Astronomers Discover Nearby Super-Earth That Future Telescopes Could Image Directly

Astronomers have discovered a super-Earth orbiting in the habitable zone of a star just 18 light-years away, and the timing couldn’t be better. The planet, designated GJ 251 c, orbits in its star’s “just right” zone, where liquid water could exist under the right atmospheric conditions. More importantly, it’s one of the few potentially habitable worlds close enough and positioned just right for the next generation of giant telescopes to photograph directly.

Most exoplanets are too far away, too small, or too close to their stars for current technology to image. GJ 251 c breaks through these barriers. When 30-meter telescopes like the Thirty Meter Telescope come online in the next decade, astronomers may be able to capture actual images of this world. If the planet has an atmosphere with clouds or other features, scientists will be able to see them.

The discovery comes from a team led by Corey Beard at the University of California, Irvine, who combined more than 20 years of observations from five different telescopes. The research was published in The Astronomical Journal.

Detecting a Whisper Against Stellar Noise

GJ 251, the host star, is an M dwarf about one-third the mass of our Sun. It’s the 74th closest star system to Earth and has been studied for decades. Previous research had identified one planet in the system, GJ 251 b, which orbits every 14.2 days and sits too close to the star for habitability.

Finding GJ 251 c was harder. The planet’s subtle gravitational tug on its star produces a radial velocity signal of roughly 1.2 meters per second (±0.2 m/s), about the speed of a leisurely walk. Separating this faint signal from the noise created by the star’s own activity required high-precision instruments and advanced methods.

Beard’s team used data from the Habitable-zone Planet Finder and NEID spectrometers, which observe in infrared wavelengths where M dwarfs like GJ 251 shine brightest. They also incorporated archival observations from three other instruments spanning back to 1997. The combined dataset included 624 velocity measurements.

Stellar activity posed the biggest obstacle. GJ 251 rotates slowly, with a period around 122 to 133 days, and starspots on its surface create periodic signals that can masquerade as planets. To account for this, the researchers used advanced computational models that treat activity as a distinct pattern from the steady orbital motion of a planet.

Orbiting in the Habitable Zone

The candidate planet GJ 251 c orbits its star every 53.6 days at a distance of 0.196 astronomical units, about one-fifth the distance from Earth to the Sun. Because GJ 251 is cooler than the Sun, this orbital distance places the planet squarely in the habitable zone, where surface temperatures could allow liquid water to exist under certain atmospheric conditions.

The planet has a minimum mass of 3.84 Earth masses, measured from its gravitational effect on the star. Since this technique only reveals the planet’s minimum mass, GJ 251 c could be somewhat more massive. Still, the measured value suggests a potentially rocky composition rather than a gas-dominated mini-Neptune.

Using generic mass-radius relations, the team estimates a super-Earth size, though the exact radius remains unknown because the planet does not transit across its star as seen from Earth.

Climate simulations run by the team explored several atmospheric scenarios. With a thick carbon dioxide atmosphere, the planet could maintain liquid water oceans and a global average surface temperature suitable for habitability. An Earth-like atmosphere with current levels of greenhouse gases would be too thin to keep the planet warm, leading to global freezing. A hydrogen-dominated atmosphere would make the planet too hot for life.

A Rare Opportunity for Direct Imaging

Thousands of exoplanets have been discovered, but only a handful meet the criteria for direct imaging with next-generation telescopes. The planet must be large enough and far enough from its star to separate its light from the stellar glare, yet the star itself must be close enough to Earth for the planet to appear sufficiently bright.

GJ 251 c fits these requirements. At 18 light-years away, GJ 251 is among the closest stars to Earth that host planets. The planet’s angular separation from its star as seen from Earth is about 0.035 arcseconds, roughly the apparent width of a quarter viewed from 110 miles away. While impossibly small for current telescopes, it falls within the expected capabilities of instruments being designed for 30-meter telescopes.

The Planetary Sciences Imager planned for the Thirty Meter Telescope aims to achieve contrasts of 100 million to one, with an inner working angle of about 0.017 arcseconds. GJ 251 c’s angular separation is twice this limit, potentially placing it within reach.

Actual imaging feasibility depends on several factors the team cannot yet know precisely: the planet’s radius, its atmospheric reflectivity, and the final performance of the telescope and its instruments. Under optimistic assumptions, the planet could be detected. Under pessimistic assumptions, it would remain out of reach.

Still, among all known potentially terrestrial planets in habitable zones, GJ 251 c ranks as the best candidate for direct imaging from the Northern Hemisphere.

Why Scientists Remain Cautious

Evidence supports the planetary interpretation of the 54-day signal, but uncertainties remain. The signal does not appear in stellar activity indicators, which typically track starspot activity. Its period doesn’t match the stellar rotation period or its common multiples.

When the researchers tested the signal’s stability by analyzing different time periods separately, they found consistent orbital parameters across most subsets of data. Planetary signals show this stability, while activity-driven artifacts tend to vary as stellar cycles evolve.

However, the preference for a two-planet model over a one-planet model only marginally exceeds the conventional threshold for strong evidence, which is why the authors classify GJ 251 c as a “candidate planet” rather than a confirmed planet. False positives have fooled astronomers before, particularly for planets orbiting active M dwarfs.

The candidate planet does not transit, so astronomers cannot measure the planet’s radius or atmospheric composition through conventional methods. Confirmation of GJ 251 c will require continued monitoring with high-precision instruments.

If the planet is real, characterizing its atmosphere will require direct imaging observations that are probably a decade away at minimum. The Thirty Meter Telescope, currently under construction in Hawaii, aims for first light in the early 2030s.

GJ 251 c sits at the intersection of what astronomers can detect with current technology and what they might soon be able to see with the next generation of telescopes. Whether it turns out to be a rocky world with a breathable atmosphere, a frozen wasteland, or something else entirely, the discovery shows how close we are to answering one of humanity’s oldest questions: Are we alone?

Disclaimer: This article is based on peer-reviewed research published in The Astronomical Journal. The content provides general information about astronomical discoveries and should not be considered as definitive proof of planetary characteristics until confirmed through independent observations.