Astronomers have refined the characteristics of a nearby exoplanet that could be one of the most promising candidates for habitability beyond the Solar System. GJ 3378b, located just 25 light-years away in the northern constellation Camelopardalis (the Giraffe), orbits a small, cool red dwarf star within the so-called habitable zone — the region where temperatures might allow liquid water to exist on a planetary surface.
New observations conducted with the Hobby-Eberly Telescope at the McDonald Observatory and published this week in The Astrophysical Journal have significantly revised the planet's estimated mass and orbital period. When GJ 3378b was first discovered in 2024, scientists calculated its mass at roughly five times that of Earth. The latest measurements, led by Paul Robertson of the University of California, Irvine, have reduced that figure to approximately 2.3 Earth masses. This downward revision makes it far more likely that GJ 3378b is a rocky super-Earth rather than a world with a thick, crushing atmosphere that would preclude habitability.
The research team also refined the planet's orbital period from 25 to 21 days. While this orbit is extremely close compared to Earth's 365-day journey around the Sun, the proximity is necessary for the planet to remain within the habitable zone of its host star, which is only about one-third the size of the Sun. At this distance, GJ 3378b receives roughly the same amount of stellar light as Earth does from the Sun.
To study the planet, the astronomers used the Habitable-zone Planet Finder (HPF), a specialized spectrograph installed on the Hobby-Eberly Telescope. The HPF is optimized to detect the tiny gravitational wobble that an orbiting planet induces in its host star. By measuring this minuscule shift in the star's motion, scientists can calculate the planet's mass and orbital characteristics. Because red dwarfs are small and cool, they emit most of their energy in infrared wavelengths, making the HPF's infrared sensitivity particularly valuable for detecting Earth-like planets around such stars.
Red dwarfs are the most common type of star in the Milky Way, making them prime targets in the search for habitable exoplanets. However, their low luminosity and the small size of terrestrial planets make these worlds difficult to detect, requiring extremely sensitive and specialized instruments. GJ 3378b is classified as a super-Earth — a rocky planet larger than our own but not so massive that it would retain a dense, crushing atmosphere.
Despite the promising findings, the planet's close orbit raises questions about its long-term habitability. The intense stellar radiation from the red dwarf could strip away the planet's atmosphere over time. Further observations will be needed to determine the actual conditions on GJ 3378b and whether it can retain the ingredients necessary for life.
Since 2018, the Habitable-zone Planet Finder has supported astronomers in cataloging exoplanets that could potentially host life. The refined data on GJ 3378b will help prioritize targets for the next generation of telescopes, including the Giant Magellan Telescope, the Extremely Large Telescope, and the Habitable Worlds Observatory. These instruments, equipped with giant mirrors for light collection, will be capable of directly imaging exoplanets and searching for the first long-awaited signs of life beyond Earth.
