HD 101581 c is an Earth-size exoplanet orbiting a nearby K-type main-sequence star located in the constellation Centaurus, approximately 12.8 parsecs (about 41–42 light-years) from Earth. It is part of a compact planetary system discovered through high-precision photometric observations from the Transiting Exoplanet Survey Satellite (TESS), which has revealed at least two confirmed Earth-sized planets and a possible third candidate in closely spaced, short-period orbits. The system has become a notable example of the so-called “peas-in-a-pod” architecture, where planets are similar in size and regularly spaced in orbital period ratios, a pattern that may offer important clues about how small planets assemble and evolve around Sun-like and slightly cooler stars.
The host star, HD 101581 (also catalogued as TOI-6276 and GJ 435), is a K5V-type dwarf star with a mass of roughly 0.65–0.74 times that of the Sun and a surface temperature around 4600 K, giving it an orange hue. It is older than the Sun at an estimated age of several billion years and exhibits relatively modest magnetic activity compared to younger K dwarfs. Its brightness (apparent magnitude around 7.8) makes it too faint to be seen with the naked eye but accessible with small telescopes. Its relatively quiet nature and proximity make it a valuable target for detailed exoplanet characterization, particularly atmospheric studies using transmission spectroscopy.
HD 101581 c itself has a radius very close to that of Earth, estimated at about 0.99 Earth radii, placing it firmly in the terrestrial planet regime rather than as a mini-Neptune or sub-Neptune. Its orbital period is approximately 6.2 days, meaning it completes a full orbit around its host star in less than a week. This places it extremely close to the star, at a distance of only about 0.057 astronomical units, far inside the orbit of Mercury in our own Solar System. As a result, HD 101581 c is classified as a hot Earth or ultra-short-period terrestrial planet candidate, receiving intense stellar irradiation that would likely render its surface extremely hot and inhospitable to Earth-like life as we know it.
The planet was detected via the transit method, in which astronomers measure the slight dimming of a star’s light when a planet passes in front of it from our line of sight. Follow-up analysis of TESS data confirmed the periodic nature of the signal and helped distinguish it from stellar variability or instrumental noise. Its discovery was part of a broader effort to characterize small, rocky exoplanets around nearby bright stars, which are particularly valuable because they allow more precise follow-up observations than planets around distant or faint stars.
HD 101581 c orbits in a dynamically interesting configuration with at least one other confirmed Earth-sized planet, HD 101581 b, which has a slightly shorter orbital period of about 4.47 days. The two planets are close to a 4:3 mean-motion resonance, meaning their orbital periods form a near-integer ratio that suggests past orbital migration and gravitational interaction during their formation or early evolution. A third signal, TOI-6276.03, has also been reported with a period of about 7.9 days, though its planetary status has been considered a strong candidate rather than fully confirmed in earlier analyses.
Despite their small sizes and close-in orbits, the planets in the HD 101581 system are remarkably uniform, with radii clustered around Earth size. This uniformity, combined with their regular spacing, supports emerging theories that many compact planetary systems form in relatively calm, disk-driven environments where planets grow and migrate without violent scattering events. Such systems contrast with the more chaotic architectures seen in some other exoplanetary systems containing large gas giants or highly eccentric orbits.
Although HD 101581 c is not considered habitable due to its extreme proximity to its star and likely high surface temperatures, its scientific importance lies in its composition and accessibility. Earth-sized exoplanets around nearby bright stars are prime candidates for atmospheric studies using current and future space telescopes. If HD 101581 c retains any atmosphere, it could be probed for its chemical composition, thermal structure, and potential signs of atmospheric loss driven by stellar radiation. These measurements would contribute to a broader understanding of how rocky planets evolve under intense stellar irradiation.
Overall, HD 101581 c represents a key data point in the rapidly expanding census of small exoplanets in our galactic neighborhood. Its combination of Earth-like size, ultra-short orbital period, and membership in a compact multi-planet system makes it an important target for comparative planetology, helping scientists refine models of planet formation and the diversity of terrestrial worlds beyond our Solar System.
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