A collaboration of international astronomers has uncovered a rare super-Earth exoplanet located just 19.5 light-years away. Known as GJ 251 c, this world resides inside the habitable zone of its star—a region where conditions may allow liquid water to exist. It stands as the nearest planet of its kind and one of the few accessible for detailed study with upcoming telescope technology.
The team, led by Corey Beard, documented their findings in the Astronomical Journal. Their research highlights the potential for direct imaging of rocky exoplanets, a feat expected to be achievable with the new generation of 30-meter-class telescopes. This makes GJ 251 c a prime candidate for the first atmospheric characterization of a non-transiting habitable planet.
The discovery rests on a robust foundation of more than twenty years of radial velocity measurements from leading instruments such as HPF, NEID, alongside archival data from Keck/HIRES, SPIRou, and CARMENES. Among northern sky targets, GJ 251 c is now viewed as the premier candidate for rocky planet imaging through reflected light.
The significance goes beyond its habitability potential, as the planet's proximity enhances its observability. As Beard and colleagues emphasize,
“GJ 251 c falls in a very narrow range of parameter space wherein a terrestrial, HZ exoplanet may be directly imaged via reflected light with the upcoming next generation of extremely large telescopes (ELTs).“
Two Decades of Observations Unveil a Second Planet
The star at the center of this discovery, GJ 251, is a quiet M3 red dwarf positioned just 5.58 parsecs from Earth. While the inner planet GJ 251 b was known, a newly detected 54-day signal — distinct from stellar activity — surfaced after thorough analysis of over 900 high-precision radial velocity observations.
Characterized by a minimum mass of approximately 3.84 ± 0.75 Earth masses, GJ 251 c orbits within the temperate habitable zone, where theorized conditions might allow liquid water on its surface. Its inferred rocky nature and orbit make it an excellent target for future atmospheric investigations.
The group employed a comprehensive Bayesian model comparison testing over 50 distinct scenarios, weighing planetary models against stellar activity explanations. The findings strongly support the presence of GJ 251 c as a genuine planet rather than stellar-induced noise.
Distinguishing Planetary Signals from Stellar Activity
Identifying small exoplanets orbiting red dwarfs is challenging due to the stars’ magnetic variability, which can mimic planetary Doppler shifts. To avoid false positives, researchers applied chromatic Gaussian process models alongside multi-instrument radial velocity data, confirming signals in both visible and infrared light spectra.
As stated in their publication,
“We perform a color-dependent analysis of the system and a detailed comparison of more than 50 models that describe the nature of the planets and stellar activity in the system.”
Additionally, the team examined several stellar activity indicators such as H-alpha, calcium infrared triplet, and differential line widths. The 54-day periodic signal was absent from these proxies, reinforcing its planetary origin.
GJ 251’s calm stellar environment — supported by photometric rotation periods near 122 days — permits highly accurate RV measurements. The authors highlight that GJ 251 is “bright (V = 9.9 ± 0.1) and nearby (d = 5.58 ± 0.01 pc), making it an outstanding candidate for future direct imaging efforts.”
Prime Candidate for Upcoming Telescope Missions
Unlike many known exoplanets detected via transits or indirect methods, GJ 251 c lies close enough to be directly observed through its reflected starlight. This opens the door to analyzing its atmosphere, temperature, and possible biosignatures such as oxygen, methane, and carbon dioxide.
“GJ 251 c is currently the best candidate for terrestrial, HZ planet imaging in the northern sky”, According to the authors
Successful direct imaging requires ideal conditions, including sufficient angular separation, planet-to-star brightness contrast, and luminosity. GJ 251 c meets these criteria, making it detectable by upcoming facilities such as the Extremely Large Telescope (ELT) and Habitable Worlds Observatory (HWO) within the next ten years.
This discovery also highlights the importance of studying non-transiting planets, which often escape transit surveys like TESS. Planets such as GJ 251 c are more effectively uncovered via radial velocity measurements, expanding our understanding of nearby habitable worlds.
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