Kepler-338 d
Astrophysical Synthesis & Habitability Characterization
Kepler-338 d is a confirmed sub-neptune exoplanet orbiting the star Kepler-338, located approximately 1803 light-years (552.8 parsecs) from the Solar System. First announced in 2014 by Kepler using the Transit detection technique (producing an estimated primary transit depth of 0.0251% across the stellar disk), this world measures 3 Earth radii (0.27 Jupiter radii) and has a measured mass of 9.27 Earth masses. Combining its mass and radius yields a mean bulk density of 1.89 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 1.03 g⊕ (10.1 m/s²), and an escape velocity of 19.7 km/s.
In its orbital architecture around Kepler-338, Kepler-338 d completes one full revolution every 44.431 Earth days at a semi-major axis of 0.257 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 62.9 km/s (compared to Earth's 29.8 km/s). With a near-circular eccentricity of e = 0.03 (ranging from 0.2493 AU at periapsis to 0.2647 AU at apoapsis), the planet experiences minimal orbital distance variation.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 680 K (+407°C / +765°F) and receiving 53.56 S⊕ (~72,890 W/m²), Kepler-338 d orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star Kepler-338 is classified as a stellar host with an effective photospheric temperature of 5,923 K, a radius of 1.735 R☉, a mass of 1.105 M☉, an estimated bolometric luminosity of 3.324 L☉. Stellar evolution modeling indicates an estimated stellar age of 6.46 Gyr and a metallicity of [Fe/H] = -0.029 dex (near-solar heavy-element abundance). From Earth, Kepler-338 shines at apparent V-band magnitude 12.19 (faint and requiring research-grade or CCD telescopic photometry) in the Northern celestial hemisphere. The Kepler-338 system is a multi-planet architecture hosting 4 confirmed worlds, with Kepler-338 d sharing its stellar neighborhood alongside Kepler-338 e, Kepler-338 b, Kepler-338 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: Kepler · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | Kepler-338 |
| Spectral Type | Unclassified |
| Effective Temp (Teff) | 5,923 K |
| Stellar Radius | 1.7350 R☉ |
| Stellar Mass | 1.1050 M☉ |
| Bolometric Luminosity | 3.324 L☉ |
| Metallicity [Fe/H] | -0.029 dex |
| Stellar Age | 6.460 Gyr |
| Apparent Magnitude | V: 12.190 · G: 12.083 |
| System Architecture | 1 Star, 4 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.257000 AU |
| Orbital Period | 44.431 days |
| Mean Orbital Velocity | 62.9 km/s |
| Orbital Eccentricity (e) | 0.03000 |
| Orbital Inclination (i) | 89.9600° |
| Mean Bulk Density | 1.89 g/cm³ (19.7 km/s vesc) |
| Insolation Flux | 53.5560 S⊕ (~72,890 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 1803.000 LY (552.798 pc) |