HAT-P-21 b
Astrophysical Synthesis & Habitability Characterization
HAT-P-21 b is a confirmed gas giant exoplanet orbiting the star HAT-P-21, located approximately 904.4 light-years (277.28 parsecs) from the Solar System. First announced in 2010 by HATNet using the Transit detection technique (producing an estimated primary transit depth of 0.8887% across the stellar disk), this world measures 12.442 Earth radii (1.11 Jupiter radii) and has a measured mass of 1547.832 Earth masses (4.87 Jupiter masses). Combining its mass and radius yields a mean bulk density of 4.43 g/cm³ (a terrestrial silicate-iron composition comparable to Earth (5.51 g/cm³)), a surface gravity of 10 g⊕ (98.07 m/s²), and an escape velocity of 124.8 km/s.
In its orbital architecture around HAT-P-21, HAT-P-21 b completes one full revolution every 4.124 Earth days at a semi-major axis of 0.05037 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 132.9 km/s (compared to Earth's 29.8 km/s). Its moderate orbital eccentricity of e = 0.23 carries the world between 0.0388 AU at periapsis and 0.062 AU at apoapsis, driving measurable seasonal insolation cycles.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 1367 K (+1094°C / +2001°F) and receiving 459.54 S⊕ (~625,437 W/m²), HAT-P-21 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star HAT-P-21 is classified as a solar-type yellow star of spectral type G3 with an effective photospheric temperature of 5,588 K, a radius of 1.21 R☉, a mass of 1.24 M☉, an estimated bolometric luminosity of 1.281 L☉. Stellar evolution modeling indicates an estimated stellar age of 10.2 Gyr and a metallicity of [Fe/H] = 0.01 dex (near-solar heavy-element abundance). From Earth, HAT-P-21 shines at apparent V-band magnitude 11.459 (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: HATNet · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | HAT-P-21 |
| Spectral Type | G3 |
| Effective Temp (Teff) | 5,588 K |
| Stellar Radius | 1.2100 R☉ |
| Stellar Mass | 1.2400 M☉ |
| Bolometric Luminosity | 1.281 L☉ |
| Metallicity [Fe/H] | 0.010 dex |
| Stellar Age | 10.200 Gyr |
| Apparent Magnitude | V: 11.459 · G: 11.558 |
| System Architecture | 1 Star, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.050370 AU |
| Orbital Period | 4.124 days |
| Mean Orbital Velocity | 132.9 km/s |
| Orbital Eccentricity (e) | 0.23000 |
| Orbital Inclination (i) | 87.2000° |
| Mean Bulk Density | 4.43 g/cm³ (124.8 km/s vesc) |
| Insolation Flux | 459.5420 S⊕ (~625,437 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 904.400 LY (277.278 pc) |