GJ 3378b’s mass drops to 2.3 Earths, putting a 25-light-year world in reach

Astronomers have revised the mass of GJ 3378b, a potentially habitable planet just 25 light-years away, and the change is not cosmetic. What started as a “mini-Neptune” estimate of 5.26 times Earth’s mass now lands at 2.3 times Earth’s mass, pulling the planet much closer to rocky super-Earth territory. That matters because “rocky” is the gateway category for habitability arguments like liquid water, while “mostly gaseous” weakens them fast.

The other timeline-shifting detail is orbital. The planet’s orbital period is now measured at 21 days, not the originally reported 25. With that shorter orbit, GJ 3378b sits comfortably within the habitable zone where surface temperatures could allow liquid water, assuming it has an atmosphere. As Paul Robertson of the University of California, Irvine, put it, “This one’s exciting,” and he anchored the perspective by noting that 25 light-years may sound far, but it is our next-door neighborhood in Milky Way scale terms.

So what did the astronomers actually do to get here? GJ 3378b was first discovered in 2024 by French astronomers using the Canada-France-Hawaii Telescope in Mauna Kea, but American researchers then re-examined the system and corrected the planet’s characteristics. The work hinges on how the planet reveals itself. GJ 3378b does not transit its star, meaning it never passes in front of the host from our viewpoint to block light. Instead, the planet was detected through the gravitational tug it exerts on its parent star, causing the star to wobble around the shared center of mass. That wobble shows up as a Doppler shift in the star’s light, measurable in the star’s spectrum, the wavelengths the star emits.

In plain English: because the planet does not transit, the team cannot use the most straightforward habitability tool, transit spectroscopy. With a transiting planet, an atmosphere absorbs specific wavelengths, leaving identifiable dark absorption lines. But here, there’s no transit geometry to exploit. As a result, “All we know for sure is the mass and the orbit of GJ 3378b,” and that is both powerful and frustrating. Powerful because the revised numbers sharpen what kind of planet it probably is. Frustrating because the big questions remain unanswered: do we get an atmosphere, clouds, oceans, or anything like life?

The radiation complication adds another layer of uncertainty. The planet orbits a faint red dwarf in the constellation of Camelopardalis, the Giraffe, and red dwarfs can produce hostile bursts of radiation in strong stellar winds. Those conditions can strip an exposed planet of its atmosphere over time. The good news, Robertson’s team highlights, is that GJ 3378b appears to be on the edge of the zone where planets are expected to be seriously battered by radiation. Being on the edge is not a guarantee, but it raises the odds that it may have escaped the worst of the atmospheric erosion.

From a habitability-energy standpoint, the planet is in a sweet spot. Robertson said GJ 3378b receives about 90% of the radiation Earth gets from the sun. That is a striking figure because it directly ties orbital position to potential surface conditions. However, energy alone does not build an atmosphere. Without transit data, astronomers can’t yet confirm whether the planet has enough air to support liquid water or protect anything living. And because of that observational constraint, the next major checkpoint is delayed.

According to the source, the James Webb Space Telescope (JWST) is probing atmospheres around other rocky worlds orbiting red dwarfs, such as those in the TRAPPIST-1 system, by using transit spectroscopy. For GJ 3378b, the same approach is off the table until a future mission. NASA’s Habitable Worlds Observatory, hoped to launch in the 2040s, is expected to be able to answer whether the planet has an atmosphere. In other words: the science question is sharp, but the measurement access is gated by whether the planet ever lines up to transit, which it does not do from our current vantage point.

If you zoom out, this discovery is a reminder of how habitability searches operate at scale. We are in what University of Texas at Austin astronomer Michael Endl called the reconnaissance phase, trying to find planets around the nearest stars because those systems are the easiest targets for biosignature detection. The ultimate goal, Endl said, is biosignatures, and the logic is straightforward: if we can identify worlds that are close enough and likely rocky enough, the next generation of instruments can do deeper characterization. GJ 3378b, with its revised mass and 21-day orbit, is one step closer to that shortlist.

For executives and board-level decision-makers, the second-order takeaway is about timing and optionality. Missions like JWST have shown what’s possible, but non-transiting systems force longer planning horizons and increase the value of future observatories. When a planet’s mass estimate changes by a factor that changes its category from mini-Neptune-like to rocky super-Earth-like, it does not just update a headline. It reshapes which targets justify follow-on investment in observational time, instrumentation priorities, and long-lead mission planning. In the race to understand whether we are alone, the winners are the teams that can turn “closest neighbor” discoveries into actionable measurement plans, even when the answers are years away.