Kepler-81 b
Astrophysical Synthesis & Habitability Characterization
Kepler-81 b is a confirmed sub-neptune exoplanet orbiting the star Kepler-81, located approximately 1136 light-years (348.29 parsecs) from the Solar System. First announced in 2012 by Kepler using the Transit detection technique (producing an estimated primary transit depth of 0.1414% across the stellar disk), this world measures 2.42 Earth radii and has a measured mass of 17.2 Earth masses. Combining its mass and radius yields a mean bulk density of 6.69 g/cm³ (an iron-enriched, high-compression rocky core), a surface gravity of 2.94 g⊕ (28.83 m/s²), and an escape velocity of 29.8 km/s.
In its orbital architecture around Kepler-81, Kepler-81 b completes one full revolution every 5.955 Earth days at a semi-major axis of 0.0522 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 95.4 km/s (compared to Earth's 29.8 km/s). The orbit is modeled as circular (e = 0.00), maintaining a constant stellar distance throughout its year.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 635 K (+362°C / +684°F) and receiving 39.95 S⊕ (~54,372 W/m²), Kepler-81 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star Kepler-81 is classified as a low-mass red dwarf of spectral type M0 V with an effective photospheric temperature of 4,385 K, a radius of 0.59 R☉, a mass of 0.64 M☉, an estimated bolometric luminosity of 0.115 L☉. Stellar evolution modeling indicates an estimated stellar age of 3.63 Gyr and a metallicity of [Fe/H] = -0.18 dex (metal-poor relative to the Sun). From Earth, Kepler-81 shines at apparent V-band magnitude 15.258 (faint and requiring research-grade or CCD telescopic photometry) in the Northern celestial hemisphere. The Kepler-81 system is a multi-planet architecture hosting 3 confirmed worlds, with Kepler-81 b sharing its stellar neighborhood alongside Kepler-81 c, Kepler-81 d.
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: Kepler · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | Kepler-81 |
| Spectral Type | M0 V |
| Effective Temp (Teff) | 4,385 K |
| Stellar Radius | 0.5900 R☉ |
| Stellar Mass | 0.6400 M☉ |
| Bolometric Luminosity | 0.115 L☉ |
| Metallicity [Fe/H] | -0.180 dex |
| Stellar Age | 3.630 Gyr |
| Apparent Magnitude | V: 15.258 · G: 15.004 |
| System Architecture | 1 Star, 3 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.052200 AU |
| Orbital Period | 5.955 days |
| Mean Orbital Velocity | 95.4 km/s |
| Orbital Eccentricity (e) | 0.00000 |
| Orbital Inclination (i) | 89.0000° |
| Mean Bulk Density | 6.69 g/cm³ (29.8 km/s vesc) |
| Insolation Flux | 39.9500 S⊕ (~54,372 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 1136.000 LY (348.291 pc) |