K2-28 b

Astrophysical Synthesis & Habitability Characterization

K2-28 b is a confirmed sub-neptune exoplanet orbiting the star K2-28, located approximately 205.5 light-years (63 parsecs) from the Solar System. First announced in 2016 by K2 using the Transit detection technique (producing an estimated primary transit depth of 0.5454% across the stellar disk), this world measures 2.32 Earth radii and has a measured mass of 5.99 Earth masses. Combining its mass and radius yields a mean bulk density of 2.65 g/cm³ (an intermediate density indicative of a deep water-ice mantle or dense volatile envelope), a surface gravity of 1.11 g⊕ (10.89 m/s²), and an escape velocity of 18 km/s.

In its orbital architecture around K2-28, K2-28 b completes one full revolution every 2.26 Earth days at a semi-major axis of 0.0214 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 103 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 568 K (+295°C / +563°F) and receiving 30.10 S⊕ (~40,966 W/m²), K2-28 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.

The host star K2-28 is classified as a low-mass red dwarf of spectral type M4 with an effective photospheric temperature of 3,214 K, a radius of 0.288 R☉, a mass of 0.257 M☉, an estimated bolometric luminosity of 0.00794 L☉. Stellar evolution modeling indicates a metallicity of [Fe/H] = 0.26 dex (metal-rich relative to the Sun). From Earth, K2-28 shines at apparent V-band magnitude 16.526 (faint and requiring research-grade or CCD telescopic photometry) in the Southern celestial hemisphere.

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