HD 101581 b is a confirmed terrestrial exoplanet orbiting the K-type main-sequence star HD 101581, also catalogued as GJ 435, TOI-6276, and TIC 397362481. Located at a distance of roughly 12.8 parsecs from Earth, or about 41.7 light-years, the system has drawn significant scientific attention because it hosts multiple Earth-size planets in tightly packed orbits that resemble a “peas-in-a-pod” architectural pattern commonly seen in compact exoplanetary systems. HD 101581 b itself represents one of the most precisely characterized members of this system and provides an important example of a small, close-in terrestrial world orbiting a relatively bright nearby star.
The planet was identified through transit observations, primarily using data from NASA’s Transiting Exoplanet Survey Satellite (TESS), which detects periodic dips in starlight caused when a planet passes in front of its host star. HD 101581 b has a measured radius of approximately 0.96 Earth radii and a mass of about 0.83 Earth masses, making it slightly smaller and less massive than Earth. These measurements place it firmly in the category of Earth-size rocky planets, rather than mini-Neptunes or volatile-rich sub-Neptunes. Its orbital period is exceptionally short at roughly 4.5 days, meaning it completes a full revolution around its star in less than a week.
The planet orbits extremely close to its host star, at a distance of about 0.046 astronomical units, which is roughly 4.6% of the Earth–Sun distance. At such proximity, HD 101581 b is expected to be heavily irradiated by its star, leading to equilibrium temperatures that likely place it far outside the traditional habitable zone. Estimates suggest a surface equilibrium temperature on the order of hundreds of kelvin above Earth’s average, making it a hot, likely airless or thin-atmosphere rocky world depending on its atmospheric retention and composition. Despite these harsh conditions, planets like HD 101581 b are scientifically valuable because they allow researchers to study the physical properties of terrestrial planets under extreme stellar irradiation, helping refine models of atmospheric loss and planetary geology.
HD 101581 b is part of a tightly packed multi-planet system that includes at least one other confirmed Earth-size planet, HD 101581 c, with a third candidate signal also reported in the same system. The orbital periods of these planets are close to a 4:3 mean-motion resonance, meaning their orbital periods are in a ratio that suggests long-term gravitational interactions and a dynamically ordered architecture. This resonance-like spacing is of particular interest to astronomers because it provides evidence of planetary migration during system formation, where planets likely formed farther out and moved inward while maintaining stable orbital relationships. The uniformity in planetary sizes and spacing has made the system a textbook example of compact planetary system formation.
The host star, HD 101581, is a K5V dwarf star that is cooler and smaller than the Sun. Its relatively modest size and brightness make it an excellent target for transit studies, as planetary signals are easier to detect and characterize compared to those around larger or more variable stars. The system’s brightness, combined with its proximity to Earth, enhances its value for follow-up observations, including potential atmospheric characterization using current and future space telescopes. Because HD 101581 is one of the brightest known stars hosting multiple transiting Earth-size planets, it stands out as a benchmark system for comparative planetology of small rocky worlds.
The discovery and validation of HD 101581 b were part of a broader effort to identify Earth-size planets around nearby stars using high-precision photometry and statistical validation techniques. Rather than relying on a single detection method, researchers combined space-based transit data with ground-based follow-up observations to rule out false positives such as eclipsing binary stars or instrumental artifacts. This multi-layered validation approach has become standard in modern exoplanet science, especially for small planets where signals are subtle and require careful confirmation.
In the broader context of exoplanet research, HD 101581 b contributes to a growing catalog of small, rocky planets orbiting K-type stars, which are often considered particularly interesting targets in the search for potentially habitable environments. Although HD 101581 b itself is too close to its star to host liquid water on its surface under Earth-like atmospheric assumptions, studying such planets helps constrain the boundaries of planetary habitability and improves understanding of how rocky planets evolve under different stellar conditions.
As observational capabilities improve, systems like HD 101581 are expected to become prime targets for atmospheric spectroscopy. If the planet retains any atmosphere, future instruments may be able to probe its composition, searching for signatures of volcanic outgassing, atmospheric escape, or even unexpected chemical disequilibrium. Even in the absence of a substantial atmosphere, precise measurements of its mass and radius contribute to refining models of rocky planet structure and composition, helping to place Earth in a broader galactic context.
HD 101581 b therefore represents more than just a single exoplanet discovery; it is a key data point in understanding how small, rocky planets form, migrate, and survive in tightly packed systems around common K-type stars.
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