A potentially habitable super-Earth 25 light-years from Earth has moved sharply up the priority list for future life-hunting telescopes, according to a paper published in The Astrophysical Journal on June 30, 2026. The planet, GJ 3378b, orbits a small red dwarf star in the northern constellation Camelopardalis, and a new analysis of its mass and orbit suggests it is far more rocky, and far more Earth-like, than the version first announced in 2024.
The re-measurement dropped the planet’s minimum mass from 5.26 Earth masses to 2.3 Earth masses and trimmed its orbital period from 24.73 days to 21.45 days. That combination pushed the planet from the fuzzy boundary where rocky worlds give way to mini-Neptunes, and parked it squarely inside the conservative habitable zone of its host star. The catch, as the authors are quick to underline, is that the obvious follow-up question, whether GJ 3378b actually has an atmosphere, is one no existing instrument can answer.
What Changed, and Why It Matters
GJ 3378b was first flagged in 2024 as a candidate planet around a faint red dwarf called GJ 3378, an M4-type star about 7.7 parsecs (roughly 25 light-years) from the Sun. The new study, led by Paul Robertson of the University of California, Irvine, kept the original detection but redrew almost every number that defines the world.
Around 5 Earth masses sits the line that planetary scientists use to separate two kinds of planets. Below it, worlds are more likely to be rocky super-Earths with thin atmospheres, or no atmosphere at all. Above it, the same-sized worlds are more likely to be mini-Neptunes, planets wrapped in thick, hazy gas envelopes that would crush any chance of surface life. The original 5.26-Earth-mass estimate put GJ 3378b on the wrong side of that line.
The revised 2.3-Earth-mass figure is comfortably on the rocky side. Combined with the tighter 21.45-day orbit, which places the planet a little closer to its star than first thought but still well inside the habitable zone, the picture now reads like a small, temperate, plausibly rocky world. The UC Irvine announcement framing the work called the result “one of the most potentially Earth-like exoplanets known within the 10-parsec solar neighborhood.” Robertson, in the same release, put it more plainly: “This super-Earth gets about 90 percent of the radiation from its host star as Earth gets from its sun, so it’s right in the sweet spot.”
How Two Telescopes Caught the Signal
The new numbers come from the same technique that produced the original candidate: a Doppler wobble. As GJ 3378b orbits, its gravity tugs the host star back and forth, and that motion shows up as a tiny shift in the wavelengths of the star’s light. Measuring that shift at high precision gives the planet’s mass and orbital period.
Robertson’s team added 137 new infrared observations from the Habitable-zone Planet Finder (HPF), a near-infrared spectrometer on the 10-meter Hobby-Eberly Telescope at McDonald Observatory in Texas, taken between November 2018 and March 2025. They supplemented those with 18 high-cadence measurements from the NEID spectrometer on the 3.5-meter WIYN Telescope at Kitt Peak National Observatory in Arizona, a campaign that ran in the first half of 2025 specifically to confirm or rule out the 2024 candidate. They also folded in previously published velocities from the CARMENES spectrograph at Calar Alto in Spain and the SPIRou instrument on the Canada-France-Hawaii Telescope.
- 137 HPF observations across 2,328 days
- 18 NEID observations across 69 days
- ~1.3 m/s RV amplitude on the revised orbit
- 2,328-day combined baseline for the HPF time series
That combination is what let the team shrink the planet’s Doppler signal from a noisy hint into a clean curve. Michael Endl, an astronomer at the University of Texas at Austin and a co-author, framed the difficulty in a McDonald Observatory write-up of the work: “The name of the game is precision. In order to find those low-mass planets, you’re always looking for tiny signals. If your instruments aren’t precise enough, you won’t find them. You can’t find them.”
The revised solution, published under the title “A Revised Mass and Period for the Habitable Zone super-Earth GJ 3378 b: A Planet Straddling the Cosmic Shoreline,” runs to a circular orbit at 21.45 days and a minimum mass that the team describes as “potentially consistent with a terrestrial composition.” That language is conservative on purpose; the minimum mass is a lower bound, not a fixed value, because the planet’s tilt relative to our line of sight is unknown.
The 2024 Detection That Started It
The signal around GJ 3378 was first reported in 2024 by a French-led team using SPIRou, the same near-infrared spectropolarimeter on the Canada-France-Hawaii Telescope whose archival data helped Robertson’s group cross-check the new result. The original characterization, led by Claire Moutou and collaborators, described a candidate with a 24.73-day period and a minimum mass near 5.26 Earth masses.
At that mass, the most likely interpretation was a mini-Neptune, a planet that had somehow held onto a thick hydrogen-helium envelope rather than losing it. The new measurements move the planet out of that category by more than halving its estimated mass. The detection itself holds up; only the parameters have been redrawn, an increasingly common pattern as more precise instruments get pointed at candidates first spotted by their predecessors. GJ 3378, the parent star, is small enough and faint enough that initial measurements carried wide error bars, and the new paper’s whole point is to shrink them.
Inside the Cosmic Shoreline
The phrase the authors picked for their title, “a planet straddling the cosmic shoreline,” refers to a concept developed in recent years by planetary scientists: the dividing line inside a habitable zone beyond which a planet’s atmosphere is expected to be stripped away by stellar radiation. Planets that sit comfortably inside the shoreline keep their air; planets outside lose it.
