A small, rocky world orbiting a red dwarf star just 25 light-years from Earth now looks more like a plausible candidate for liquid water than astronomers previously thought. New radial-velocity measurements have cut the estimated minimum mass of GJ 3378 b nearly in half, from roughly 5.26 Earth masses to about 2.3 Earth masses, while also tightening its orbital period from about 24.73 days to roughly 21 days. The revision places the planet squarely in its star’s habitable zone, where it receives about 90 percent of the sunlight Earth gets, and raises a pointed question: how many other nearby exoplanets have been mischaracterized because early data came from a single instrument?
Why the revised mass of GJ 3378 b changes the search for nearby habitable worlds
The difference between a 5.26-Earth-mass planet and a 2.3-Earth-mass planet is not a rounding error. At the higher mass, GJ 3378 b sat in the range astronomers associate with sub-Neptunes, worlds likely wrapped in thick hydrogen-helium envelopes hostile to surface liquid water. At the lower mass, the planet falls into the super-Earth category, where rocky compositions and thinner atmospheres become far more plausible. That single measurement shift moves GJ 3378 b from a curiosity to a priority target for future atmospheric follow-up with instruments like the James Webb Space Telescope.
The original signal came from SPIRou, a near-infrared spectrograph that monitors radial-velocity wobbles in cool, dim stars. SPIRou detected the planet candidate and reported a minimum mass of about 5.26 Earth masses with an orbital period near 24.73 days. Those numbers were reasonable given the data in hand, but they rested on observations from a single instrument observing a fully convective M dwarf, a type of star whose magnetic activity can mimic or distort planetary signals.
The updated analysis added data from two additional spectrographs: HPF, mounted on the Hobby-Eberly Telescope, and NEID, mounted on the WIYN telescope at Kitt Peak. By combining optical and near-infrared radial-velocity datasets, the team could separate genuine planetary signals from stellar noise more effectively. The result was a revised minimum mass near 2.3 Earth masses and a shorter orbital period of about 21 days, as described in the new radial-velocity study. The planet’s irradiation level, about 90 percent of what Earth receives, stayed consistent with habitability.
How cross-instrument data reshaped the planet’s profile
The pattern here matters beyond a single planet. Fully convective M dwarfs are the most common type of star in the galaxy, and they are prime targets for radial-velocity planet searches because their low masses make planetary wobbles easier to detect. But their surfaces are magnetically active, producing starspots and flares that inject false signals into radial-velocity time series. When astronomers rely on data from only one wavelength band, those stellar signals can inflate or distort the apparent mass of a planet.
Joint modeling across optical and near-infrared wavelengths helps break that degeneracy. Stellar activity signals tend to behave differently at different wavelengths, while a genuine planetary tug shows the same signature regardless of color. In the case of GJ 3378 b, the multi-instrument approach dropped the minimum mass estimate by more than 55 percent. That scale of revision suggests a broader pattern: other planets initially characterized by single-instrument campaigns around M dwarfs may also carry inflated mass estimates, and some fraction of worlds currently classified as sub-Neptunes could turn out to be terrestrial.
The NASA Exoplanet Catalog entry for GJ 3378 b confirms the planet’s designation and host star, though the catalog’s adopted parameters will reflect the most recent peer-reviewed values once the new preprint completes review. The planet’s proximity, just 25 light-years away, makes it one of the closer habitable-zone candidates known, and its host star’s brightness in the infrared makes atmospheric characterization technically feasible with current or near-future instruments.
The GJ 3378 system also illustrates how the culture of rapid, open dissemination of results shapes exoplanet science. Teams often post their findings as preprints so other researchers can scrutinize methods, reanalyze data, or plan follow-up observations even before journal publication. Platforms like arXiv have become central to this workflow, especially for fast-moving fields where new data arrive every observing season. For a nearby, potentially habitable world, that speed matters: telescope time is scarce, and early access to candidate parameters can determine whether a target makes it into upcoming observing campaigns.
Sustaining that open infrastructure requires ongoing support. The servers, curation, and moderation behind preprint archives are not invisible or cost-free, and the volume of submissions from astronomy and planetary science continues to grow. Researchers and institutions that rely on rapid access to preprints are increasingly encouraged to contribute financially, whether through institutional memberships or individual gifts, to keep resources like arXiv robust and independent. Readers who value that open pipeline from telescope to public preprint can learn how to help through arXiv’s own donation page, which outlines options for sustaining the service.
Open questions about GJ 3378 b and what to watch next
Several pieces of the puzzle are still missing. The revised mass is a minimum value, derived from the radial-velocity method, which measures only the component of a planet’s gravitational tug along the line of sight. The true mass depends on the orbital inclination, which radial velocities alone cannot determine. If the orbit is nearly face-on, the actual mass could be significantly higher, potentially pushing the planet back toward sub-Neptune territory. Transit observations, which would pin down the inclination and also reveal the planet’s radius, have not yet been reported.
Without a measured radius, researchers must infer composition statistically. At 2.3 Earth masses and an irradiation level similar to Earth’s, a rocky interior with a relatively thin atmosphere is plausible, but not guaranteed. A modest hydrogen-helium envelope could still produce a mini-Neptune, while a dense, iron-rich world might end up smaller and more compact than Earth. Future transit searches targeting GJ 3378 could clarify which of these scenarios applies, and also test whether the system hosts additional, lower-mass planets that current radial-velocity data cannot yet cleanly resolve.
The preprint also positions GJ 3378 b along what researchers call the “cosmic shoreline,” a boundary in mass–irradiation space that separates worlds expected to retain atmospheres from those stripped bare by stellar radiation. At 2.3 Earth masses and 90 percent of Earth’s irradiation, the planet sits close to that dividing line. On one side lie airless, baked rocks; on the other, planets massive and cool enough to hold onto substantial atmospheres over billions of years. Exactly where GJ 3378 b falls will depend on its atmospheric composition, magnetic field, and the detailed history of its star’s high-energy output.
That uncertainty is part of what makes the planet so scientifically valuable. If future observations show that GJ 3378 b has retained a temperate, possibly water-bearing atmosphere, it will provide a key data point for understanding how common truly habitable environments are around red dwarfs. If, instead, the planet turns out to be stripped or swaddled in a thick, uninhabitable envelope, it will still sharpen the empirical boundaries of the cosmic shoreline and refine models of atmospheric escape.
Either way, the story of GJ 3378 b already highlights a methodological lesson: robust characterization of small exoplanets around active stars demands multiple instruments, multiple wavelengths, and transparent sharing of data and analysis. As more nearby systems receive this level of scrutiny, some will shift categories just as GJ 3378 b did, moving from gas-rich to rocky, or from seemingly inhospitable to potentially life-friendly. In that evolving landscape, this small world around a faint red star has become an outsized test case for how we find-and correctly weigh-the planets closest to home.
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*This article was researched with the help of AI, with human editors creating the final content.
