K2-123 b

Astrophysical Synthesis & Habitability Characterization

K2-123 b is a confirmed sub-neptune exoplanet orbiting the star K2-123, located approximately 526.9 light-years (161.56 parsecs) from the Solar System. First announced in 2017 by K2 using the Transit detection technique (producing an estimated primary transit depth of 0.1839% across the stellar disk), this world measures 2.76 Earth radii and has a measured mass of 8.05 Earth masses. Combining its mass and radius yields a mean bulk density of 2.11 g/cm³ (an intermediate density indicative of a deep water-ice mantle or dense volatile envelope), a surface gravity of 1.06 g⊕ (10.4 m/s²), and an escape velocity of 19.1 km/s.

In its orbital architecture around K2-123, K2-123 b completes one full revolution every 30.952 Earth days at a semi-major axis of 0.1649 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 58 km/s (compared to Earth's 29.8 km/s). Its moderate orbital eccentricity of e = 0.06 carries the world between 0.155 AU at periapsis and 0.1748 AU at apoapsis, driving measurable seasonal insolation cycles.

☁️ Atmospheric Regime (Volatile / Gas Envelope): With a radius of 2.760 R⊕, K2-123 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 K2-123 is classified as a stellar host with an effective photospheric temperature of 4,035 K, a radius of 0.59 R☉, a mass of 0.62 M☉, an estimated bolometric luminosity of 0.083 L☉. Stellar evolution modeling indicates a metallicity of [Fe/H] = 0.03 dex (near-solar heavy-element abundance). From Earth, K2-123 shines at apparent V-band magnitude 14.781 (faint and requiring research-grade or CCD telescopic photometry) in the Northern celestial hemisphere.

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