HATS-18 b
Astrophysical Synthesis & Habitability Characterization
HATS-18 b is a confirmed gas giant exoplanet orbiting the star HATS-18, located approximately 2036.4 light-years (624.37 parsecs) from the Solar System. First announced in 2016 by Multiple Observatories using the Transit detection technique (producing an estimated primary transit depth of 1.7928% across the stellar disk), this world measures 14.897 Earth radii (1.33 Jupiter radii) and has a measured mass of 629.144 Earth masses (1.98 Jupiter masses). Combining its mass and radius yields a mean bulk density of 1.05 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 2.83 g⊕ (27.75 m/s²), and an escape velocity of 72.7 km/s.
In its orbital architecture around HATS-18, HATS-18 b completes one full revolution every 0.838 Earth days at a semi-major axis of 0.0174 AU (an ultra-close orbit less than one-seventh of Mercury's 0.387 AU distance from the Sun, where strong stellar tidal forces typically induce synchronous rotation). Its mean orbital velocity is approximately 225.9 km/s (compared to Earth's 29.8 km/s). Its moderate orbital eccentricity of e = 0.166 carries the world between 0.0145 AU at periapsis and 0.0203 AU at apoapsis, driving measurable seasonal insolation cycles.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 2069 K (+1796°C / +3265°F) and receiving 2,990.00 S⊕ (~4,069,390 W/m²), HATS-18 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star HATS-18 is classified as a solar-type yellow star of spectral type G V with an effective photospheric temperature of 5,600 K, a radius of 1.02 R☉, a mass of 1.037 M☉, an estimated bolometric luminosity of 0.918 L☉. Stellar evolution modeling indicates an estimated stellar age of 4.2 Gyr and a metallicity of [Fe/H] = 0.28 dex (metal-rich relative to the Sun). From Earth, HATS-18 shines at apparent V-band magnitude 14.188 (faint and requiring research-grade or CCD telescopic photometry) in the Southern 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: Multiple Observatories · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | HATS-18 |
| Spectral Type | G V |
| Effective Temp (Teff) | 5,600 K |
| Stellar Radius | 1.0200 R☉ |
| Stellar Mass | 1.0370 M☉ |
| Bolometric Luminosity | 0.918 L☉ |
| Metallicity [Fe/H] | 0.280 dex |
| Stellar Age | 4.200 Gyr |
| Apparent Magnitude | V: 14.188 · G: 13.879 |
| System Architecture | 1 Star, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.017400 AU |
| Orbital Period | 0.838 days |
| Mean Orbital Velocity | 225.9 km/s |
| Orbital Eccentricity (e) | 0.16600 |
| Orbital Inclination (i) | 85.2900° |
| Mean Bulk Density | 1.05 g/cm³ (72.7 km/s vesc) |
| Insolation Flux | 2990.0000 S⊕ (~4,069,390 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 2036.400 LY (624.368 pc) |