Kepler-218 d
Astrophysical Synthesis & Habitability Characterization
Kepler-218 d is a confirmed sub-neptune exoplanet orbiting the star Kepler-218, located approximately 2140.8 light-years (656.38 parsecs) from the Solar System. First announced in 2016 by Kepler using the Transit detection technique (producing an estimated primary transit depth of 0.0572% across the stellar disk), this world measures 2.66 Earth radii and has a measured mass of 7.56 Earth masses. Combining its mass and radius yields a mean bulk density of 2.21 g/cm³ (an intermediate density indicative of a deep water-ice mantle or dense volatile envelope), a surface gravity of 1.07 g⊕ (10.49 m/s²), and an escape velocity of 18.9 km/s.
In its orbital architecture around Kepler-218, Kepler-218 d completes one full revolution every 124.524 Earth days at a semi-major axis of 0.49884 AU (situated at an orbital separation between Mercury's 0.39 AU and Venus's 0.72 AU). Its mean orbital velocity is approximately 43.6 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 356 K (+83°C / +181°F) and receiving 4.12 S⊕ (~5,605 W/m²), Kepler-218 d orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star Kepler-218 is classified as a stellar host with an effective photospheric temperature of 5,542 K, a radius of 1.02 R☉, a mass of 1.01 M☉, an estimated bolometric luminosity of 0.881 L☉. Stellar evolution modeling indicates an estimated stellar age of 5.62 Gyr and a metallicity of [Fe/H] = 0.31 dex (metal-rich relative to the Sun). From Earth, Kepler-218 shines at apparent V-band magnitude 14.035 (faint and requiring research-grade or CCD telescopic photometry) in the Northern celestial hemisphere. The Kepler-218 system is a multi-planet architecture hosting 3 confirmed worlds, with Kepler-218 d sharing its stellar neighborhood alongside Kepler-218 b, Kepler-218 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-218 |
| Spectral Type | Unclassified |
| Effective Temp (Teff) | 5,542 K |
| Stellar Radius | 1.0200 R☉ |
| Stellar Mass | 1.0100 M☉ |
| Bolometric Luminosity | 0.881 L☉ |
| Metallicity [Fe/H] | 0.310 dex |
| Stellar Age | 5.620 Gyr |
| Apparent Magnitude | V: 14.035 · G: 13.907 |
| System Architecture | 1 Star, 3 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.498840 AU |
| Orbital Period | 124.524 days |
| Mean Orbital Velocity | 43.6 km/s |
| Orbital Eccentricity (e) | 0.00000 |
| Orbital Inclination (i) | 90.0000° |
| Mean Bulk Density | 2.21 g/cm³ (18.9 km/s vesc) |
| Insolation Flux | 4.1180 S⊕ (~5,605 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 2140.800 LY (656.380 pc) |