A Super-Earth in the Neighborhood: GJ 3378b Offers a Window Into Habitability Around Red Dwarfs
Featured image: [Artist’s impression of a rocky super-Earth orbiting a red dwarf star; credit: NASA/JPL-Caltech]
A team of astronomers has confirmed that GJ 3378b, a rocky super-Earth just 25 light-years from Earth, sits squarely in the habitable zone of its red dwarf star. The planet receives about 90 percent of the stellar radiation Earth gets from the Sun, placing it in a “sweet spot” for potential habitability.
The discovery, published in The Astrophysical Journal by a team led by Dr. Paul Robertson of the University of California, Irvine, revises earlier estimates of the planet’s mass and orbit. Originally detected in 2024 with a mass of 5.26 Earth masses and a 25-day orbital period, new observations from two premier spectrographs revealed a much lighter world: 2.3 Earth masses on a 21.45-day orbit around the red dwarf GJ 3378 in the constellation Camelopardalis.
“This one’s exciting,” Robertson said. “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.”
A Rocky World in the Habitable Zone
The mass revision is critical. A 5.26-Earth-mass planet would likely be a mini-Neptune with a thick gaseous envelope unsuitable for life. At 2.3 Earth masses, GJ 3378b is a solid, rocky super-Earth, the kind of world where liquid water could exist on the surface, given the right atmospheric conditions.
The team used the Habitable-zone Planet Finder (HPF) on the Hobby-Eberly Telescope at McDonald Observatory in Texas and the NEID spectrograph on the WIYN Telescope at Kitt Peak National Observatory in Arizona. Independent confirmation came from the CARMENES instrument at Calar Alto Observatory in Spain.
“This super-Earth gets about 90 percent of the radiation from its host star as Earth gets from the Sun, so it’s right in the sweet spot,” Robertson said. “This is a real benefit for habitability.”
Straddling the Cosmic Shoreline
GJ 3378b sits directly on what astronomers call the “cosmic shoreline”, the theoretical boundary where stellar radiation can strip a planet of its atmosphere. If the planet has retained an atmosphere, it becomes a prime target for future biosignature searches. If not, its fate may resemble Mars.
“About 70 percent of stars in our galaxy are red dwarfs, so they represent the standard,” said co-author Dr. Michael Endl of the University of Texas at Austin. “It’s really important that we understand the planet population around these stars.”
Since GJ 3378b does not transit its star, astronomers cannot use the transit spectroscopy method employed by the James Webb Space Telescope to analyze its atmosphere. Direct imaging will have to wait for future observatories such as NASA’s Habitable Worlds Observatory, expected in the 2040s.
One of the Closest Known Habitable Zone Worlds
At 25 light-years, GJ 3378b joins a small group of nearby potentially habitable exoplanets. Only a handful of worlds, including Proxima Centauri b at 4.2 light-years, are closer. But Proxima Centauri b’s habitability is heavily debated due to violent stellar flares from its host star, while GJ 3378’s red dwarf appears relatively quiet.
The research paper, titled “A Revised Mass and Period for the Habitable Zone super-Earth GJ 3378b: A Planet Straddling the Cosmic Shoreline,” appears in The Astrophysical Journal (DOI: 10.3847/1538-4357/ae732b). The work was supported by NASA and the National Science Foundation.
“The ultimate goal is biosignatures,” Endl said. “We really want to know: are we alone in the universe?”