Kepler-721 b

Astrophysical Synthesis & Habitability Characterization

Kepler-721 b is a confirmed sub-neptune exoplanet orbiting the star Kepler-721, located approximately 3022.2 light-years (926.6 parsecs) from the Solar System. First announced in 2016 by Kepler using the Transit detection technique (producing an estimated primary transit depth of 0.0963% across the stellar disk), this world measures 2.64 Earth radii and has a measured mass of 7.46 Earth masses. Combining its mass and radius yields a mean bulk density of 2.24 g/cm³ (an intermediate density indicative of a deep water-ice mantle or dense volatile envelope), a surface gravity of 1.07 g⊕ (10.49 m/s²), and an escape velocity of 18.8 km/s.

In its orbital architecture around Kepler-721, Kepler-721 b completes one full revolution every 5.392 Earth days at a semi-major axis of 0.0583 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 117.6 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 850 K (+577°C / +1071°F) and receiving 138.75 S⊕ (~188,840 W/m²), Kepler-721 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.

The host star Kepler-721 is classified as a stellar host with an effective photospheric temperature of 5,075 K, a radius of 0.78 R☉, a mass of 0.82 M☉, an estimated bolometric luminosity of 0.362 L☉. Stellar evolution modeling indicates an estimated stellar age of 4.9 Gyr and a metallicity of [Fe/H] = -0.01 dex (near-solar heavy-element abundance). From Earth, Kepler-721 shines at apparent V-band magnitude 15.963 (faint and requiring research-grade or CCD telescopic photometry) in the Northern celestial hemisphere.

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