A potentially habitable rocky world has been found in the habitable zone around a red dwarf just 25 light-years from us.
However, faced with a hostile wind of radiation from its host star, it remains unclear whether this new exoplanet supports an atmosphere, or the possibility of life. Nevertheless, astronomers are celebrating the discovery.
“This one’s exciting,” said Paul Robertson of the University of California, Irvine, 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 planet, designated GJ 3378b, orbits the faint red dwarf star in the constellation of Camelopardalis, the Giraffe. It was discovered in 2024 by French astronomers using the Canada–France–Hawaii Telescope in Mauna Kea, but American astronomers have revised those initial findings, revealing that the planet is possibly more like Earth than we realized.
All we know for sure is the mass and the orbit of GJ 3378b. We do not yet know whether it is like Earth or not – it could have land and sea and clouds and life, or it could be airless and cratered.
The planet is not seen to transit, or pass in front of its star, blocking its light from our vantage point. Instead, GJ 3378b was detected by the effects of its gravity tugging on its parent star. This causes the star to wobble around the center of mass that it shares with the planet, a wobbling that is betrayed by a Doppler shift in the star’s light that can be measured by its spectra, the wavelengths of light that it emits.
When it was discovered in 2024, its mass was measured to be 5.26 times the mass of Earth, putting it in mini-Neptune territory of being a larger, mostly gaseous world. However, by taking a second look at the planet using two different telescopes, Robertson’s team was able to show that the planet’s true mass is 2.3 times the mass of Earth. This means that it is closer to being a rocky super-Earth instead.
Furthermore, the same observations found that GJ 3378b’s orbital period is 21 days, not the 25 days that had originally been measured. This means that the planet is closer to the star than had been thought, sitting comfortably within the habitable zone where temperatures will be suitable for liquid water on the surface of a planet with an atmosphere. So from that point of view, the chance of GJ 3378b being habitable, if not inhabited, seems fair.
“This super-Earth gets about 90% of the radiation from its host star that Earth gets from its sun, so it’s right in the sweet spot,” said Robertson.
One significant problem, however, is that red dwarfs spit out harmful torrents of radiation in fierce gusts of their stellar winds, which can strip away a planet’s atmosphere. This raises the question, does GJ 3378b even have an atmosphere?
Currently there is no way to tell. The James Webb Space Telescope (JWST) has been probing for atmospheres around other rocky worlds orbiting red dwarfs, such as those in the TRAPPIST-1 system. It does so by transit spectroscopy, where an atmosphere wrapped around a planet absorbs some of the star’s light filtering through it, leaving dark absorption lines in the star’s spectrum.
Unfortunately, GJ 3378b does not transit its star. This means that astronomers will have to wait until the 2040s, when NASA’s Habitable Worlds Observatory will hopefully launch, to answer the question of whether GJ 3378b really does have an atmosphere or not.
Still, astronomers are hopeful. GJ 3378b is right on the edge of the zone where planets are expected to be seriously battered by radiation, meaning it could have escaped the worst. If so, there might be more than just an atmosphere for the Habitable Worlds Observatory to discover.
“The ultimate goal is biosignatures,” said University of Texas at Austin astronomer Michael Endl in a separate statement. “We really want to know, are we alone in the universe? 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 findings were reported on June 30 in The Astrophysical Journal.
