GJ 238 b

Astrophysical Synthesis & Habitability Characterization

GJ 238 b is a confirmed sub-earth exoplanet orbiting the star GJ 238, located approximately 49.6 light-years (15.21 parsecs) from the Solar System. First announced in 2024 by Transiting Exoplanet Survey Satellite (TESS) using the Transit detection technique (producing an estimated primary transit depth of 0.0145% across the stellar disk), this world measures 0.566 Earth radii and has a measured mass of 0.126 Earth masses. Combining its mass and radius yields a mean bulk density of 3.83 g/cm³ (an intermediate density indicative of a deep water-ice mantle or dense volatile envelope), a surface gravity of 0.39 g⊕ (3.82 m/s²), and an escape velocity of 5.3 km/s.

In its orbital architecture around GJ 238, GJ 238 b completes one full revolution every 1.745 Earth days at a semi-major axis of 0.02123 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 132.4 km/s (compared to Earth's 29.8 km/s).

⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 758 K (+485°C / +905°F) and receiving 229.46 S⊕ (~312,292 W/m²), GJ 238 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.

The host star GJ 238 is classified as a low-mass red dwarf of spectral type M2.5 V with an effective photospheric temperature of 3,485 K, a radius of 0.4314 R☉, a mass of 0.4193 M☉, an estimated bolometric luminosity of 0.025 L☉. Stellar evolution modeling indicates a metallicity of [Fe/H] = 0.15 dex (metal-rich relative to the Sun). From Earth, GJ 238 shines at apparent V-band magnitude 11.62 (faint and requiring research-grade or CCD telescopic photometry) in the Southern celestial hemisphere.

Discovery Facility: Transiting Exoplanet Survey Satellite (TESS) · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).