HAT-P-11 b
Astrophysical Synthesis & Habitability Characterization
HAT-P-11 b is a confirmed sub-neptune exoplanet orbiting the star HAT-P-11, located approximately 123.2 light-years (37.77 parsecs) from the Solar System. First announced in 2010 by HATNet using the Transit detection technique (producing an estimated primary transit depth of 0.3425% across the stellar disk), this world measures 4.36 Earth radii (0.39 Jupiter radii) and has a measured mass of 25 Earth masses. Combining its mass and radius yields a mean bulk density of 1.66 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 1.32 g⊕ (12.94 m/s²), and an escape velocity of 26.8 km/s.
In its orbital architecture around HAT-P-11, HAT-P-11 b completes one full revolution every 4.888 Earth days at a semi-major axis of 0.05258 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 117 km/s (compared to Earth's 29.8 km/s). The planet follows a highly eccentric trajectory (e = 0.251, swinging from 0.0394 AU at periapsis out to 0.0658 AU at apoapsis), pointing to past dynamical scattering or secular gravitational perturbations.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 838 K (+565°C / +1049°F) and receiving 100.77 S⊕ (~137,148 W/m²), HAT-P-11 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star HAT-P-11 is classified as an orange K-type star of spectral type K4 with an effective photospheric temperature of 4,653 K, a radius of 0.683 R☉, a mass of 0.811 M☉, an estimated bolometric luminosity of 0.196 L☉. Stellar evolution modeling indicates an estimated stellar age of 2.69 Gyr and a metallicity of [Fe/H] = 0.31 dex (metal-rich relative to the Sun). From Earth, HAT-P-11 shines at apparent V-band magnitude 9.46 (accessible to binoculars and small amateur telescopes) in the Northern celestial hemisphere. The HAT-P-11 system is a multi-planet architecture hosting 2 confirmed worlds, with HAT-P-11 b sharing its stellar neighborhood alongside HAT-P-11 c.
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-11 |
| Spectral Type | K4 |
| Effective Temp (Teff) | 4,653 K |
| Stellar Radius | 0.6830 R☉ |
| Stellar Mass | 0.8110 M☉ |
| Bolometric Luminosity | 0.196 L☉ |
| Metallicity [Fe/H] | 0.310 dex |
| Stellar Age | 2.690 Gyr |
| Apparent Magnitude | V: 9.460 · G: 9.151 |
| System Architecture | 1 Star, 2 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.052580 AU |
| Orbital Period | 4.888 days |
| Mean Orbital Velocity | 117 km/s |
| Orbital Eccentricity (e) | 0.25100 |
| Orbital Inclination (i) | 89.0270° |
| Mean Bulk Density | 1.66 g/cm³ (26.8 km/s vesc) |
| Insolation Flux | 100.7700 S⊕ (~137,148 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 123.200 LY (37.765 pc) |