K2-137 b

Astrophysical Synthesis & Habitability Characterization

K2-137 b is a confirmed sub-earth exoplanet orbiting the star K2-137, located approximately 322.5 light-years (98.88 parsecs) from the Solar System. First announced in 2017 by K2 using the Transit detection technique (producing an estimated primary transit depth of 0.0409% across the stellar disk), this world measures 0.64 Earth radii and has a measured mass of 158.915 Earth masses (0.5 Jupiter masses). Combining its mass and radius yields a mean bulk density of 3342.66 g/cm³ (an iron-enriched, high-compression rocky core), a surface gravity of 387.98 g⊕ (3804.78 m/s²), and an escape velocity of 176.3 km/s.

In its orbital architecture around K2-137, K2-137 b completes one full revolution every 0.18 Earth days at a semi-major axis of 0.0058 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 351.1 km/s (compared to Earth's 29.8 km/s). The orbit is modeled as circular (e = 0.00), maintaining a constant stellar distance throughout its year.

⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 1471 K (+1198°C / +2188°F) and receiving 975.57 S⊕ (~1,327,755 W/m²), K2-137 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.

The host star K2-137 is classified as a low-mass red dwarf of spectral type M3 V with an effective photospheric temperature of 3,697 K, a radius of 0.29 R☉, a mass of 0.463 M☉, an estimated bolometric luminosity of 0.014 L☉. Stellar evolution modeling indicates a metallicity of [Fe/H] = -0.059 dex (near-solar heavy-element abundance). From Earth, K2-137 shines at apparent V-band magnitude 15.94 (faint and requiring research-grade or CCD telescopic photometry) in the Southern celestial hemisphere.

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