K2-89 b

Astrophysical Synthesis & Habitability Characterization

K2-89 b is a confirmed sub-earth exoplanet orbiting the star K2-89, located approximately 280.6 light-years (86.04 parsecs) from the Solar System. First announced in 2016 by K2 using the Transit detection technique (producing an estimated primary transit depth of 0.0314% across the stellar disk), this world measures 0.615 Earth radii and has a measured mass of 0.17 Earth masses. Combining its mass and radius yields a mean bulk density of 4.03 g/cm³ (a terrestrial silicate-iron composition comparable to Earth (5.51 g/cm³)), a surface gravity of 0.45 g⊕ (4.41 m/s²), and an escape velocity of 5.9 km/s.

In its orbital architecture around K2-89, K2-89 b completes one full revolution every 1.096 Earth days at a semi-major axis of 0.01462 AU (an ultra-close orbit less than one-seventh of Mercury's 0.387 AU distance from the Sun, where strong stellar tidal forces typically induce synchronous rotation). Its mean orbital velocity is approximately 145.1 km/s (compared to Earth's 29.8 km/s). Its moderate orbital eccentricity of e = 0.15 carries the world between 0.0124 AU at periapsis and 0.0168 AU at apoapsis, driving measurable seasonal insolation cycles.

⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 830 K (+557°C / +1035°F) and receiving 78.00 S⊕ (~106,158 W/m²), K2-89 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.

The host star K2-89 is classified as a stellar host with an effective photospheric temperature of 3,691 K, a radius of 0.318 R☉, a mass of 0.347 M☉, an estimated bolometric luminosity of 0.017 L☉. From Earth, K2-89 shines at apparent V-band magnitude 14.162 (faint and requiring research-grade or CCD telescopic photometry) in the Northern celestial hemisphere.

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