A nearby star just 25 light-years from Earth hosts a rocky world that sits inside the orbital band where liquid water could exist on its surface. The planet, GJ 3378 b, has been reclassified as a super-Earth with a minimum mass of 2.3 Earth masses after a new joint analysis combined data from four separate spectrographs. The revised measurements cut the planet’s estimated mass by more than half compared to its original detection, placing it squarely on the boundary between worlds that can hold atmospheres and those that cannot.
Why a 25-light-year super-Earth demands attention right now
GJ 3378 b orbits a low-mass red dwarf star at roughly 0.09673 AU, close enough that it completes one orbit every 21.45 days, according to a recent preprint posted to arXiv by Robertson and collaborators. That tight orbit around a small, cool star places the planet inside the conservative habitable zone, the narrow ring where temperatures could allow liquid water to persist on a rocky surface. The combination of the star’s faintness and the planet’s short period produces a comparatively large radial-velocity signal, which is why four different instruments were able to measure it independently.
Proximity matters as much as orbital geometry. At 25 light-years, GJ 3378 b is one of the closest known habitable-zone planets. That short distance means ground-based high-resolution spectrographs can realistically search for signs of atmospheric escape, the process by which stellar radiation strips gas from a planet’s surface. Whether GJ 3378 b retains an atmosphere depends heavily on its true mass. The revised estimate of 2.3 Earth masses places it near what researchers call the “cosmic shoreline,” a conceptual dividing line between worlds massive enough to hold onto their gas envelopes and smaller bodies that lose them. If the planet’s mass sits above that threshold, it could retain a substantial atmosphere. If it falls below, the star’s radiation may have already stripped it bare.
The practical window for testing this question is narrow but real. Several next-generation spectrographs are already operational, and the planet’s geometry favors detection of helium or hydrogen outflows during transit. Within roughly 18 months, targeted observations could determine whether GJ 3378 b possesses an atmosphere at all, turning a statistical data point into a direct measurement of habitability conditions. Because the star is relatively dim and cool, the contrast between the star and any escaping gas may be favorable, especially in near-infrared wavelengths where many atmospheric features appear.
Four instruments and a halved mass estimate for GJ 3378 b
The planet was first flagged as a candidate using near-infrared radial velocities from the CFHT/SPIRou instrument. That initial analysis, led by Moutou et al., reported a minimum mass of approximately 5.26 Earth masses and an orbital period near 24.73 days. The detection was explicitly labeled a candidate because no optical radial-velocity confirmation existed at the time, and red dwarf stars like GJ 3378 are notorious for magnetic activity that can masquerade as planetary signals.
The new study by Robertson et al. added data from three additional instruments: HPF on the Hobby-Eberly Telescope, NEID on the WIYN telescope, and published CARMENES measurements. Combining all four datasets produced a sharply different picture. The orbital period tightened to 21.45 plus or minus 0.01 days, and the minimum mass dropped to 2.3 plus or minus 0.4 Earth masses. That mass revision is significant because it moves GJ 3378 b from the upper range of super-Earths, where compositions can be gas-rich and Neptune-like, down into territory more consistent with rocky or water-rich interiors.
NASA’s exoplanet catalog now lists GJ 3378 b as a super-Earth planet with a mass of roughly 2.3 Earths, an orbital distance of about 0.09673 AU, and a discovery year of 2026. The “super-Earth” label, as used in this catalog, refers strictly to size and mass range. It does not imply Earth-like conditions, surface water, or biological activity. Many super-Earths discovered so far have turned out to be scorching, airless, or shrouded in thick hydrogen envelopes with no resemblance to Earth, underscoring how limited mass and orbit alone can be as guides to habitability.
The two competing mass estimates highlight how sensitive radial-velocity measurements are to the number and quality of data points. The earlier SPIRou-only solution used a single instrument operating in the near-infrared, where stellar activity from red dwarfs can mimic or distort planetary signals. Adding optical-wavelength data from HPF, NEID, and CARMENES allowed the team to separate the planet’s signal from stellar noise more effectively, producing a lower and more precise mass. In particular, activity indicators such as chromospheric lines can be monitored in the optical, helping to distinguish genuine orbital motion from starspots or flares rotating in and out of view.
Open questions about GJ 3378 b’s atmosphere and true composition
No transmission or emission spectroscopy data exist for GJ 3378 b yet. Without those measurements, researchers cannot confirm whether the planet holds any atmosphere, let alone one with water vapor or other molecules relevant to habitability. The habitable zone is a useful screening tool, but as NASA’s explainer on the habitable-zone concept emphasizes, many other factors determine whether a planet is actually habitable, including atmospheric composition, magnetic field strength, and the intensity of stellar flares from the host star.
For a world orbiting a red dwarf, stellar activity is a central concern. Young and mid-aged red dwarfs can unleash frequent flares and high-energy radiation that erode atmospheres over time. If GJ 3378 b formed with a thick hydrogen-helium envelope, that same radiation could have stripped away the outer layers, potentially leaving behind a denser, more compact core. Alternatively, if the planet began as a smaller, rocky body with only a thin atmosphere, it might have lost most of its gas early and now exist as a bare, irradiated surface despite its location in the habitable zone.
The current mass estimate alone cannot distinguish among these scenarios. A 2.3-Earth-mass planet could be a dense, iron-rich world with a thin or absent atmosphere, a more Earth-like mix of rock and metal with a secondary atmosphere from volcanic outgassing, or a water-rich planet with deep global oceans capped by a substantial atmosphere. Radius measurements from future transit detections, if the orbital alignment proves favorable, would be critical for narrowing down its bulk composition by providing a density estimate.
Even without a transit, high-resolution spectroscopy during the planet’s orbit could reveal hints of atmospheric escape. Observers can search for narrow absorption features in lines associated with hydrogen or helium that vary in step with the planet’s orbital phase. A detection would confirm the presence of at least some gas and offer clues about how quickly the atmosphere is leaking into space. Conversely, a stringent non-detection would push models toward a compact, possibly airless world.
For now, GJ 3378 b sits at an intriguing crossroads. It is close enough for detailed follow-up, small enough to be plausibly rocky, and bathed in just the right amount of starlight to allow liquid water under the right conditions. Yet every one of those advantages comes with caveats. The same proximity that aids observation also means any atmospheric escape signatures must be teased out of complex stellar behavior. The same modest mass that supports a rocky interpretation also raises the risk that the planet could not hang on to its air in the long term.
Upcoming observations will determine which of these possibilities is closest to reality. If GJ 3378 b turns out to host a stable atmosphere, it will become a prime target for future telescopes seeking to characterize nearby potentially habitable worlds. If it proves to be stripped and barren, it will still offer a valuable data point for understanding how red dwarf environments sculpt the fates of close-in planets. Either outcome will sharpen the picture of what it really means for a world to reside in the habitable zone-and how often those worlds can actually stay habitable over billions of years.
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*This article was researched with the help of AI, with human editors creating the final content.
