Nearby “Super-Earth” GJ 3378b is more Earth-like than previously thought

Astronomers using the Hobby-Eberly Telescope at the McDonald Observatory have re-analysed a nearby exoplanet, revealing that it is far more Earth-like than initial measurements suggested

The planet, named GJ 3378b, sits 25 light-years away in the constellation Camelopardalis and orbits within the habitable zone of its host star—the region where planetary temperatures could allow liquid surface water to exist.

The study, published in The Astrophysical Journal, was led by researchers from the University of California, Irvine, and the University of Texas at Austin.

Target: Red dwarf stars

GJ 3378b orbits a red dwarf, a class of stars that are significantly smaller, cooler, and dimmer than our Sun. Red dwarfs are the most common stellar bodies in the Milky Way, accounting for roughly 70% of all stars in the galaxy. Because they represent the standard stellar environment, understanding the types of planets that orbit them is a primary goal in the search for extraterrestrial life.

Because red dwarfs are inherently faint, finding small planets around them requires specialised equipment. To isolate GJ 3378b’s signature, astronomers used the Habitable-zone Planet Finder (HPF). The instrument utilises an infrared spectrometer mounted on a 10-meter telescope. Because cool red dwarfs emit the vast majority of their energy in infrared wavelengths, the HPF is uniquely optimised to collect this faint light and measure the subtle gravitational “wobble” the planet exerts on its host star.

Revising the mass of a “Super-Earth”

GJ 3378b belongs to a planetary class known as “Super-Earths”, rocky worlds that are larger than Earth but smaller than gas giants like Neptune.

The new HPF data radically revised the planet’s physical profile:

• Initial 2024 discovery baseline:

  • The planet was originally estimated to be roughly 5 times the mass of Earth. A planet that heavy risks holding onto a dense, volatile gas envelope that would crush surface environments.

• New 2026 refined measurements:

  • The updated analysis proves the planet is actually closer to 2.3 times the mass of Earth. This significant downward revision vastly increases the probability that GJ 3378b is a genuinely rocky world with a thin, unsmothering atmosphere.

Orbit refinement and potential climate risks

The research team also refined the planet’s orbital period, adjusting it down from 25 days to a tight 21 days.

While a 21-day orbit is incredibly swift compared to Earth’s 365-day year, this extreme proximity is necessary for the planet to stay warm. Because its red dwarf host star is only about one-third the size of our Sun, a planet must orbit closely to absorb enough starlight to stay in the habitable zone.

However, this proximity introduces a distinct environmental threat: being so close to a red dwarf could subject GJ 3378b to intense stellar radiation. This radiation risk could potentially strip or evaporate the planet’s atmosphere entirely. Scientists note that more observations are required to determine if the planet has successfully retained its atmosphere.

Mapping the neighbourhood for future flagships

The cosmic reconnaissance on GJ 3378b is part of a broader effort to catalogue nearby candidate worlds for the next generation of mega-telescopes.

Data gathered by the HPF is currently building a target list for upcoming observatories, including the Giant Magellan Telescope, the Extremely Large Telescope, and the Habitable Worlds Observatory. Armed with massive light-collecting mirrors, these future facilities will be capable of directly imaging nearby worlds to search for distinct atmospheric biosignatures, the chemical fingerprints of alien life.