A study in The Astrophysical Journal has refined the picture of GJ 3378 b, a nearby rocky super-Earth that receives about 90% as much stellar radiation as Earth gets from the Sun. The planet sits just 25 light-years away, close enough on cosmic scales to make astronomers pay attention.
The world orbits a faint red dwarf star in the northern constellation Camelopardalis. Its revised mass, about 2.3 times that of Earth, places it in the super-Earth category. Its 21.45-day orbit puts it in a region where temperatures could allow liquid water on the surface if the planet has the right kind of atmosphere.
“This one’s exciting,” said Paul Robertson, an astronomer at the University of California, Irvine and lead author of the study. That excitement comes with an important caveat. The planet’s orbit is promising, but its atmosphere remains unknown.
A rocky world 25 light-years away
GJ 3378 b is part of a growing catalog of nearby planets that may help scientists study what makes a world potentially habitable. Its host star is also cataloged as Gliese 3378, LHS 1805 and TIC 322347050. The star lies about 7.7 parsecs from Earth, or roughly 25 light-years.
That sounds far by everyday standards. In the Milky Way, it counts as the astronomical neighborhood. Our galaxy stretches roughly 100,000 light-years across, so a planet 25 light-years away sits close enough for researchers to place it on the short list for detailed follow-up.
The planet’s mass matters. At about 2.3 Earth masses, GJ 3378 b is heavier than Earth and likely rocky. Astronomers call such planets super-Earths, a term that describes their size or mass range rather than their surface conditions.
Its orbital period is also compact. A year on GJ 3378 b lasts 21.45 Earth days. Around a dim red dwarf, that tight orbit can still place a planet in the star’s temperate region because the star gives off much less energy than the Sun.
From Earth, the planet cannot be seen as a bright dot beside its star. Researchers detect it through the small gravitational tug it exerts on the star. That method can reveal a planet’s minimum mass and orbital rhythm.
Why red dwarfs matter
Red dwarf stars dominate the Milky Way. They are smaller, cooler and dimmer than the Sun and they make up the majority of stars in the galaxy. That abundance makes them central to the search for nearby planets.
If habitable worlds are common around red dwarfs, the galaxy could contain vast numbers of potentially temperate rocky planets. If red dwarf environments often strip away atmospheres, that would reshape how scientists rank the best places to search for life.
These stars also offer practical advantages. Because red dwarfs are small, a planet’s gravitational pull can make a more noticeable wobble in the star’s motion. That helps instruments detect planets with masses closer to Earth’s.
There is a tradeoff. Planets in the habitable zones of red dwarfs orbit close to their stars. Close-in planets can face strong radiation and stellar activity over long periods. Those conditions can shape whether a planet keeps an atmosphere.
The GJ 3378 system gives astronomers a nearby example to test these questions. It combines a common kind of star, a small rocky planet and an orbit that receives nearly Earth-like levels of incoming energy.
A revised orbit changes the picture
The new analysis revises earlier estimates of GJ 3378 b’s orbit and mass. The team found a lower mass and shorter orbital period than previously reported. That adjustment makes the planet more Earth-like in the specific sense most important for habitability studies, its incoming energy.
The researchers combined measurements from several high-precision instruments. A key part of the work used the Habitable-zone Planet Finder, a near-infrared spectrometer on the Hobby-Eberly Telescope at McDonald Observatory in Texas.
The team also used the NEID Spectrometer on the WIYN Telescope at Kitt Peak National Observatory in Arizona. Those data were analyzed with published observations from the CARMENES and SPIRou spectrometers.
This approach relies on radial velocity, a technique that tracks tiny shifts in starlight caused by a planet tugging its star toward and away from Earth. The shifts are extremely small. With enough measurements, they can reveal the rhythm of an unseen planet’s orbit.
“This super-Earth gets about 90% of the radiation from its host star as Earth gets from the Sun, so it’s right in the sweet spot,” Robertson said. That amount of energy places the planet in the habitable zone, where surface water could persist under suitable atmospheric pressure and temperature.
The atmosphere question
The central mystery is whether GJ 3378 b still has an atmosphere. A favorable orbit can supply the right amount of heating. An atmosphere helps determine whether that energy produces stable surface conditions.
Atmospheres are fragile on planetary scales. Earth’s atmosphere feels immense from the ground, yet it forms a thin skin around the planet. “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 thin layer performs essential work. It creates pressure at the surface, moves heat around the globe and allows liquid water to remain stable across a range of temperatures. Without enough atmospheric pressure, surface water can freeze, boil away, or exist only briefly.
The study describes GJ 3378 b as lying near the cosmic shoreline. This idea compares a planet’s gravity with the radiation it receives from its host star. Worlds with enough gravity may hold onto air. Worlds exposed to too much radiation can lose it over time.
Mars offers a familiar example from our own Solar System. Scientists think it once had a thicker atmosphere and surface water. Over time, much of that atmosphere was lost, changing the planet’s climate and surface conditions. GJ 3378 b gives researchers another case to examine under a different star.
What astronomers will look for next
The next step is to learn whether GJ 3378 b has an atmosphere and what that atmosphere might contain. That will require careful observations and likely more than one technique. Nearby planets are the best candidates for such work because their signals are easier to study.
“Our mantra is ‘follow the water,'” Robertson said. For exoplanets, that means looking for worlds where temperature, pressure and chemistry could allow stable surface water. GJ 3378 b now fits that search strategy more strongly than earlier measurements suggested.
A confirmed atmosphere would make the planet especially valuable. Astronomers could then ask whether it contains molecules linked to climate, geology, or biology. Any potential biosignature would need cautious study because gases can have several possible origins.
The planet may also help researchers understand red dwarf habitability as a broader category. Since red dwarfs are so common, each nearby system adds evidence. GJ 3378 b can show how mass, stellar radiation and atmospheric survival interact around one of the galaxy’s most widespread star types.
For now, GJ 3378 b stands as a nearby rocky planet in a compelling location. It receives nearly Earth-like energy from a small star and it sits close enough for astronomers to keep watching. The world’s true promise depends on the thin layer of gas that may surround it.
