Cut the mass of a planet in half and it changes everything about how you think of it. That is roughly what happened to GJ 3378b, a world circling a small red star 25 light-years from here. Astronomers first spotted it in 2024 and pegged it at more than five times the mass of Earth: heavy, probably wrapped in gas, unremarkable. A fresh round of measurements has now dragged that number down to about 2.3 Earth masses. Small enough to be rock. Close enough to matter.
The revision, announced by a team led by University of California, Irvine astronomer Paul Robertson, was reported on June 30 in The Astrophysical Journal. It lands GJ 3378b squarely inside its star’s habitable zone — the band of orbital distance where a planet catches enough warmth for liquid water to pool on its surface, but not so much that it boils away. The planet receives about 90 percent of the starlight Earth gets from the Sun. In the arithmetic of habitability, that is almost eerily well placed.
“This one’s exciting,” Robertson said in a statement. “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 star it orbits, GJ 3378, is a red dwarf in the northern constellation Camelopardalis, the Giraffe. That detail is not incidental. Red dwarfs are the workhorses of the galaxy, and any honest search for life has to reckon with them.
“About 70% of stars in our galaxy are red dwarfs, so they represent the standard. It’s really important that we understand the planet population around these stars.”
Michael Endl, astronomer, University of Texas at Austin
Pinning down a planet twice
Getting the mass right is not a footnote. It is the whole story. A super-Earth of five-plus Earth masses tends to hold onto a thick hydrogen-helium envelope, more mini-Neptune than second Earth. At 2.3 masses, the odds tilt hard toward a rocky, terrestrial world, the kind that can, in principle, keep a thin atmosphere and a solid surface underfoot.
The team reached the new figure with two of the most precise planet-hunting instruments in operation: the Habitable-zone Planet Finder on the Hobby-Eberly Telescope at McDonald Observatory in Texas, and the NEID Spectrometer on the WIYN Telescope at Kitt Peak in Arizona. Both measure the faint gravitational tug a planet exerts on its star, a wobble read in shifts of starlight. GJ 3378b completes an orbit every 21.45 days. The refined data make it, in the researchers’ words, “among the most potentially Earth-like exoplanets known within the 10-parsec solar neighborhood.”
That word, potentially, is carrying weight.
The catch nobody can measure yet
GJ 3378b sits right on what astronomers have taken to calling the cosmic shoreline. Step inside it and a red dwarf’s radiation, ferocious in its violent youth, can strip a planet’s atmosphere clean off, taking any surface water with it. Mars is the cautionary tale close to home: a world that may once have worn an atmosphere like ours and lost it. Whether GJ 3378b weathered the first billion years of its star’s temper with its air intact is the question that decides everything, and it is exactly the question that current telescopes cannot answer for a world this small around a star this dim.
Robertson framed the guiding logic simply. “Our mantra is ‘follow the water,’” he said. “It’s the one thing every known living thing on Earth needs, so that’s the first thing we look for when trying to find environments that could support life.”
The instrument built to finish the job does not exist yet. NASA’s Habitable Worlds Observatory, a flagship telescope designed specifically to hunt for atmospheres and biosignatures on nearby Earth-size worlds, is not expected to fly until the 2040s. Which means GJ 3378b joins a shortlist: a target logged now, to be revisited by hardware still on the drawing board.
For now, the value of a planet like this is less about GJ 3378b itself than about the census it belongs to. Every rocky world confirmed around a nearby red dwarf sharpens the map of where the next generation of telescopes should point. The reconnaissance phase, as Robertson calls it, is about knowing your neighbors before you knock. Astronomers have been building that catalog steadily, the same impulse driving the sky-wide surveys now getting underway, from the decade-long cosmic survey at the Vera C. Rubin Observatory to the aging space telescopes NASA is fighting to keep aloft, including its recent scramble to rescue the falling Swift observatory.
GJ 3378b is not a second Earth. It might not even hold air. But it is close, it is rocky, and it is sitting in the right patch of light — three things that, until a couple of weeks ago, nobody was sure were all true at once.
