A planet straddling the cosmic shoreline
GJ 3378b was first flagged in 2024 as a candidate super-Earth. New observations published June 30, 2026 in The Astrophysical Journal revised its mass downward from 5.3 to 2.3 times Earth, and confirmed it sits inside its star's habitable zone, the band of distances where liquid water can persist on a rocky surface. At 2.3 Earth masses the planet is now more likely rocky than gaseous, making it one of the most compelling nearby candidates for follow-up atmosphere study. It receives roughly 90 percent of the solar radiation Earth gets from the Sun, placing it well inside the "Goldilocks" sweet spot.
Distance from Earth
25 light-years (7.7 parsecs)
Planet mass
2.3 times Earth (revised down from 5.3)
Orbital period
21.45 days around GJ 3378
Solar flux received
about 90% of Earth's from the Sun
Host star mass
0.3 times the Sun (cool M-dwarf)
Total radial velocity observations
137 (2018 to 2025)
25light-years: among the closest potentially Earth-like worlds known
The "cosmic shoreline" is the theoretical boundary where a planet's gravity is too weak to hold an atmosphere against the stellar radiation stripping it away. GJ 3378b sits right on that edge. Mars crossed to the wrong side billions of years ago and lost its air. Whether GJ 3378b has any atmosphere at all will only be known once a telescope like JWST or a future mission observes it in transit. That observation has not yet been made.
The Habitable-zone Planet Finder (HPF) on the Hobby-Eberly Telescope in Texas provided the bulk of the 137 radial velocity measurements. HPF was built specifically to find rocky planets around cool red dwarf stars, where Earth-like worlds orbit in just days or weeks rather than years.
Red dwarf stars like GJ 3378 flare frequently and fire coronal mass ejections toward close-in planets. This high-energy radiation can strip atmospheres over billions of years. The question for any habitable-zone planet around an M-dwarf is not just "is it the right distance?" but "has it held on to its air?"
"The ultimate goal is biosignatures. We really want to know: are we alone in the universe? This planet brings us one step closer to knowing all of our neighbors.
"Dr. Paul Robertson