NGTS-21 b
Astrophysical Synthesis & Habitability Characterization
NGTS-21 b is a confirmed gas giant exoplanet orbiting the star NGTS-21, located approximately 1933.7 light-years (592.89 parsecs) from the Solar System. First announced in 2022 by Next-Generation Transit Survey (NGTS) using the Transit detection technique (producing an estimated primary transit depth of 2.5257% across the stellar disk), this world measures 14.908 Earth radii (1.33 Jupiter radii) and has a measured mass of 750.075 Earth masses (2.36 Jupiter masses). Combining its mass and radius yields a mean bulk density of 1.25 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 3.37 g⊕ (33.05 m/s²), and an escape velocity of 79.3 km/s.
In its orbital architecture around NGTS-21, NGTS-21 b completes one full revolution every 1.543 Earth days at a semi-major axis of 0.0236 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 166.4 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 1357 K (+1084°C / +1983°F) and receiving 514.29 S⊕ (~699,942 W/m²), NGTS-21 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star NGTS-21 is classified as an orange K-type star of spectral type K3 V with an effective photospheric temperature of 4,660 K, a radius of 0.86 R☉, a mass of 0.72 M☉, an estimated bolometric luminosity of 0.313 L☉. Stellar evolution modeling indicates an estimated stellar age of 10.02 Gyr and a metallicity of [Fe/H] = -0.26 dex (metal-poor relative to the Sun). From Earth, NGTS-21 shines at apparent V-band magnitude 15.506 (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-21 |
| Spectral Type | K3 V |
| Effective Temp (Teff) | 4,660 K |
| Stellar Radius | 0.8600 R☉ |
| Stellar Mass | 0.7200 M☉ |
| Bolometric Luminosity | 0.313 L☉ |
| Metallicity [Fe/H] | -0.260 dex |
| Stellar Age | 10.020 Gyr |
| Apparent Magnitude | V: 15.506 · G: 15.224 |
| System Architecture | 1 Star, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.023600 AU |
| Orbital Period | 1.543 days |
| Mean Orbital Velocity | 166.4 km/s |
| Orbital Eccentricity (e) | 0.00000 |
| Orbital Inclination (i) | 83.8500° |
| Mean Bulk Density | 1.25 g/cm³ (79.3 km/s vesc) |
| Insolation Flux | 514.2853 S⊕ (~699,942 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 1933.700 LY (592.891 pc) |