UC Irvine-Led Team Revises Nearby Exoplanet GJ 3378b to a Lighter, More Earth-Like Mass Inside the Habitable Zone

Overview

A team led by astronomers at the University of California, Irvine has substantially revised the mass and orbit of a nearby exoplanet first identified in 2024, finding it is lighter, closer to its star, and more likely to be a rocky, Earth-like world than originally measured. The planet, GJ 3378b, orbits a red dwarf star about 25 light-years from Earth in the constellation Camelopardalis, according to UC Irvine News. The revised measurements, published June 30 in The Astrophysical Journal, place the planet within its star’s habitable zone, the region where surface temperatures could allow liquid water.

What We Know

• GJ 3378b was discovered in 2024 by French astronomers using the SPIRou spectrometer on the Canada–France–Hawaii Telescope in Mauna Kea, who measured its mass at 5.26 times that of Earth, “putting it in mini-Neptune territory of being a larger, mostly gaseous world,” with an orbital period of about 25 days, according to Space.com.
• The original announcement, credited to a team led by C. Moutou, put the planet’s minimum mass at 5.26 (+0.94/-0.97) Earth masses with an orbital period of 24.73 ± 0.06 days, based on SPIRou radial-velocity data, according to The Astrophysical Journal.
• The new study, led by UC Irvine associate professor of astronomy Paul Robertson, combined fresh radial-velocity data from 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 National Observatory in Arizona with the previously published CARMENES and SPIRou measurements, according to The Astrophysical Journal.
• The joint analysis reduced GJ 3378b’s minimum mass to 2.3 ± 0.4 Earth masses and its orbital period to 21.45 ± 0.01 days, according to The Astrophysical Journal.
• The lower mass “increases the likelihood that the planet has a terrestrial composition,” and the shorter orbital distance “remains within the conservative circumstellar liquid-water habitable zone,” according to The Astrophysical Journal.
• The planet is roughly twice the size of Earth and sits inside its host star’s habitable zone, according to UC Irvine News.
• GJ 3378b does not transit its star as seen from Earth; it was instead detected through the radial-velocity method, which measures the tiny gravitational wobble a planet induces in its host star, betrayed by a Doppler shift in the star’s light, according to Space.com.
• “This super-Earth gets about 90 percent of the radiation from its host star as Earth gets from its sun, so it’s right in the sweet spot,” said Robertson, according to UC Irvine News.
• Robertson said, “This one’s exciting. It’s one of our closest cosmic neighbors. 25 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,” according to UC Irvine News.
• Michael Endl, an astronomer at the University of Texas at Austin and a co-author of the study, said, “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,” according to Sci.News.

What We Don’t Know

• Whether GJ 3378b actually has an atmosphere remains unknown. The planet sits near what researchers call the “cosmic shoreline” — the region around a star where solar radiation can strip a planet’s atmosphere away — with Mars cited as an example of a planet that may have lost its atmosphere this way, according to UC Irvine News.
• Because GJ 3378b does not transit its star, astronomers cannot use the transit-spectroscopy technique the James Webb Space Telescope has applied to probe atmospheres of planets such as those in the TRAPPIST-1 system, according to Space.com.
• Confirming whether the planet holds an atmosphere will likely require NASA’s planned Habitable Worlds Observatory, a mission slated to launch sometime in the 2040s that could directly image planets like GJ 3378b, according to UC Irvine News.

Analysis

Endl framed the revised measurements in terms of the broader search for life beyond the solar system: “The ultimate goal is biosignatures. 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,” he said, according to Space.com.

Robertson offered a sense of scale for what a viable atmosphere on GJ 3378b might look like: “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. That’s just enough to maintain the kinds of surface pressures where you can have liquid water. It’s enough that there’ll be breathable air, and it provides maybe a little bit of protection from the harsh radiation environment of space,” according to UC Irvine News. Gogod James, a UC Irvine student in Robertson’s group who worked to characterize the planet’s size, said such an atmosphere would justify a deeper look: “If a planet in the habitable zone has a proper atmosphere, we can justify further research looking for biosignatures, liquid water or other signs of life that require both an atmosphere and the right amount of heating from the host star,” according to UC Irvine News.

The study, titled “A Revised Mass and Period for the Habitable Zone super-Earth GJ 3378 b: A Planet Straddling the Cosmic Shoreline,” was funded by National Science Foundation Astronomy & Astrophysics Research Grants and NASA’s Interdisciplinary Consortia for Astrobiology Research, and lists Robertson, Endl, William Cochran, Gudmundur Stefansson and Suvrath Mahadevan among its authors, according to UC Irvine News.