K2-17 b

Astrophysical Synthesis & Habitability Characterization

K2-17 b is a confirmed sub-neptune exoplanet orbiting the star K2-17, located approximately 408 light-years (125.11 parsecs) from the Solar System. First announced in 2015 by K2 using the Transit detection technique (producing an estimated primary transit depth of 0.096% across the stellar disk), this world measures 2.23 Earth radii and has a measured mass of 5.6 Earth masses. Combining its mass and radius yields a mean bulk density of 2.78 g/cm³ (an intermediate density indicative of a deep water-ice mantle or dense volatile envelope), a surface gravity of 1.13 g⊕ (11.08 m/s²), and an escape velocity of 17.7 km/s.

In its orbital architecture around K2-17, K2-17 b completes one full revolution every 17.968 Earth days at a semi-major axis of 0.119 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 72.1 km/s (compared to Earth's 29.8 km/s). Its moderate orbital eccentricity of e = 0.18 carries the world between 0.0976 AU at periapsis and 0.1404 AU at apoapsis, driving measurable seasonal insolation cycles.

⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 487 K (+214°C / +417°F) and receiving 4.91 S⊕ (~6,683 W/m²), K2-17 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.

The host star K2-17 is classified as an orange K-type star of spectral type K7 with an effective photospheric temperature of 4,320 K, a radius of 0.66 R☉, a mass of 0.71 M☉, an estimated bolometric luminosity of 0.136 L☉. Stellar evolution modeling indicates a metallicity of [Fe/H] = 0.15 dex (metal-rich relative to the Sun). From Earth, K2-17 shines at apparent V-band magnitude 13.411 (faint and requiring research-grade or CCD telescopic photometry) in the Northern celestial hemisphere.

Discovery Facility: K2 · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).