The team’s own framing, in a statement carried by the UC Irvine press release, leans on a Mars-style analogy: a planet that once had an Earth-like envelope but lost it to radiation. “If you scale the Earth down to the size of an apple, its atmosphere would be about as thick as the skin of the apple,” Robertson said. “That’s just enough to maintain the kinds of surface pressures where you can have liquid water.”
This one’s exciting. It’s one of our closest cosmic neighbors. Twenty-five light-years sounds like a long way, but the Milky Way is about 100,000 light-years across, so in that respect it’s our next-door neighbor.
The numbers that define the planet, and that define its risk, sit on a narrow shelf:
| Parameter | 2024 estimate (Moutou et al.) | 2026 revision (Robertson et al.) |
|---|---|---|
| Orbital period | 24.73 days | 21.45 days |
| Minimum mass | 5.26 Earth masses | 2.3 Earth masses |
| Likely composition | mini-Neptune (gaseous) | rocky super-Earth |
| Habitable zone status | Inside, but borderline | Inside, “right in the sweet spot” |
Red dwarfs make the calculation harder. They are far more active than the Sun, flaring frequently and throwing off coronal mass ejections that can sandblast nearby worlds. About 70 percent of the stars in the Milky Way are red dwarfs, Endl noted in the McDonald write-up, so the planet population around them sets the default for what “habitable” might mean in the galaxy at large. A super-Earth right on the cosmic shoreline is the kind of world that could be either a museum of life’s ingredients or a bare rock, and there is no way to tell from the current data.
Gogod James, a UC Irvine graduate student in Robertson’s group who worked on the size characterization, said in the same release that the next test is atmospheric: “If a planet in the habitable zone has a proper atmosphere, we can justify further research looking for biosignatures, liquid water, or other signs of life that require both an atmosphere and the right amount of heating from the host star.” The new paper’s measurements meet the first half of that test. The second half is still open.
Why the Answer Has to Wait for the 2040s
The natural next step for any rocky, temperate, nearby exoplanet is to point the James Webb Space Telescope at it and look for atmospheric gases. Webb can do that today, and it has been doing it for planets in the TRAPPIST-1 system. GJ 3378b, however, is not seen to transit, meaning it never passes in front of its star from our line of sight, and transit spectroscopy is the technique Webb relies on. Without a transit, the planet’s atmosphere stays out of Webb’s reach. The same constraint applies to the upcoming Nancy Grace Roman Space Telescope and to ESA’s Ariel mission, both of which lean on the same transit geometry.
The facility designed specifically to break that constraint is NASA’s planned Habitable Worlds Observatory, a large space telescope currently expected to launch in the 2040s. HWO is being designed to directly image Earth-sized planets around nearby stars and to read the chemistry of their atmospheres for biosignatures. Ground-based giants are queued up for the same job on a faster schedule, including the Giant Magellan Telescope and the Extremely Large Telescope, both of which should see first light within the next decade.
Until any of those instruments turn on, GJ 3378b is a paper target, a world whose habitability status has been refined but not resolved. Endl framed the gap, again in the McDonald Observatory write-up, as a reconnaissance problem: “We are still in the reconnaissance phase of our solar neighborhood, trying to find the planets around the nearest stars because those will be the easiest ones to detect a biosignature on. This planet brings us one step closer to knowing all of our neighbors and, ultimately, which might be hospitable for life.” The closest known analog to Earth just got a tighter map; the trip to find out whether it is actually one will take another generation of telescopes.
Frequently Asked Questions
What is a super-Earth?
A super-Earth is a planet larger than Earth but smaller than Neptune, typically rocky, with a mass somewhere between about 1.5 and 10 times that of Earth. GJ 3378b is on the smaller end of that range at 2.3 Earth masses and is described in the Robertson paper as a “potentially rocky” super-Earth.
Why does the planet’s mass matter for habitability?
Planets above roughly 5 Earth masses tend to hold onto thick hydrogen-helium envelopes and become mini-Neptunes, while planets below that line are more likely to be rocky with thin or no atmospheres. Dropping the estimate from 5.26 to 2.3 Earth masses moves GJ 3378b from the gaseous side of that boundary to the rocky side.
How close is 25 light-years, really?
It is close on a galactic scale. The Milky Way is roughly 100,000 light-years across, so 25 light-years is, as Robertson put it, “our next-door neighbor.” In human terms, 25 light-years is about 235 trillion kilometers, far beyond anything current spacecraft can reach.
Could GJ 3378b have already lost its atmosphere?
It is possible. The planet sits on the edge of the cosmic shoreline, the region where red-dwarf radiation can strip a planet’s air over time. Mars, in our own solar system, is the textbook example of a world that likely once had an atmosphere and lost it. Astronomers cannot yet tell which side of that line GJ 3378b is on.
When will we know if GJ 3378b is habitable?
Not soon. The James Webb Space Telescope cannot probe its atmosphere because the planet does not transit its star, and the same constraint rules out most current and near-term observatories. NASA’s planned Habitable Worlds Observatory, currently scheduled for a 2040s launch, is being designed specifically to image and characterize nearby planets like GJ 3378b.
