GJ 3138 d
Astrophysical Synthesis & Habitability Characterization
GJ 3138 d is a confirmed sub-neptune exoplanet orbiting the star GJ 3138, located approximately 92.9 light-years (28.47 parsecs) from the Solar System. First announced in 2017 by La Silla Observatory using the Radial Velocity detection technique (measuring periodic Doppler reflex motion in the host star's spectrum), this world measures 3.23 Earth radii (0.29 Jupiter radii) and has a measured mass of 10.5 Earth masses. Combining its mass and radius yields a mean bulk density of 1.72 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 1.01 g⊕ (9.9 m/s²), and an escape velocity of 20.2 km/s.
In its orbital architecture around GJ 3138, GJ 3138 d completes one full revolution every 257.8 Earth days at a semi-major axis of 0.698 AU (situated at an orbital separation between Mercury's 0.39 AU and Venus's 0.72 AU). Its mean orbital velocity is approximately 29.5 km/s (compared to Earth's 29.8 km/s). The planet follows a highly eccentric trajectory (e = 0.32, swinging from 0.4746 AU at periapsis out to 0.9214 AU at apoapsis), pointing to past dynamical scattering or secular gravitational perturbations.
❄️ Thermal Regime (Cryogenic Zone): With an equilibrium temperature of 152 K (-121°C / -186°F) and an incident flux of 0.10 S⊕ (~136 W/m²), surface water and volatile species on GJ 3138 d would remain locked in cryogenic ice phases unless warmed by subsurface tidal or radiogenic heating.
The host star GJ 3138 is classified as a low-mass red dwarf of spectral type M0 with an effective photospheric temperature of 3,717 K, a radius of 0.5 R☉, a mass of 0.681 M☉, an estimated bolometric luminosity of 0.043 L☉. Stellar evolution modeling indicates a metallicity of [Fe/H] = -0.3 dex (metal-poor relative to the Sun). From Earth, GJ 3138 shines at apparent V-band magnitude 10.828 (faint and requiring research-grade or CCD telescopic photometry) in the Southern celestial hemisphere. The GJ 3138 system is a multi-planet architecture hosting 3 confirmed worlds, with GJ 3138 d sharing its stellar neighborhood alongside GJ 3138 b, GJ 3138 c.
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: La Silla Observatory · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | GJ 3138 |
| Spectral Type | M0 |
| Effective Temp (Teff) | 3,717 K |
| Stellar Radius | 0.5000 R☉ |
| Stellar Mass | 0.6810 M☉ |
| Bolometric Luminosity | 0.043 L☉ |
| Metallicity [Fe/H] | -0.300 dex |
| Stellar Age | — |
| Apparent Magnitude | V: 10.828 · G: 10.192 |
| System Architecture | 1 Star, 3 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.698000 AU |
| Orbital Period | 257.8 days |
| Mean Orbital Velocity | 29.5 km/s |
| Orbital Eccentricity (e) | 0.32000 |
| Orbital Inclination (i) | — |
| Mean Bulk Density | 1.72 g/cm³ (20.2 km/s vesc) |
| Insolation Flux | 0.1000 S⊕ (~136 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 92.900 LY (28.471 pc) |