Kepler-1721 b
Astrophysical Synthesis & Habitability Characterization
Kepler-1721 b is a confirmed sub-neptune exoplanet orbiting the star Kepler-1721, located approximately 2091.9 light-years (641.37 parsecs) from the Solar System. First announced in 2021 by Kepler using the Transit detection technique (producing an estimated primary transit depth of 0.129% across the stellar disk), this world measures 3.119 Earth radii (0.28 Jupiter radii) and has a measured mass of 9.9 Earth masses. Combining its mass and radius yields a mean bulk density of 1.8 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 1.02 g⊕ (10 m/s²), and an escape velocity of 19.9 km/s.
In its orbital architecture around Kepler-1721, Kepler-1721 b completes one full revolution every 4.592 Earth days at a semi-major axis of 0.0519 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 122.9 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 924 K (+651°C / +1204°F) and receiving 172.60 S⊕ (~234,909 W/m²), Kepler-1721 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star Kepler-1721 is classified as a stellar host with an effective photospheric temperature of 5,351 K, a radius of 0.796 R☉, a mass of 0.89 M☉, an estimated bolometric luminosity of 0.466 L☉. Stellar evolution modeling indicates an estimated stellar age of 0.7 Gyr and a metallicity of [Fe/H] = -0.04 dex (near-solar heavy-element abundance). From Earth, Kepler-1721 shines at apparent V-band magnitude 15.085 (faint and requiring research-grade or CCD telescopic photometry) in the Northern celestial hemisphere.
Methodology: Computed using the Schulze-Makuch et al. (2011) weighted metric comparing physical interior (radius, estimated density) and surface thermal regime (escape velocity, equilibrium temperature).
Discovery Facility: Kepler · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | Kepler-1721 |
| Spectral Type | Unclassified |
| Effective Temp (Teff) | 5,351 K |
| Stellar Radius | 0.7960 R☉ |
| Stellar Mass | 0.8900 M☉ |
| Bolometric Luminosity | 0.466 L☉ |
| Metallicity [Fe/H] | -0.040 dex |
| Stellar Age | 0.700 Gyr |
| Apparent Magnitude | V: 15.085 · G: 14.876 |
| System Architecture | 1 Star, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.051900 AU |
| Orbital Period | 4.592 days |
| Mean Orbital Velocity | 122.9 km/s |
| Orbital Eccentricity (e) | 0.00000 |
| Orbital Inclination (i) | 89.9300° |
| Mean Bulk Density | 1.8 g/cm³ (19.9 km/s vesc) |
| Insolation Flux | 172.6000 S⊕ (~234,909 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 2091.900 LY (641.374 pc) |