GJ 338 B b
Astrophysical Synthesis & Habitability Characterization
GJ 338 B b is a confirmed sub-neptune exoplanet orbiting the star GJ 338 B, located approximately 20.7 light-years (6.33 parsecs) from the Solar System. First announced in 2020 by Calar Alto Observatory using the Radial Velocity detection technique (measuring periodic Doppler reflex motion in the host star's spectrum), this world measures 3.25 Earth radii (0.29 Jupiter radii) and has a measured mass of 10.6 Earth masses. Combining its mass and radius yields a mean bulk density of 1.7 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 1 g⊕ (9.81 m/s²), and an escape velocity of 20.2 km/s.
In its orbital architecture around GJ 338 B, GJ 338 B b completes one full revolution every 24.422 Earth days at a semi-major axis of 0.142 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 63.3 km/s (compared to Earth's 29.8 km/s). Its moderate orbital eccentricity of e = 0.109 carries the world between 0.1265 AU at periapsis and 0.1575 AU at apoapsis, driving measurable seasonal insolation cycles.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 391 K (+118°C / +244°F) and receiving 3.93 S⊕ (~5,346 W/m²), GJ 338 B b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star GJ 338 B is classified as a low-mass red dwarf of spectral type M0 V with an effective photospheric temperature of 4,014 K, a radius of 0.58 R☉, a mass of 0.64 M☉, an estimated bolometric luminosity of 0.078 L☉. Stellar evolution modeling indicates an estimated stellar age of 4 Gyr and a metallicity of [Fe/H] = -0.03 dex (near-solar heavy-element abundance). From Earth, GJ 338 B shines at apparent V-band magnitude 7.7 (accessible to binoculars and small amateur telescopes) 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: Calar Alto Observatory · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | GJ 338 B |
| Spectral Type | M0 V |
| Effective Temp (Teff) | 4,014 K |
| Stellar Radius | 0.5800 R☉ |
| Stellar Mass | 0.6400 M☉ |
| Bolometric Luminosity | 0.078 L☉ |
| Metallicity [Fe/H] | -0.030 dex |
| Stellar Age | 4.000 Gyr |
| Apparent Magnitude | V: 7.700 · G: 7.048 |
| System Architecture | 2 Stars, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.142000 AU |
| Orbital Period | 24.422 days |
| Mean Orbital Velocity | 63.3 km/s |
| Orbital Eccentricity (e) | 0.10900 |
| Orbital Inclination (i) | — |
| Mean Bulk Density | 1.7 g/cm³ (20.2 km/s vesc) |
| Insolation Flux | 3.9278 S⊕ (~5,346 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 20.700 LY (6.333 pc) |