GJ 3998 c
Astrophysical Synthesis & Habitability Characterization
GJ 3998 c is a confirmed sub-neptune exoplanet orbiting the star GJ 3998, located approximately 59.2 light-years (18.15 parsecs) from the Solar System. First announced in 2016 by Roque de los Muchachos Observatory using the Radial Velocity detection technique (measuring periodic Doppler reflex motion in the host star's spectrum), this world measures 2.5 Earth radii and has a measured mass of 6.82 Earth masses. Combining its mass and radius yields a mean bulk density of 2.41 g/cm³ (an intermediate density indicative of a deep water-ice mantle or dense volatile envelope), a surface gravity of 1.09 g⊕ (10.69 m/s²), and an escape velocity of 18.5 km/s.
In its orbital architecture around GJ 3998, GJ 3998 c completes one full revolution every 13.727 Earth days at a semi-major axis of 0.09 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 71.3 km/s (compared to Earth's 29.8 km/s). The orbit is modeled as circular (e = 0.00), maintaining a constant stellar distance throughout its year.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 423 K (+150°C / +302°F) and receiving 5.40 S⊕ (~7,349 W/m²), GJ 3998 c orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star GJ 3998 is classified as a low-mass red dwarf of spectral type M1 with an effective photospheric temperature of 3,726 K, a radius of 0.5 R☉, a mass of 0.52 M☉, an estimated bolometric luminosity of 0.043 L☉. Stellar evolution modeling indicates an estimated stellar age of 8.38 Gyr and a metallicity of [Fe/H] = -0.13 dex (metal-poor relative to the Sun). From Earth, GJ 3998 shines at apparent V-band magnitude 10.844 (faint and requiring research-grade or CCD telescopic photometry) in the Northern celestial hemisphere. The GJ 3998 system is a multi-planet architecture hosting 3 confirmed worlds, with GJ 3998 c sharing its stellar neighborhood alongside GJ 3998 b, GJ 3998 d.
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: Roque de los Muchachos Observatory · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | GJ 3998 |
| Spectral Type | M1 |
| Effective Temp (Teff) | 3,726 K |
| Stellar Radius | 0.5000 R☉ |
| Stellar Mass | 0.5200 M☉ |
| Bolometric Luminosity | 0.043 L☉ |
| Metallicity [Fe/H] | -0.130 dex |
| Stellar Age | 8.380 Gyr |
| Apparent Magnitude | V: 10.844 · G: 9.983 |
| System Architecture | 1 Star, 3 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.090000 AU |
| Orbital Period | 13.727 days |
| Mean Orbital Velocity | 71.3 km/s |
| Orbital Eccentricity (e) | 0.00000 |
| Orbital Inclination (i) | — |
| Mean Bulk Density | 2.41 g/cm³ (18.5 km/s vesc) |
| Insolation Flux | 5.4000 S⊕ (~7,349 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 59.200 LY (18.146 pc) |