Kepler Data Reveal a One-Transit Candidate: An Earth-Size Planet on a Near-Earth-Year Orbit

On a quiet night in 2017, NASA’s aging Kepler space telescope stared at an unremarkable orange star in the constellation Libra. For about 10 hours, that star, HD 137010, dimmed by a fraction of a percent—then brightened again and stayed steady for the rest of the three-month observing run.

Years later, that fleeting dip in starlight has been traced to what may be one of the most tantalizing planet candidates yet: an Earth-size world with an almost Earth-length year, circling a relatively calm, Sunlike star about 146 light-years away.

In a study posted Jan. 27 and now published in The Astrophysical Journal Letters, an international team led by University of Southern Queensland Ph.D. student Alexander Venner reports evidence for a planet candidate, dubbed HD 137010 b, based on that single, high-quality transit captured by Kepler’s extended K2 mission.

A temperate-size world—on paper

The world, if it exists, would be just slightly larger than Earth, receive about 30% as much sunlight and orbit near the outer edge of its star’s habitable zone—the region where liquid water could exist on a rocky planet’s surface under the right atmospheric conditions.

“It’s a cool, Earth-sized exoplanet with radius and orbital properties similar to our own planet,” Venner and colleagues write, adding that the planet “receives 0.29 times Earth’s insolation and lies close to the outer edge of the habitable zone.”

Scientists caution that HD 137010 b is still a candidate, not a confirmed planet. Only one transit has been seen, and the planet’s mass, atmosphere and exact orbit remain unknown. But the brightness and relative proximity of its host star make it one of the more promising targets for detailed studies of an Earth-sized, temperate world—if astronomers can catch it crossing its star a second time.

HD 137010 is a K3.5V “orange dwarf,” somewhat smaller and cooler than the Sun. It has about 72% of the Sun’s mass, 71% of its radius and around 23% of its luminosity, with a surface temperature near 4,770 Kelvin—roughly 1,000 degrees cooler than our star. It appears at magnitude 10.1 in visible light, bright compared with many planet-hosting stars discovered by Kepler.

What a single transit can (and can’t) tell astronomers

From the depth of the 2017 transit—about 225 parts per million, or a dimming of 0.0225%—the team derived a planetary radius of 1.06 times Earth’s, with uncertainties of a few percent. The shape and duration of the transit, combined with precise measurements of the star’s size and density from the European Space Agency’s Gaia mission, allowed the researchers to statistically infer an orbital period of about 355 days, plus or minus several months.

That period is strikingly close to Earth’s 365-day year. The analysis suggests HD 137010 b orbits at an average distance of about 0.88 astronomical units, a bit closer to its star than Earth is to the Sun, but around a significantly dimmer star.

Overall, the team estimates the planet receives 29% of the sunlight Earth does. In simple energy-balance terms, that translates to an equilibrium temperature of about 190 Kelvin (minus 83 degrees Celsius), before any greenhouse heating is taken into account.

Co-author Chelsea Huang, an astrophysicist at the University of Southern Queensland, said in media interviews that even with a realistic greenhouse effect, the planet’s mean surface temperature could be below minus 70 C, closer to conditions on Mars than on modern Earth.

“It likely sits near the outer edge of the habitable zone,” Huang said, estimating roughly a 50% chance that the planet’s orbit falls within the model-dependent boundaries of that zone. “You would need a fairly thick, CO₂-rich atmosphere to maintain liquid water on the surface.”

That nuance is critical. Labels such as “Earth-like” and “potentially habitable,” already attached to HD 137010 b in some headlines, can be misleading. The planet appears Earth-like in size and roughly in orbital distance once the star’s lower luminosity is factored in. But its likely climate, absent a substantial greenhouse blanket, would be much colder than Earth’s and possibly globally glaciated.

Mining Kepler’s archives for long-period planets

The new study also underscores how much information can be extracted from what astronomers call single-transit events. Most transiting exoplanets have been found by watching for multiple periodic dips in starlight, which readily yield the orbital period. Kepler’s K2 mission, however, observed fields along the ecliptic for only about 80 days at a time, making it unlikely to see more than one transit for planets with year-long orbits.

Venner’s team set out to mine those K2 data for one-off events and to demonstrate that, under the right conditions, a single transit can still identify high-priority planet candidates.

To build their case for HD 137010 b, the researchers first ruled out common sources of false positives. They tested whether the dip could be caused by instrumental glitches, background eclipsing binary stars, or a faint, unresolved companion. They used high-resolution imaging to search for nearby stars in the same line of sight and checked Gaia astrometry and archival radial-velocity measurements for signs of hidden stellar companions.

After that vetting, the simplest explanation remained a small planet crossing the face of HD 137010. The authors stress, though, that the period they infer—355 days, with a plausible range from about 296 to 555 days—is statistical, not directly measured.

Because no second transit has yet been recorded and the radial-velocity “wobble” induced by an Earth-mass planet at that distance would be extremely small, the team is careful to call HD 137010 b a planet candidate.

“We report the discovery of the first planet candidate with Earth-like radius and orbital properties that transits a Sun-like star bright enough for substantial follow-up observations,” the authors write.

A bright target—and a ticking clock

For scientists planning the next decade of observations, HD 137010 b presents both an opportunity and a deadline. Confirming the planet and pinning down its orbit will likely require years of precise radial-velocity monitoring with top-tier spectrographs, as well as targeted space-based photometry timed to catch another transit within the broad prediction window.

Without additional observations, uncertainty in the orbital period will grow, making future transits increasingly difficult to predict and observe. A similar issue nearly derailed follow-up of the sub-Neptune K2-18b before additional space-based data narrowed its transit times.

If astronomers can secure HD 137010 b’s ephemeris—the schedule of when it passes in front of its star—the system could become a tempting target for the James Webb Space Telescope and future extremely large ground-based telescopes. In principle, those observatories could probe the planet’s atmosphere, searching for carbon dioxide, water vapor and other gases that shape surface conditions. Detecting clear signs of biological activity would be far more challenging and remains a long-term aspiration.

For now, HD 137010 b is a single, precise flicker in an old dataset, transformed into a carefully argued portrait of a distant, frozen world. Whether it proves to be a rocky, ice-covered planet or something more complex, astronomers say it already points to a future in which many of the most informative planets will be found not by launching new spacecraft, but by looking more deeply at the light we have already captured.