GJ 251c: A New Exoplanet Candidate for Extraterrestrial Life

A newly identified exoplanet candidate has entered the spotlight, and it could rank among the most promising alien worlds for the search for extraterrestrial life.

Named GJ 251c, this super-Earth lies only 18 light-years away and has a minimum mass roughly 3.84 times that of Earth. Most importantly, it sits squarely within its star’s habitable zone: an orbital region that is neither too distant nor too close to sustain life.

"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," explains astronomer Suvrath Mahadevan of Pennsylvania State University.

What makes GJ 251c a promising exoplanet

Researchers are still working to understand the complicated combination of conditions needed for life to arise on a world. Before studying a candidate in greater depth, however, they look for several broad, essential features.

The first two criteria are whether a world is solid and rocky like Earth - the only planet definitively known to harbour life - and whether it orbits at a distance from its star where temperatures could permit liquid water, often called the “solvent of life”.

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Among the thousands of worlds found so far, exoplanets meeting both requirements are remarkably uncommon. A planet that fulfils them while also being close enough for detailed examination is therefore an exceptionally valuable find.

"What makes [GJ 251c] especially valuable is that its host star is close by, at just about 18 light-years away," says astronomer Paul Robertson of the University of California, Irvine. "Cosmically speaking, it's practically next door."

GJ 251c and its nearby red dwarf star

The exoplanet circles GJ 251, a red dwarf with around one-third of the Sun’s mass and diameter. As the star is both cool and small, its habitable zone lies far nearer to it than the Solar System’s habitable zone does. That proximity can make worlds in this region easier to detect, since their shorter orbits produce signals from repeated journeys that can be more readily identified and combined.

A group of astronomers headed by Corey Beard of UC Irvine observed GJ 251 while searching for nearby, low-mass worlds that could be suitable candidates for direct imaging. The star was selected because astronomers already knew it had an exoplanet: GJ 251b, a super-Earth with 3.85 times Earth’s mass that completes an orbit in 14.2 days, far too close to the star to be habitable. More than 20 years of observations of the star had also already been collected.

Building on those records, the team gathered fresh observations at higher resolution to analyse the star’s minute motions as orbiting planets pull on it gravitationally. To their great excitement, they detected not only the signal of the known planet but also evidence for a second world. It has an orbital period of 53.6 days and lies within the star’s habitable zone.

Direct imaging could reveal more about GJ 251c

Very little is currently known about GJ 251c. Its gravitational effect on the star has allowed scientists to estimate its mass, but because the planet does not cross in front of its star, its diameter and other characteristics cannot be determined without additional data.

Fortunately, it is an excellent target for follow-up work. The researchers expect the exoplanet to be detectable through future direct-imaging campaigns, in which astronomers study a world by observing the exoplanet itself rather than its influence on its host star.

Achieving this is difficult, but emerging technology is on the verge of opening a new era for direct-imaging investigations of alien worlds.

"We are at the cutting edge of technology and analysis methods with this system," Beard says. "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."

The next generation of telescopes is tantalisingly close, potentially allowing astronomers to establish whether GJ 251c has an atmosphere capable of retaining enough heat for oceans of water to exist across its surface.

"While we can't yet confirm the presence of an atmosphere or life on GJ 251c, the planet represents a promising target for future exploration," Mahadevan adds. "We made an exciting discovery, but there's still much more to learn about this planet."

The research has been published in The Astronomical Journal.