Kepler-121 b
Astrophysical Synthesis & Habitability Characterization
Kepler-121 b is a confirmed sub-neptune exoplanet orbiting the star Kepler-121, located approximately 1665.1 light-years (510.52 parsecs) from the Solar System. First announced in 2014 by Kepler using the Transit detection technique (producing an estimated primary transit depth of 0.0937% across the stellar disk), this world measures 2.34 Earth radii and has a measured mass of 6.08 Earth masses. Combining its mass and radius yields a mean bulk density of 2.62 g/cm³ (an intermediate density indicative of a deep water-ice mantle or dense volatile envelope), a surface gravity of 1.11 g⊕ (10.89 m/s²), and an escape velocity of 18 km/s.
In its orbital architecture around Kepler-121, Kepler-121 b completes one full revolution every 3.177 Earth days at a semi-major axis of 0.039 AU (an ultra-close orbit less than one-seventh of Mercury's 0.387 AU distance from the Sun, where strong stellar tidal forces typically induce synchronous rotation). Its mean orbital velocity is approximately 133.5 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 997 K (+724°C / +1335°F) and receiving 220.49 S⊕ (~300,086 W/m²), Kepler-121 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star Kepler-121 is classified as a stellar host with an effective photospheric temperature of 5,311 K, a radius of 0.701 R☉, a mass of 0.859 M☉, an estimated bolometric luminosity of 0.351 L☉. Stellar evolution modeling indicates an estimated stellar age of 0.5 Gyr and a metallicity of [Fe/H] = -0.08 dex (near-solar heavy-element abundance). From Earth, Kepler-121 shines at apparent V-band magnitude 14.904 (faint and requiring research-grade or CCD telescopic photometry) in the Northern celestial hemisphere. The Kepler-121 system is a multi-planet architecture hosting 2 confirmed worlds, with Kepler-121 b sharing its stellar neighborhood alongside Kepler-121 c.
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-121 |
| Spectral Type | Unclassified |
| Effective Temp (Teff) | 5,311 K |
| Stellar Radius | 0.7010 R☉ |
| Stellar Mass | 0.8590 M☉ |
| Bolometric Luminosity | 0.351 L☉ |
| Metallicity [Fe/H] | -0.080 dex |
| Stellar Age | 0.500 Gyr |
| Apparent Magnitude | V: 14.904 · G: 14.677 |
| System Architecture | 1 Star, 2 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.039000 AU |
| Orbital Period | 3.177 days |
| Mean Orbital Velocity | 133.5 km/s |
| Orbital Eccentricity (e) | 0.00000 |
| Orbital Inclination (i) | 85.7900° |
| Mean Bulk Density | 2.62 g/cm³ (18 km/s vesc) |
| Insolation Flux | 220.4890 S⊕ (~300,086 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 1665.100 LY (510.523 pc) |