HD 137010 b

Lore & Background

HD 137010 b was first flagged by citizen scientists in the Planet Hunters project, after automated algorithms overlooked the signal. Astrophysicist Alexander Venner re-examined the K2 Campaign 15 data during his Ph.D. research at the University of Southern Queensland, leading to the identification of the single 10-hour transit recorded in 2017. The discovery team, including collaborators from the Max Planck Institute for Astronomy, ruled out false positives such as stellar binaries through detailed modeling. The findings were published on January 27, 2026, in The Astrophysical Journal Letters, and Venner presented the results at the Rocky Worlds conference.

The host star HD 137010 is a K-type dwarf with a visual magnitude of 10.1, observable with amateur telescopes. It has an effective temperature about 1,000 K cooler than the Sun, with roughly 70% of the Sun's mass and radius, and correspondingly lower luminosity. This shifts the habitable zone inward. HD 137010 b has an estimated radius of 1.06 R_Earth, a nearly circular orbit, and receives only 29% of the incident flux Earth does, resulting in a blackbody equilibrium temperature of about −68°C. No mass or density measurements are available, but its size suggests a rocky composition.

Positioned at the outer boundary of its star's habitable zone, HD 137010 b may support liquid water under a thick, greenhouse-enhanced atmosphere rich in CO2, potentially resembling a super-Venus or early Martian environment. Atmospheric models indicate a 40% probability of residing in the conservative habitable zone and 51% in the optimistic zone, but a comparable chance of being entirely too cold for surface habitability without extreme greenhouse forcing. The planet remains an unconfirmed candidate.

Reader's Guide

HD 137010 b is significant as a rare example of an Earth-sized exoplanet candidate discovered from a single transit event, highlighting the value of citizen science and manual re-examination of archival data. Its orbital period nearly identical to Earth's and its location near the outer edge of the habitable zone make it a compelling target for follow-up studies, despite the challenges posed by its long orbital period. The candidate's relatively bright host star (magnitude 10.1) facilitates potential spectroscopic characterization of any atmosphere using future observatories like the James Webb Space Telescope, which could detect biosignatures from subsurface oceans or geothermal activity if present. However, confirmation as a genuine exoplanet requires additional transits or alternative observations, which may be pursued with missions like TESS or CHEOPS. Its legacy lies in demonstrating that small, cool planets around K-dwarfs can be detected even with limited transit data, and in motivating targeted monitoring strategies for long-period candidates.

Did You Know?

• Its orbital period of about 355 days is nearly identical to Earth's.
• The planet receives only 29% of the incident flux that Earth does, giving it an equilibrium temperature of −68°C.

Discovery and Data Provenance

The identification of HD 137010 b emerged from archival Kepler K2 photometric data collected in 2017, yet the finding was not formally reported until 27 January 2026. This roughly nine-year interval between data acquisition and public announcement is characteristic of exoplanet discovery pipelines, where raw time-series light curves must be carefully reprocessed, vetted, and cross-checked before a signal is elevated to the status of a candidate. The planet was flagged specifically as a transiting exoplanet candidate rather than a confirmed detection, a distinction that reflects the scientific community's ongoing caution in separating genuine planetary signals from stellar variability or instrumental artifacts. The reliance on the K2 mission's legacy dataset also underscores how earlier space-based photometry continues to yield new targets long after the spacecraft's operational phase has ended, extending the scientific return of the investment originally made in the mission.

Physical Characterization

HD 137010 b is described as a cool, Earth-sized world, placing it in a size category of particular interest to astronomers searching for terrestrial planets beyond the Solar System. The designation cool indicates that the planet's equilibrium temperature sits at a lower end compared to many other transiting exoplanets, a trait consistent with its orbital placement near the outer boundary of the habitable zone. Its Earth-sized radius is broadly consistent with a rocky bulk composition, though the candidate status means that composition has not been independently verified through atmospheric characterization or dynamical mass measurement. The transiting nature of the planet—meaning it periodically passes in front of its host star as seen from Earth—provides the geometric alignment necessary for transit photometry, the very method that revealed its presence in the K2 light curves. This alignment also opens the door to future transmission spectroscopy should the candidate be confirmed and the geometry permits.

Orbital Position and the Habitable Zone

One of the most notable aspects of HD 137010 b's orbit is its placement near the outer edge of its host star's habitable zone. This region represents the orbital distance at which a planet with a suitable atmosphere could, in principle, maintain liquid water on its surface. Being situated at the outer boundary rather than the center or inner edge carries specific implications: the planet receives less stellar flux, which aligns with its classification as cool, and it sits in a regime where even modest changes in the star's luminosity or the planet's atmospheric composition could shift it in or out of the liquid-water window. The outer-edge position also implies a relatively long orbital period compared to close-in super-Earths or hot Jupiters, granting it a more temperate seasonal rhythm if it possesses an atmosphere. This orbital context is what elevates HD 137010 b from a mere catalog entry to a target of genuine astrobiological interest.

Distance and Observational Context

At a distance of 146 light years, HD 137010 b resides in the galactic neighborhood—close enough to be within the reach of current and near-future ground-based and space-based telescopes for follow-up observations, yet far enough that any light we observe from it today left the system roughly a century and a half ago. This distance places it well within the Milky Way's disk but far beyond the reach of any conceivable direct imaging with present technology; the planet's signal was instead extracted from the subtle dimming of its host star during transits recorded by the Kepler K2 photometer. The 146-light-year separation also means the planet belongs to a different stellar system entirely, orbiting a star unrelated to the Sun, and that any future characterization—whether confirming its candidate status, measuring its mass, or probing for an atmosphere—will depend on the angular resolution and sensitivity of instruments that must work at interstellar distances.