NGTS-18 b
Astrophysical Synthesis & Habitability Characterization
NGTS-18 b is a confirmed gas giant exoplanet orbiting the star NGTS-18, located approximately 3575.3 light-years (1096.19 parsecs) from the Solar System. First announced in 2021 by Next-Generation Transit Survey (NGTS) using the Transit detection technique (producing an estimated primary transit depth of 0.7979% across the stellar disk), this world measures 13.563 Earth radii (1.21 Jupiter radii) and has a measured mass of 129.992 Earth masses (0.41 Jupiter masses). Combining its mass and radius yields a mean bulk density of 0.29 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 0.71 g⊕ (6.96 m/s²), and an escape velocity of 34.6 km/s.
In its orbital architecture around NGTS-18, NGTS-18 b completes one full revolution every 3.051 Earth days at a semi-major axis of 0.0448 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 159.7 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 1381 K (+1108°C / +2026°F) and receiving 845.00 S⊕ (~1,150,045 W/m²), NGTS-18 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star NGTS-18 is classified as a solar-type yellow star of spectral type G5 V with an effective photospheric temperature of 5,610 K, a radius of 1.392 R☉, a mass of 1.003 M☉, an estimated bolometric luminosity of 1.722 L☉. Stellar evolution modeling indicates an estimated stellar age of 10.84 Gyr and a metallicity of [Fe/H] = 0.15 dex (metal-rich relative to the Sun). From Earth, NGTS-18 shines at apparent V-band magnitude 14.588 (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: Next-Generation Transit Survey (NGTS) · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | NGTS-18 |
| Spectral Type | G5 V |
| Effective Temp (Teff) | 5,610 K |
| Stellar Radius | 1.3920 R☉ |
| Stellar Mass | 1.0030 M☉ |
| Bolometric Luminosity | 1.722 L☉ |
| Metallicity [Fe/H] | 0.150 dex |
| Stellar Age | 10.840 Gyr |
| Apparent Magnitude | V: 14.588 · G: 14.390 |
| System Architecture | 1 Star, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.044800 AU |
| Orbital Period | 3.051 days |
| Mean Orbital Velocity | 159.7 km/s |
| Orbital Eccentricity (e) | 0.00000 |
| Orbital Inclination (i) | — |
| Mean Bulk Density | 0.29 g/cm³ (34.6 km/s vesc) |
| Insolation Flux | 845.0000 S⊕ (~1,150,045 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 3575.300 LY (1096.190 pc) |