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A new exoplanet candidate has emerged as one of the most promising nearby worlds for habitability studies. At just 18 light-years away, GJ 251c appears to be a super-Earth in the habitable zone of a nearby red dwarf — close enough that follow-up observations could reveal whether it has an atmosphere or even the right conditions for liquid water.
A super-Earth practically next door
GJ 251c is reported as a candidate super-Earth with a minimum mass of roughly 3.84 times Earth's. What makes this discovery noteworthy is not only the planet's size but its orbital position: it orbits within the so-called habitable zone, the range of distances from a star where temperatures could allow liquid water to exist on a planet's surface if that world has an appropriate atmosphere.
"The exoplanet is in the habitable or the 'Goldilocks Zone,' the right distance from its star that liquid water could exist on its surface, if it has the right atmosphere," said astronomer Suvrath Mahadevan of Pennsylvania State University, summarizing why GJ 251c has sparked interest.
Planets that combine a likely rocky mass and an orbit inside a star's habitable zone are surprisingly rare among the thousands of exoplanets detected so far. Finding one this close — cosmically speaking — is a major opportunity: proximity means a brighter host star and stronger signals for the instruments astronomers plan to use next.
How astronomers found it and what they measured
GJ 251 is a red dwarf about one-third the mass and diameter of the Sun. Small, cool stars like this place their habitable zones much closer in than the Sun's habitable zone, so planets in those zones complete orbits more quickly — and repeated orbital cycles make radial velocity signals easier to detect and confirm.
Animation showing how radial velocity is measured, one of the ways a planet can affect the light of its star. (Alysa Obertas/Wikimedia Commons, CC BY-SA 4.0)
A team led by Corey Beard of the University of California, Irvine, re-examined GJ 251 after decades of observations. The star was already known to host a closer-in exoplanet, GJ 251b, a super-Earth with a 14.2-day orbit. By combining more than 20 years of archival radial velocity data with new, higher-resolution measurements, the researchers detected an additional periodic signal with a 53.6-day period — consistent with a second planet sitting in the star's habitable zone.
Radial velocity detects the tiny back-and-forth motion of a star caused by the gravitational tug of orbiting planets. For GJ 251c, the wobble reveals a minimum mass but cannot provide a radius because the planet does not transit (pass in front of) its star from Earth’s perspective. That leaves key properties — density, surface gravity and atmosphere — unknown for now.
Why GJ 251c is a valuable target
There are several reasons why this candidate is exciting for exoplanet science. First, its proximity at ~18 light-years puts it within reach of upcoming instruments designed for direct imaging and detailed spectroscopy. Second, being in the habitable zone makes the possibility of temperate surface conditions plausible. Third, its host star’s relative quietness compared to more active red dwarfs may reduce the destructive effects of stellar flares and high-energy radiation, improving chances an atmosphere could survive.
Paul Robertson of UC Irvine noted the system’s accessibility: "What makes [GJ 251c] especially valuable is that its host star is close by, at just about 18 light-years away. Cosmically speaking, it's practically next door." That nearness matters: it increases the angular separation between planet and star on the sky and boosts the planet-to-star flux ratio for direct-imaging attempts.
Limits and unknowns
Important caveats remain. The radial velocity signal gives only a minimum mass — the true mass depends on the planet’s orbital inclination. Without a measured radius, researchers cannot confirm whether GJ 251c is rocky like Earth or more like a mini-Neptune with a thick volatile envelope. Also, red dwarfs can have variable magnetic activity; detailed monitoring is required to separate stellar noise from planetary signals and to assess whether a planet could retain an atmosphere long-term.
What next: direct imaging and atmosphere hunting
Researchers view GJ 251c as a prime candidate for next-generation direct imaging and spectroscopic study. Direct imaging aims to separate the planet’s faint light from the blinding glare of its star using coronagraphs, adaptive optics, or starshade technology. Planned giant ground telescopes — the Extremely Large Telescope (ELT), Giant Magellan Telescope (GMT), and Thirty Meter Telescope (TMT) — plus future space missions with advanced coronagraphs, are expected to open a new window on nearby small planets.
Beard emphasized the need for continued technological progress and community resources: "We are at the cutting edge of technology and analysis methods with this system. While its discovery is quite statistically significant, we are still determining the status of the planet due to the uncertainty of our instruments and methods. We need the next generation of telescopes to directly image this candidate, but what we also need is community investment."
Expert Insight
Dr. Elena Marquez, an exoplanet astronomer at a major research observatory (commentary prepared for this article), adds: "GJ 251c occupies the sweet spot we dream about — close enough for future telescopes to study, and in the star’s temperate zone. Before we get too excited about habitability, we'll need a detection of an atmosphere and measurements of composition. If spectroscopy reveals molecules like water vapor, oxygen, or biosignature gases in a stable atmosphere, this system could become one of the best laboratories to study potentially habitable super-Earths."
Looking forward
GJ 251c is an important reminder that some of the most intriguing exoplanet targets may lie in our stellar neighborhood. While confirmation and characterization will require careful follow-up with the next generation of observatories, the discovery demonstrates how long-term investments in radial velocity monitoring plus targeted high-resolution observations continue to pay dividends. For astronomers planning the next missions and instruments, a close, habitable-zone super-Earth is a compelling case study — and possibly, in time, a world we can study directly rather than infer from starlight alone.
Source: sciencealert
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