K2-398 c
Astrophysical Synthesis & Habitability Characterization
K2-398 c is a confirmed sub-neptune exoplanet orbiting the star K2-398, located approximately 1979.7 light-years (606.99 parsecs) from the Solar System. First announced in 2022 by K2 using the Transit detection technique (producing an estimated primary transit depth of 0.1423% across the stellar disk), this world measures 5.098 Earth radii (0.45 Jupiter radii) and has a measured mass of 22.8 Earth masses. Combining its mass and radius yields a mean bulk density of 0.95 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 0.88 g⊕ (8.63 m/s²), and an escape velocity of 23.7 km/s.
In its orbital architecture around K2-398, K2-398 c completes one full revolution every 18.788 Earth days.
☁️ Atmospheric Regime (Volatile / Gas Envelope): With a radius of 5.098 R⊕, K2-398 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 K2-398 is classified as a solar-type yellow star of spectral type G2 with an effective photospheric temperature of 5,946 K, a radius of 1.239 R☉, a mass of 0.945 M☉, an estimated bolometric luminosity of 1.722 L☉. Stellar evolution modeling indicates a metallicity of [Fe/H] = -0.257 dex (metal-poor relative to the Sun). From Earth, K2-398 shines at apparent V-band magnitude 13.287 (faint and requiring research-grade or CCD telescopic photometry) in the Northern celestial hemisphere. The K2-398 system is a multi-planet architecture hosting 2 confirmed worlds, with K2-398 c sharing its stellar neighborhood alongside K2-398 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: K2 · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | K2-398 |
| Spectral Type | G2 |
| Effective Temp (Teff) | 5,946 K |
| Stellar Radius | 1.2390 R☉ |
| Stellar Mass | 0.9450 M☉ |
| Bolometric Luminosity | 1.722 L☉ |
| Metallicity [Fe/H] | -0.257 dex |
| Stellar Age | — |
| Apparent Magnitude | V: 13.287 · G: 13.009 |
| System Architecture | 1 Star, 2 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | — |
| Orbital Period | 18.788 days |
| Orbital Eccentricity (e) | Circular (0.00) |
| Orbital Inclination (i) | — |
| Mean Bulk Density | 0.95 g/cm³ (23.7 km/s vesc) |
| Insolation Flux | — |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 1979.700 LY (606.992 pc) |