GJ 876 d
Astrophysical Synthesis & Habitability Characterization
GJ 876 d is a confirmed sub-neptune exoplanet orbiting the star GJ 876, located approximately 15.2 light-years (4.68 parsecs) from the Solar System. First announced in 2005 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 2.51 Earth radii and has a measured mass of 6.83 Earth masses. Combining its mass and radius yields a mean bulk density of 2.38 g/cm³ (an intermediate density indicative of a deep water-ice mantle or dense volatile envelope), a surface gravity of 1.08 g⊕ (10.59 m/s²), and an escape velocity of 18.5 km/s.
In its orbital architecture around GJ 876, GJ 876 d completes one full revolution every 1.938 Earth days at a semi-major axis of 0.020807 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 116.8 km/s (compared to Earth's 29.8 km/s). Its moderate orbital eccentricity of e = 0.207 carries the world between 0.0165 AU at periapsis and 0.0251 AU at apoapsis, driving measurable seasonal insolation cycles.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 603 K (+330°C / +626°F) and receiving 28.64 S⊕ (~38,983 W/m²), GJ 876 d orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star GJ 876 is classified as a low-mass red dwarf of spectral type M2.5V with an effective photospheric temperature of 3,294 K, a radius of 0.3 R☉, a mass of 0.32 M☉, an estimated bolometric luminosity of 0.00951 L☉. Stellar evolution modeling indicates an estimated stellar age of 1 Gyr and a metallicity of [Fe/H] = 0.213 dex (metal-rich relative to the Sun). From Earth, GJ 876 shines at apparent V-band magnitude 10.16 (faint and requiring research-grade or CCD telescopic photometry) in the Southern celestial hemisphere. The GJ 876 system is a multi-planet architecture hosting 4 confirmed worlds, with GJ 876 d sharing its stellar neighborhood alongside GJ 876 c, GJ 876 b, GJ 876 e.
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 876 |
| Spectral Type | M2.5V |
| Effective Temp (Teff) | 3,294 K |
| Stellar Radius | 0.3000 R☉ |
| Stellar Mass | 0.3200 M☉ |
| Bolometric Luminosity | 0.00951 L☉ |
| Metallicity [Fe/H] | 0.213 dex |
| Stellar Age | 1.000 Gyr |
| Apparent Magnitude | V: 10.160 · G: 8.875 |
| System Architecture | 1 Star, 4 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.020807 AU |
| Orbital Period | 1.938 days |
| Mean Orbital Velocity | 116.8 km/s |
| Orbital Eccentricity (e) | 0.20700 |
| Orbital Inclination (i) | 59.0000° |
| Mean Bulk Density | 2.38 g/cm³ (18.5 km/s vesc) |
| Insolation Flux | 28.6429 S⊕ (~38,983 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 15.200 LY (4.675 pc) |