K2-259 b
Astrophysical Synthesis & Habitability Characterization
K2-259 b is a confirmed sub-neptune exoplanet orbiting the star K2-259, located approximately 1358.3 light-years (416.46 parsecs) from the Solar System. First announced in 2018 by K2 using the Transit detection technique (producing an estimated primary transit depth of 0.0354% across the stellar disk), this world measures 2.32 Earth radii and has a measured mass of 5.99 Earth masses. Combining its mass and radius yields a mean bulk density of 2.65 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 K2-259, K2-259 b completes one full revolution every 15.48 Earth days at a semi-major axis of 0.1271 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 89.3 km/s (compared to Earth's 29.8 km/s).
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 795 K (+522°C / +972°F) and receiving 90.95 S⊕ (~123,787 W/m²), K2-259 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star K2-259 is classified as a stellar host with an effective photospheric temperature of 6,059 K, a radius of 1.13 R☉, a mass of 1.14 M☉, an estimated bolometric luminosity of 1.544 L☉. Stellar evolution modeling indicates a metallicity of [Fe/H] = 0.16 dex (metal-rich relative to the Sun). From Earth, K2-259 shines at apparent V-band magnitude 12.511 (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 | K2-259 |
| Spectral Type | Unclassified |
| Effective Temp (Teff) | 6,059 K |
| Stellar Radius | 1.1300 R☉ |
| Stellar Mass | 1.1400 M☉ |
| Bolometric Luminosity | 1.544 L☉ |
| Metallicity [Fe/H] | 0.160 dex |
| Stellar Age | — |
| Apparent Magnitude | V: 12.511 · G: 12.375 |
| System Architecture | 1 Star, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.127100 AU |
| Orbital Period | 15.48 days |
| Mean Orbital Velocity | 89.3 km/s |
| Orbital Eccentricity (e) | Circular (0.00) |
| Orbital Inclination (i) | 89.0272° |
| Mean Bulk Density | 2.65 g/cm³ (18 km/s vesc) |
| Insolation Flux | 90.9529 S⊕ (~123,787 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 1358.300 LY (416.457 pc) |