K2-81 b
Astrophysical Synthesis & Habitability Characterization
K2-81 b is a confirmed super-earth exoplanet orbiting the star K2-81, located approximately 823 light-years (252.32 parsecs) from the Solar System. First announced in 2016 by K2 using the Transit detection technique (producing an estimated primary transit depth of 0.081% across the stellar disk), this world measures 1.97 Earth radii and has a measured mass of 4.54 Earth masses. Combining its mass and radius yields a mean bulk density of 3.27 g/cm³ (an intermediate density indicative of a deep water-ice mantle or dense volatile envelope), a surface gravity of 1.17 g⊕ (11.47 m/s²), and an escape velocity of 17 km/s.
In its orbital architecture around K2-81, K2-81 b completes one full revolution every 6.102 Earth days at a semi-major axis of 0.059 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 105.2 km/s (compared to Earth's 29.8 km/s). Its moderate orbital eccentricity of e = 0.06 carries the world between 0.0555 AU at periapsis and 0.0625 AU at apoapsis, driving measurable seasonal insolation cycles.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 739 K (+466°C / +871°F) and receiving 62.90 S⊕ (~85,607 W/m²), K2-81 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star K2-81 is classified as a stellar host with an effective photospheric temperature of 4,674 K, a radius of 0.6347 R☉, a mass of 0.7453 M☉, an estimated bolometric luminosity of 0.172 L☉. From Earth, K2-81 shines at apparent V-band magnitude 14.314 (faint and requiring research-grade or CCD telescopic photometry) in the Northern celestial hemisphere.
Methodology: Computed using the Schulze-Makuch et al. (2011) weighted metric comparing physical interior (radius, estimated density) and surface thermal regime (escape velocity, equilibrium temperature).
Discovery Facility: K2 · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | K2-81 |
| Spectral Type | Unclassified |
| Effective Temp (Teff) | 4,674 K |
| Stellar Radius | 0.6347 R☉ |
| Stellar Mass | 0.7453 M☉ |
| Bolometric Luminosity | 0.172 L☉ |
| Metallicity [Fe/H] | — |
| Stellar Age | — |
| Apparent Magnitude | V: 14.314 · G: 13.904 |
| System Architecture | 1 Star, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.059000 AU |
| Orbital Period | 6.102 days |
| Mean Orbital Velocity | 105.2 km/s |
| Orbital Eccentricity (e) | 0.06000 |
| Orbital Inclination (i) | 89.2900° |
| Mean Bulk Density | 3.27 g/cm³ (17 km/s vesc) |
| Insolation Flux | 62.9000 S⊕ (~85,607 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 823.000 LY (252.318 pc) |