GJ 436 b
Astrophysical Synthesis & Habitability Characterization
GJ 436 b is a confirmed sub-neptune exoplanet orbiting the star GJ 436, located approximately 31.8 light-years (9.75 parsecs) from the Solar System. First announced in 2004 by W. M. Keck Observatory using the Radial Velocity detection technique (measuring periodic Doppler reflex motion in the host star's spectrum), this world measures 4.17 Earth radii (0.37 Jupiter radii) and has a measured mass of 22.1 Earth masses. Combining its mass and radius yields a mean bulk density of 1.68 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 1.27 g⊕ (12.45 m/s²), and an escape velocity of 25.8 km/s.
In its orbital architecture around GJ 436, GJ 436 b completes one full revolution every 2.644 Earth days at a semi-major axis of 0.0291 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 119.7 km/s (compared to Earth's 29.8 km/s). Its moderate orbital eccentricity of e = 0.13827 carries the world between 0.0251 AU at periapsis and 0.0331 AU at apoapsis, driving measurable seasonal insolation cycles.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 686 K (+413°C / +775°F) and receiving 29.43 S⊕ (~40,057 W/m²), GJ 436 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star GJ 436 is classified as a low-mass red dwarf of spectral type M2.5 V with an effective photospheric temperature of 3,586 K, a radius of 0.455 R☉, a mass of 0.47 M☉, an estimated bolometric luminosity of 0.031 L☉. Stellar evolution modeling indicates an estimated stellar age of 6 Gyr and a metallicity of [Fe/H] = 0.099 dex (metal-rich relative to the Sun). From Earth, GJ 436 shines at apparent V-band magnitude 10.67 (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: W. M. Keck Observatory · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | GJ 436 |
| Spectral Type | M2.5 V |
| Effective Temp (Teff) | 3,586 K |
| Stellar Radius | 0.4550 R☉ |
| Stellar Mass | 0.4700 M☉ |
| Bolometric Luminosity | 0.031 L☉ |
| Metallicity [Fe/H] | 0.099 dex |
| Stellar Age | 6.000 Gyr |
| Apparent Magnitude | V: 10.670 · G: 9.571 |
| System Architecture | 1 Star, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.029100 AU |
| Orbital Period | 2.644 days |
| Mean Orbital Velocity | 119.7 km/s |
| Orbital Eccentricity (e) | 0.13827 |
| Orbital Inclination (i) | 86.4400° |
| Mean Bulk Density | 1.68 g/cm³ (25.8 km/s vesc) |
| Insolation Flux | 29.4322 S⊕ (~40,057 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 31.800 LY (9.753 pc) |