K2-121 b
Astrophysical Synthesis & Habitability Characterization
K2-121 b is a confirmed gas giant exoplanet orbiting the star K2-121, located approximately 552.5 light-years (169.4 parsecs) from the Solar System. First announced in 2017 by K2 using the Transit detection technique (producing an estimated primary transit depth of 0.9457% across the stellar disk), this world measures 7.16 Earth radii (0.64 Jupiter radii) and has a measured mass of 51 Earth masses (0.16 Jupiter masses). Combining its mass and radius yields a mean bulk density of 0.77 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 0.99 g⊕ (9.71 m/s²), and an escape velocity of 29.9 km/s.
In its orbital architecture around K2-121, K2-121 b completes one full revolution every 5.186 Earth days at a semi-major axis of 0.0525 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 110.1 km/s (compared to Earth's 29.8 km/s). Its moderate orbital eccentricity of e = 0.22 carries the world between 0.041 AU at periapsis and 0.0641 AU at apoapsis, driving measurable seasonal insolation cycles.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 716 K (+443°C / +829°F) and receiving 62.50 S⊕ (~85,063 W/m²), K2-121 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star K2-121 is classified as a stellar host with an effective photospheric temperature of 4,526 K, a radius of 0.675 R☉, a mass of 0.71 M☉, an estimated bolometric luminosity of 0.172 L☉. Stellar evolution modeling indicates a metallicity of [Fe/H] = 0.04 dex (near-solar heavy-element abundance). From Earth, K2-121 shines at apparent V-band magnitude 13.229 (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 | K2-121 |
| Spectral Type | Unclassified |
| Effective Temp (Teff) | 4,526 K |
| Stellar Radius | 0.6750 R☉ |
| Stellar Mass | 0.7100 M☉ |
| Bolometric Luminosity | 0.172 L☉ |
| Metallicity [Fe/H] | 0.040 dex |
| Stellar Age | — |
| Apparent Magnitude | V: 13.229 · G: 12.925 |
| System Architecture | 1 Star, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.052500 AU |
| Orbital Period | 5.186 days |
| Mean Orbital Velocity | 110.1 km/s |
| Orbital Eccentricity (e) | 0.22000 |
| Orbital Inclination (i) | 89.0266° |
| Mean Bulk Density | 0.77 g/cm³ (29.9 km/s vesc) |
| Insolation Flux | 62.5000 S⊕ (~85,063 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 552.500 LY (169.398 pc) |