HAT-P-18 b
Astrophysical Synthesis & Habitability Characterization
HAT-P-18 b is a confirmed gas giant exoplanet orbiting the star HAT-P-18, located approximately 526.4 light-years (161.4 parsecs) from the Solar System. First announced in 2010 by HATNet using the Transit detection technique (producing an estimated primary transit depth of 1.8736% across the stellar disk), this world measures 10.705 Earth radii (0.96 Jupiter radii) and has a measured mass of 55.525 Earth masses (0.17 Jupiter masses). Combining its mass and radius yields a mean bulk density of 0.25 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 0.48 g⊕ (4.71 m/s²), and an escape velocity of 25.5 km/s.
In its orbital architecture around HAT-P-18, HAT-P-18 b completes one full revolution every 5.508 Earth days at a semi-major axis of 0.0559 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 110.4 km/s (compared to Earth's 29.8 km/s). With a near-circular eccentricity of e = 0.03 (ranging from 0.0542 AU at periapsis to 0.0576 AU at apoapsis), the planet experiences minimal orbital distance variation.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 841 K (+568°C / +1054°F) and receiving 82.24 S⊕ (~111,930 W/m²), HAT-P-18 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star HAT-P-18 is classified as an orange K-type star of spectral type K2 with an effective photospheric temperature of 4,870 K, a radius of 0.717 R☉, a mass of 0.77 M☉, an estimated bolometric luminosity of 0.259 L☉. Stellar evolution modeling indicates an estimated stellar age of 12.4 Gyr and a metallicity of [Fe/H] = 0.1 dex (metal-rich relative to the Sun). From Earth, HAT-P-18 shines at apparent V-band magnitude 12.597 (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-18 |
| Spectral Type | K2 |
| Effective Temp (Teff) | 4,870 K |
| Stellar Radius | 0.7170 R☉ |
| Stellar Mass | 0.7700 M☉ |
| Bolometric Luminosity | 0.259 L☉ |
| Metallicity [Fe/H] | 0.100 dex |
| Stellar Age | 12.400 Gyr |
| Apparent Magnitude | V: 12.597 · G: 12.368 |
| System Architecture | 1 Star, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.055900 AU |
| Orbital Period | 5.508 days |
| Mean Orbital Velocity | 110.4 km/s |
| Orbital Eccentricity (e) | 0.03000 |
| Orbital Inclination (i) | 88.6600° |
| Mean Bulk Density | 0.25 g/cm³ (25.5 km/s vesc) |
| Insolation Flux | 82.2407 S⊕ (~111,930 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 526.400 LY (161.400 pc) |