K2-321 b
Astrophysical Synthesis & Habitability Characterization
K2-321 b is a confirmed sub-neptune exoplanet orbiting the star K2-321, located approximately 253.4 light-years (77.69 parsecs) from the Solar System. First announced in 2020 by K2 using the Transit detection technique (producing an estimated primary transit depth of 0.0999% across the stellar disk), this world measures 2.051 Earth radii and has a measured mass of 4.86 Earth masses. Combining its mass and radius yields a mean bulk density of 3.11 g/cm³ (an intermediate density indicative of a deep water-ice mantle or dense volatile envelope), a surface gravity of 1.16 g⊕ (11.38 m/s²), and an escape velocity of 17.2 km/s.
In its orbital architecture around K2-321, K2-321 b completes one full revolution every 2.298 Earth days at a semi-major axis of 0.02839 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 134.4 km/s (compared to Earth's 29.8 km/s). The planet follows a highly eccentric trajectory (e = 0.44, swinging from 0.0159 AU at periapsis out to 0.0409 AU at apoapsis), pointing to past dynamical scattering or secular gravitational perturbations.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 830 K (+557°C / +1035°F) and receiving 68.55 S⊕ (~93,299 W/m²), K2-321 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star K2-321 is classified as a stellar host with an effective photospheric temperature of 3,750 K, a radius of 0.595 R☉, a mass of 0.578 M☉, an estimated bolometric luminosity of 0.063 L☉. Stellar evolution modeling indicates a metallicity of [Fe/H] = 0.148 dex (metal-rich relative to the Sun). From Earth, K2-321 shines at apparent V-band magnitude 13.677 (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 | K2-321 |
| Spectral Type | Unclassified |
| Effective Temp (Teff) | 3,750 K |
| Stellar Radius | 0.5950 R☉ |
| Stellar Mass | 0.5780 M☉ |
| Bolometric Luminosity | 0.063 L☉ |
| Metallicity [Fe/H] | 0.148 dex |
| Stellar Age | — |
| Apparent Magnitude | V: 13.677 · G: 12.824 |
| System Architecture | 1 Star, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.028390 AU |
| Orbital Period | 2.298 days |
| Mean Orbital Velocity | 134.4 km/s |
| Orbital Eccentricity (e) | 0.44000 |
| Orbital Inclination (i) | 86.1000° |
| Mean Bulk Density | 3.11 g/cm³ (17.2 km/s vesc) |
| Insolation Flux | 68.5516 S⊕ (~93,299 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 253.400 LY (77.689 pc) |