, Joe P. Ninan, Claire J. Rogers, Arpita Roy, Christian Schwab

Comments: Accepted for publication in AJ
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2605.16499 [astro-ph.EP] (or arXiv:2605.16499v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2605.16499
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From: Paul Robertson
[v1] Fri, 15 May 2026 18:00:14 UTC (8,984 KB)
https://arxiv.org/abs/2605.16499
Astrobiology,