Kepler-681 b
Astrophysical Synthesis & Habitability Characterization
Kepler-681 b is a confirmed sub-neptune exoplanet orbiting the star Kepler-681, located approximately 2240.4 light-years (686.92 parsecs) from the Solar System. First announced in 2016 by Kepler using the Transit detection technique (producing an estimated primary transit depth of 0.138% across the stellar disk), this world measures 3.08 Earth radii (0.27 Jupiter radii) and has a measured mass of 9.69 Earth masses. Combining its mass and radius yields a mean bulk density of 1.83 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 1.02 g⊕ (10 m/s²), and an escape velocity of 19.8 km/s.
In its orbital architecture around Kepler-681, Kepler-681 b completes one full revolution every 26.394 Earth days at a semi-major axis of 0.1635 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 67.4 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 489 K (+216°C / +421°F) and receiving 14.15 S⊕ (~19,260 W/m²), Kepler-681 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star Kepler-681 is classified as a stellar host with an effective photospheric temperature of 5,021 K, a radius of 0.76 R☉, a mass of 0.8 M☉, an estimated bolometric luminosity of 0.329 L☉. Stellar evolution modeling indicates an estimated stellar age of 5.01 Gyr and a metallicity of [Fe/H] = -0.04 dex (near-solar heavy-element abundance). From Earth, Kepler-681 shines at apparent V-band magnitude 15.584 (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: Kepler · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | Kepler-681 |
| Spectral Type | Unclassified |
| Effective Temp (Teff) | 5,021 K |
| Stellar Radius | 0.7600 R☉ |
| Stellar Mass | 0.8000 M☉ |
| Bolometric Luminosity | 0.329 L☉ |
| Metallicity [Fe/H] | -0.040 dex |
| Stellar Age | 5.010 Gyr |
| Apparent Magnitude | V: 15.584 · G: 15.227 |
| System Architecture | 1 Star, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.163500 AU |
| Orbital Period | 26.394 days |
| Mean Orbital Velocity | 67.4 km/s |
| Orbital Eccentricity (e) | 0.00000 |
| Orbital Inclination (i) | 89.2800° |
| Mean Bulk Density | 1.83 g/cm³ (19.8 km/s vesc) |
| Insolation Flux | 14.1510 S⊕ (~19,260 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 2240.400 LY (686.923 pc) |