K2-261 b
Astrophysical Synthesis & Habitability Characterization
K2-261 b is a confirmed gas giant exoplanet orbiting the star K2-261, located approximately 695.6 light-years (213.28 parsecs) from the Solar System. First announced in 2018 by K2 using the Transit detection technique (producing an estimated primary transit depth of 0.2612% across the stellar disk), this world measures 9.27 Earth radii (0.83 Jupiter radii) and has a measured mass of 68.969 Earth masses (0.22 Jupiter masses). Combining its mass and radius yields a mean bulk density of 0.48 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 0.8 g⊕ (7.85 m/s²), and an escape velocity of 30.5 km/s.
In its orbital architecture around K2-261, K2-261 b completes one full revolution every 11.633 Earth days at a semi-major axis of 0.1086 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 101.6 km/s (compared to Earth's 29.8 km/s). The planet follows a highly eccentric trajectory (e = 0.274, swinging from 0.0788 AU at periapsis out to 0.1384 AU at apoapsis), pointing to past dynamical scattering or secular gravitational perturbations.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 1036 K (+763°C / +1405°F) and receiving 180.00 S⊕ (~244,980 W/m²), K2-261 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star K2-261 is classified as a solar-type yellow star of spectral type G7 IV/V with an effective photospheric temperature of 5,490 K, a radius of 1.663 R☉, a mass of 1.107 M☉, an estimated bolometric luminosity of 2.254 L☉. Stellar evolution modeling indicates an estimated stellar age of 8.8 Gyr and a metallicity of [Fe/H] = 0.372 dex (metal-rich relative to the Sun). From Earth, K2-261 shines at apparent V-band magnitude 10.612 (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 | K2-261 |
| Spectral Type | G7 IV/V |
| Effective Temp (Teff) | 5,490 K |
| Stellar Radius | 1.6630 R☉ |
| Stellar Mass | 1.1070 M☉ |
| Bolometric Luminosity | 2.254 L☉ |
| Metallicity [Fe/H] | 0.372 dex |
| Stellar Age | 8.800 Gyr |
| Apparent Magnitude | V: 10.612 · G: 10.459 |
| System Architecture | 1 Star, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.108600 AU |
| Orbital Period | 11.633 days |
| Mean Orbital Velocity | 101.6 km/s |
| Orbital Eccentricity (e) | 0.27400 |
| Orbital Inclination (i) | 88.5800° |
| Mean Bulk Density | 0.48 g/cm³ (30.5 km/s vesc) |
| Insolation Flux | 180.0000 S⊕ (~244,980 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 695.600 LY (213.283 pc) |