K2-9 b

Astrophysical Synthesis & Habitability Characterization

K2-9 b is a confirmed sub-neptune exoplanet orbiting the star K2-9, located approximately 270.6 light-years (82.96 parsecs) from the Solar System. First announced in 2015 by K2 using the Transit detection technique (producing an estimated primary transit depth of 0.4428% across the stellar disk), this world measures 2.25 Earth radii and has a measured mass of 5.69 Earth masses. Combining its mass and radius yields a mean bulk density of 2.75 g/cm³ (an intermediate density indicative of a deep water-ice mantle or dense volatile envelope), a surface gravity of 1.12 g⊕ (10.98 m/s²), and an escape velocity of 17.8 km/s.

In its orbital architecture around K2-9, K2-9 b completes one full revolution every 18.45 Earth days at a semi-major axis of 0.091 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 53.7 km/s (compared to Earth's 29.8 km/s).

☁️ Atmospheric Regime (Volatile / Gas Envelope): With a radius of 2.250 R⊕, K2-9 b exceeds the empirical ~1.8–2.2 R⊕ Fulton radius valley threshold, indicating a substantial hydrogen-helium or supercritical volatile envelope without a conventional solid terrestrial surface.

The host star K2-9 is classified as a low-mass red dwarf of spectral type M2.5 V with an effective photospheric temperature of 3,390 K, a radius of 0.31 R☉, a mass of 0.3 M☉, an estimated bolometric luminosity of 0.011 L☉. Stellar evolution modeling indicates an estimated stellar age of 1 Gyr and a metallicity of [Fe/H] = -0.25 dex (metal-poor relative to the Sun). From Earth, K2-9 shines at apparent V-band magnitude 15.86 (faint and requiring research-grade or CCD telescopic photometry) in the Northern celestial hemisphere.

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