GJ 229 A c
Astrophysical Synthesis & Habitability Characterization
GJ 229 A c is a confirmed sub-neptune exoplanet orbiting the star GJ 229, located approximately 18.8 light-years (5.76 parsecs) from the Solar System. First announced in 2020 by Multiple Facilities using the Radial Velocity detection technique (measuring periodic Doppler reflex motion in the host star's spectrum), this world measures 2.87 Earth radii and has a measured mass of 8.581 Earth masses. Combining its mass and radius yields a mean bulk density of 2 g/cm³ (an intermediate density indicative of a deep water-ice mantle or dense volatile envelope), a surface gravity of 1.04 g⊕ (10.2 m/s²), and an escape velocity of 19.3 km/s.
In its orbital architecture around GJ 229, GJ 229 A c completes one full revolution every 121.933 Earth days at a semi-major axis of 0.384 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 34.3 km/s (compared to Earth's 29.8 km/s). The planet follows a highly eccentric trajectory (e = 0.366, swinging from 0.2435 AU at periapsis out to 0.5245 AU at apoapsis), pointing to past dynamical scattering or secular gravitational perturbations.
☁️ Atmospheric Regime (Volatile / Gas Envelope): With a radius of 2.870 R⊕, GJ 229 A c exceeds the empirical ~1.8–2.2 R⊕ Fulton radius valley threshold, indicating a substantial hydrogen-helium or supercritical volatile envelope without a conventional solid terrestrial surface.
The host star GJ 229 is classified as a low-mass red dwarf of spectral type M1.5V with an effective photospheric temperature of 3,564 K, a radius of 0.46 R☉, a mass of 0.509 M☉, an estimated bolometric luminosity of 0.031 L☉. Stellar evolution modeling indicates an estimated stellar age of 0.4 Gyr. From Earth, GJ 229 shines at apparent V-band magnitude 8.15 (accessible to binoculars and small amateur telescopes) in the Southern celestial hemisphere. The GJ 229 system is a multi-planet architecture hosting 2 confirmed worlds, with GJ 229 A c sharing its stellar neighborhood alongside GJ 229 b.
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: Multiple Facilities · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | GJ 229 |
| Spectral Type | M1.5V |
| Effective Temp (Teff) | 3,564 K |
| Stellar Radius | 0.4600 R☉ |
| Stellar Mass | 0.5090 M☉ |
| Bolometric Luminosity | 0.031 L☉ |
| Metallicity [Fe/H] | — |
| Stellar Age | 0.400 Gyr |
| Apparent Magnitude | V: 8.150 · G: 7.307 |
| System Architecture | 3 Stars, 2 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.384000 AU |
| Orbital Period | 121.933 days |
| Mean Orbital Velocity | 34.3 km/s |
| Orbital Eccentricity (e) | 0.36600 |
| Orbital Inclination (i) | — |
| Mean Bulk Density | 2 g/cm³ (19.3 km/s vesc) |
| Insolation Flux | 0.1899 S⊕ (~258 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 18.800 LY (5.756 pc) |