Archival K2 Data Reveals Earth-Sized Planet Candidate with a 355-Day Year
In the vast, digitized archives of defunct space missions, astronomers continue to find hidden gems that were missed during initial surveys. A team of researchers, led by Alexander Venner of the University of Southern Queensland, has announced the discovery of a compelling new exoplanet candidate orbiting the bright K-dwarf star HD 137010. The candidate, designated HD 137010 b, was identified through a single, ten-hour-long transit event captured during the K2 mission in 2017. What makes this find particularly significant is its striking resemblance to our own world: the planet is nearly identical in size to Earth and follows an orbit that mirrors an Earth-like year.
The discovery, detailed in a paper involving researchers from the University of Southern Queensland, Harvard & Smithsonian, and NASA Ames Research Center, highlights the enduring value of the Kepler Space Telescope’s secondary mission, K2. While the original Kepler mission spent four years staring at a single patch of sky to find Earth-like planets, the K2 mission was forced to observe different fields along the ecliptic plane for shorter durations of approximately 80 days. This restricted timeframe typically prevents the detection of long-period planets, which require multiple transits to confirm their orbits. However, by meticulously scanning the data for single transit events, the research team has pushed the boundaries of what archival data can reveal.
The Discovery of HD 137010 b
The identification of HD 137010 b began with the visual inspection of light curves from K2’s Campaign 15. Hans Martin Schwengeler, a citizen scientist working with the Planet Hunters project, first flagged the single transit-like feature. The event, which lasted roughly 10 hours, showed a shallow dip in the star’s brightness of only 225 parts per million (ppm). Such a subtle signal is notoriously difficult to detect; however, because HD 137010 is a relatively bright tenth-magnitude star, the photometric precision achieved was exceptionally high—reaching a level of 8.5 ppm, near the theoretical limit of the spacecraft's instruments.
To validate the discovery, the team—including Chelsea X. Huang and Shishir Dholakia—performed a rigorous analysis to rule out "false positives," such as background eclipsing binaries or instrumental glitches. By cross-referencing K2 photometry with historical imaging, archival radial velocity data, and astrometry, the researchers concluded that the signal was astrophysical in nature and occurred on the target star. "Our analysis strongly indicates that the event was astrophysical, occurred on-target, and can be best explained by a transiting planet candidate," the authors noted in their report.
Defining a Single-Transit Candidate
The transit method relies on a planet passing between its host star and the observer, causing a temporary drop in the star's apparent brightness. While automated pipelines are excellent at finding planets with short orbital periods that transit many times, they often overlook single events. Single-transit detections are technically challenging because they do not provide an immediate orbital period. Instead, researchers must estimate the period based on the duration of the transit and the known properties of the host star.
In the case of HD 137010 b, the 10-hour transit duration provided a vital clue. Given the size and mass of the host K-dwarf, a transit of this length implies a wide orbit. If the orbit is circular, the team estimates an orbital period of approximately 355 days—remarkably close to a terrestrial year. This makes HD 137010 b a rare example of a long-period planet detected via a single event, a feat that has rarely been achieved for Earth-sized candidates orbiting Sun-like stars.
Physical Profile: An Earth-Sized Neighbor
The physical characteristics of HD 137010 b place it in an exclusive category of "Earth-sized" exoplanets. The research team calculated a radius of 1.06 times that of Earth, making it a near-twin of our home planet in terms of scale. Its host star, HD 137010, is a K-dwarf, which is slightly smaller, cooler, and longer-lived than our Sun. This type of star is increasingly seen as an ideal host for potentially habitable worlds because they provide a more stable radiation environment over billions of years.
The estimated orbital distance for the candidate is roughly 0.88 Astronomical Units (AU), placing it at a distance from its star comparable to the Earth-Sun separation. Because the host star is less luminous than the Sun, this distance results in a much cooler environment than Earth's. The discovery represents a significant milestone, as most Earth-sized planets found by current missions like TESS orbit much smaller M-dwarf stars or have extremely short, scorching orbital periods.
Habitability and Temperature Estimates
Temperature is a critical factor in determining the habitability of a world. Based on its projected orbit, HD 137010 b receives an incident stellar flux of approximately 0.29 times that of Earth. This puts the planet near the outer edge of the star’s habitable zone—the region where liquid water could theoretically exist on a planetary surface. Depending on its atmospheric composition, HD 137010 b might be a "cool" Earth or perhaps more akin to a temperate version of Mars.
While the researchers caution that the exact orbital period remains a "projected" value due to the single-transit nature of the data, the candidate's properties are nonetheless enticing. Even if it sits at the chilly fringe of the habitable zone, it remains one of the few Earth-sized candidates orbiting a Sun-like (FGK) star that is not a "hot Earth." The presence of such a world suggests that the K2 mission’s data still contains untapped potential for identifying temperate, terrestrial environments.
The Importance of Bright Host Stars
One of the most exciting aspects of HD 137010 b is the brightness of its host star. At a V-magnitude of 10.1, HD 137010 is significantly brighter than the distant stars targeted by the original Kepler mission. Brightness is the "currency" of exoplanetary follow-up; it allows for high-precision radial velocity measurements to determine the planet's mass and enables future atmospheric characterization via spectroscopy.
Steve B. Howell of the NASA Ames Research Center and other co-authors emphasize that this is the first Earth-sized planet candidate with an Earth-like orbital period transiting a star bright enough for substantial follow-up. Most previous Kepler discoveries of this type are too faint for current telescopes to weigh the planets or probe their air. HD 137010 b, however, offers a reachable target for the next generation of astronomical instruments.
The Legacy of K2 and Future Directions
The discovery of HD 137010 b serves as a testament to the legacy of the Kepler Space Telescope. Even years after its mission ended, its data remains a goldmine for discovery when paired with modern analytical techniques and the dedication of citizen scientists. By bridging the gap between the short-period discoveries of TESS and the deep-space stare of the original Kepler mission, K2 has provided a unique window into the population of long-period terrestrial planets.
Looking ahead, the research team suggests that future missions like PLATO (Planetary Transits and Oscillations of stars), expected to launch in 2026, will be specifically designed to find more worlds like HD 137010 b. Until then, HD 137010 b stands as a high-priority candidate for ground-based observatories. Confirming its 355-day period and measuring its mass will be the next steps in understanding whether this distant world is truly an Earth-like sibling or a frozen relic in the outer reaches of its solar system.
