TOI-251 b

Astrophysical Synthesis & Habitability Characterization

TOI-251 b is a confirmed sub-neptune exoplanet orbiting the star TOI-251, located approximately 324.6 light-years (99.52 parsecs) from the Solar System. First announced in 2021 by Transiting Exoplanet Survey Satellite (TESS) using the Transit detection technique (producing an estimated primary transit depth of 0.0813% across the stellar disk), this world measures 2.74 Earth radii and has a measured mass of 317.83 Earth masses (1 Jupiter masses). Combining its mass and radius yields a mean bulk density of 85.19 g/cm³ (an iron-enriched, high-compression rocky core), a surface gravity of 42.33 g⊕ (415.12 m/s²), and an escape velocity of 120.5 km/s.

In its orbital architecture around TOI-251, TOI-251 b completes one full revolution every 4.938 Earth days at a semi-major axis of 0.05741 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 126.5 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 1041 K (+768°C / +1414°F) and receiving 277.75 S⊕ (~378,013 W/m²), TOI-251 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.

The host star TOI-251 is classified as a stellar host with an effective photospheric temperature of 5,875 K, a radius of 0.881 R☉, a mass of 1.036 M☉, an estimated bolometric luminosity of 0.83 L☉. Stellar evolution modeling indicates an estimated stellar age of 0.18 Gyr and a metallicity of [Fe/H] = -0.106 dex (metal-poor relative to the Sun). From Earth, TOI-251 shines at apparent V-band magnitude 9.887 (accessible to binoculars and small amateur telescopes) in the Southern celestial hemisphere.

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