Kepler-42 d
Astrophysical Synthesis & Habitability Characterization
Kepler-42 d is a confirmed sub-earth exoplanet orbiting the star Kepler-42, located approximately 130.7 light-years (40.06 parsecs) from the Solar System. First announced in 2011 by Kepler using the Transit detection technique (producing an estimated primary transit depth of 0.0945% across the stellar disk), this world measures 0.57 Earth radii and has a measured mass of 0.358 Earth masses. Combining its mass and radius yields a mean bulk density of 10.66 g/cm³ (an iron-enriched, high-compression rocky core), a surface gravity of 1.1 g⊕ (10.79 m/s²), and an escape velocity of 8.9 km/s.
In its orbital architecture around Kepler-42, Kepler-42 d completes one full revolution every 1.865 Earth days at a semi-major axis of 0.0154 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 89.8 km/s (compared to Earth's 29.8 km/s). With a near-circular eccentricity of e = 0.02 (ranging from 0.0151 AU at periapsis to 0.0157 AU at apoapsis), the planet experiences minimal orbital distance variation.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 450 K (+177°C / +351°F) and receiving 12.54 S⊕ (~17,064 W/m²), Kepler-42 d orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star Kepler-42 is classified as a low-mass red dwarf of spectral type M with an effective photospheric temperature of 3,068 K, a radius of 0.17 R☉, a mass of 0.13 M☉, an estimated bolometric luminosity of 0.00230 L☉. Stellar evolution modeling indicates an estimated stellar age of 4.5 Gyr and a metallicity of [Fe/H] = -0.33 dex (metal-poor relative to the Sun). From Earth, Kepler-42 shines at apparent V-band magnitude 16.7 (faint and requiring research-grade or CCD telescopic photometry) in the Northern celestial hemisphere. The Kepler-42 system is a multi-planet architecture hosting 3 confirmed worlds, with Kepler-42 d sharing its stellar neighborhood alongside Kepler-42 c, Kepler-42 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: Kepler · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | Kepler-42 |
| Spectral Type | M |
| Effective Temp (Teff) | 3,068 K |
| Stellar Radius | 0.1700 R☉ |
| Stellar Mass | 0.1300 M☉ |
| Bolometric Luminosity | 0.00230 L☉ |
| Metallicity [Fe/H] | -0.330 dex |
| Stellar Age | 4.500 Gyr |
| Apparent Magnitude | V: 16.700 · G: 14.993 |
| System Architecture | 1 Star, 3 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.015400 AU |
| Orbital Period | 1.865 days |
| Mean Orbital Velocity | 89.8 km/s |
| Orbital Eccentricity (e) | 0.02000 |
| Orbital Inclination (i) | 88.0200° |
| Mean Bulk Density | 10.66 g/cm³ (8.9 km/s vesc) |
| Insolation Flux | 12.5380 S⊕ (~17,064 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 130.700 LY (40.060 pc) |