Kepler-166 d
Astrophysical Synthesis & Habitability Characterization
Kepler-166 d is a confirmed super-earth exoplanet orbiting the star Kepler-166, located approximately 1968.4 light-years (603.53 parsecs) from the Solar System. First announced in 2016 by Kepler using the Transit detection technique (producing an estimated primary transit depth of 0.0365% across the stellar disk), this world measures 1.73 Earth radii and has a measured mass of 3.64 Earth masses. Combining its mass and radius yields a mean bulk density of 3.88 g/cm³ (an intermediate density indicative of a deep water-ice mantle or dense volatile envelope), a surface gravity of 1.22 g⊕ (11.96 m/s²), and an escape velocity of 16.2 km/s.
In its orbital architecture around Kepler-166, Kepler-166 d completes one full revolution every 1.554 Earth days at a semi-major axis of 0.02449 AU (an ultra-close orbit less than one-seventh of Mercury's 0.387 AU distance from the Sun, where strong stellar tidal forces typically induce synchronous rotation). Its mean orbital velocity is approximately 171.4 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 1313 K (+1040°C / +1904°F) and receiving 651.41 S⊕ (~886,568 W/m²), Kepler-166 d orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star Kepler-166 is classified as a stellar host with an effective photospheric temperature of 5,350 K, a radius of 0.83 R☉, a mass of 0.88 M☉, an estimated bolometric luminosity of 0.506 L☉. Stellar evolution modeling indicates an estimated stellar age of 4.47 Gyr and a metallicity of [Fe/H] = -0.03 dex (near-solar heavy-element abundance). From Earth, Kepler-166 shines at apparent V-band magnitude 14.857 (faint and requiring research-grade or CCD telescopic photometry) in the Northern celestial hemisphere. The Kepler-166 system is a multi-planet architecture hosting 3 confirmed worlds, with Kepler-166 d sharing its stellar neighborhood alongside Kepler-166 b, Kepler-166 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-166 |
| Spectral Type | Unclassified |
| Effective Temp (Teff) | 5,350 K |
| Stellar Radius | 0.8300 R☉ |
| Stellar Mass | 0.8800 M☉ |
| Bolometric Luminosity | 0.506 L☉ |
| Metallicity [Fe/H] | -0.030 dex |
| Stellar Age | 4.470 Gyr |
| Apparent Magnitude | V: 14.857 · G: 14.685 |
| System Architecture | 1 Star, 3 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.024490 AU |
| Orbital Period | 1.554 days |
| Mean Orbital Velocity | 171.4 km/s |
| Orbital Eccentricity (e) | 0.00000 |
| Orbital Inclination (i) | 89.8700° |
| Mean Bulk Density | 3.88 g/cm³ (16.2 km/s vesc) |
| Insolation Flux | 651.4090 S⊕ (~886,568 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 1968.400 LY (603.530 pc) |