K2-357 b
Astrophysical Synthesis & Habitability Characterization
K2-357 b is a confirmed sub-neptune exoplanet orbiting the star K2-357, located approximately 1688.5 light-years (517.69 parsecs) from the Solar System. First announced in 2021 by K2 using the Transit detection technique (producing an estimated primary transit depth of 0.1163% across the stellar disk), this world measures 3.72 Earth radii (0.33 Jupiter radii) and has a measured mass of 13.4 Earth masses. Combining its mass and radius yields a mean bulk density of 1.44 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 0.97 g⊕ (9.51 m/s²), and an escape velocity of 21.2 km/s.
In its orbital architecture around K2-357, K2-357 b completes one full revolution every 16.349 Earth days at a semi-major axis of 0.145 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 96.5 km/s (compared to Earth's 29.8 km/s).
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 735 K (+462°C / +864°F) and receiving 40.89 S⊕ (~55,654 W/m²), K2-357 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star K2-357 is classified as a stellar host with an effective photospheric temperature of 5,794 K, a radius of 1 R☉, a mass of 0.98 M☉, an estimated bolometric luminosity of 1.011 L☉. Stellar evolution modeling indicates a metallicity of [Fe/H] = -0.02 dex (near-solar heavy-element abundance). From Earth, K2-357 shines at apparent V-band magnitude 13.748 (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 | K2-357 |
| Spectral Type | Unclassified |
| Effective Temp (Teff) | 5,794 K |
| Stellar Radius | 1.0000 R☉ |
| Stellar Mass | 0.9800 M☉ |
| Bolometric Luminosity | 1.011 L☉ |
| Metallicity [Fe/H] | -0.020 dex |
| Stellar Age | — |
| Apparent Magnitude | V: 13.748 · G: 13.479 |
| System Architecture | 1 Star, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.145000 AU |
| Orbital Period | 16.349 days |
| Mean Orbital Velocity | 96.5 km/s |
| Orbital Eccentricity (e) | Circular (0.00) |
| Orbital Inclination (i) | — |
| Mean Bulk Density | 1.44 g/cm³ (21.2 km/s vesc) |
| Insolation Flux | 40.8918 S⊕ (~55,654 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 1688.500 LY (517.687 pc) |