Planet Hunters citizen science project participants made the discovery of a single 10-hour dip in starlight using NASA’s archived Kepler space telescope data.
This dip is characteristic of an Earth-like exoplanet moving across its host star with a unique physical appearance.
This potential planet orbits a sun-like star about 146 light-years away from Earth in a large orbit, and the entire surface of the body is expected to be permanently frozen.
Uncovering hidden planetary signals inside archival telescope data
Planet Hunters volunteers visually inspected thousands of stellar light curves recorded during NASA’s K2 / K2 Campaign 15.
Automated search algorithms missed the subtle 10-hour dimming event because the planet transited its host star only once during the multi-year observation window.
Human volunteers reviewing light graphs flagged the isolated starlight drop as a promising planetary transit candidate.
Archival cross-matching confirmed the host star sits approximately 146 light-years from Earth in our local interstellar neighborhood.
Rigorous statistical validation reduced potential false positives, including instrument artifacts, background eclipsing binary stars, and intrinsic stellar variability.
Human visual inspection pulled a hidden planetary candidate out of millions of complex astronomical data points.
Estimating physical size and orbit from a single light dip
Planet Hunters volunteers manually analyzed thousands of star light curves obtained during NASA’s Kepler space telescope’s K2 / K2 Campaign 15.
An automatic search algorithm could not detect the faint 10-hour dimming event since the planet transited its parent star only once.
The human volunteers analyzing the light curves recognized the isolated starlight decrease as a strong transit signal candidate.
Historical archival crossmatching determined the star’s distance from Earth.
Stringent statistical verification ruled out any false positives, such as instrumental anomalies, eclipsing binaries due to background stars, or variability of the star itself.
A human eye sifted the planetary candidate out of millions of astronomical data points.
Modeling thermal conditions across a frigid terrestrial world
The comparison between the new planet and Earth was carried out because both revolve around sun-like stars at similar distances.
Spectroscopy results have shown that the parent star emits slightly cooler radiation compared to our Sun.
Receiving less energy from the star results in the planet moving further away from the warm range where liquid water existence is possible.
The thermal models predict a dark planet covered by global ice layers instead of oceans.
Less energy from the star turns an Earth-like planet into an ice-covered desert.
Confronting the observational limits of single transit events
The detection of one 10-hour transit event proves that the object passed in front of its parent star while Kepler was in operation.
One observation is not sufficient to confirm the precise orbital period of the planet since two repeat transit cycles need to be observed, according to NASA Science.
The ground-based telescopes cannot spot such faint starlight dimming events due to Earth’s atmosphere.
The radial velocities of the star system cannot provide us with the mass of the planet because of insufficient gravitational pull from the orbit.
Unconfirmed single-transit objects are among the top priority targets for study.
Targeting cold rocky exoplanets with future space missions
Identifying cold terrestrial bodies will allow scientists to explore the complete range of planetary system configurations in our galaxy.
Traditional transit searches inevitably result in the discovery of hot planets close to their parent stars because they regularly move across them.
Future space missions will observe the planetary system to detect additional transits or gravitational effects.
In the future, analysis of the planet’s atmosphere will reveal if volatile gases are contained within its frozen core or atmosphere.
A one-off drop in the brightness of the star provides an opportunity to investigate cold terrestrial planets moving through space.
The details of the full study can be found here: Beard, C. (2026). A cool Earth-sized planet candidate transiting a tenth-magnitude K-dwarf from K2. The Astrophysical Journal Letters. doi, 10, 2041-8213.
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