K2-117 c
Astrophysical Synthesis & Habitability Characterization
K2-117 c is a confirmed sub-neptune exoplanet orbiting the star K2-117, located approximately 331.5 light-years (101.63 parsecs) from the Solar System. First announced in 2017 by K2 using the Transit detection technique (producing an estimated primary transit depth of 0.1371% across the stellar disk), this world measures 2.06 Earth radii and has a measured mass of 4.9 Earth masses. Combining its mass and radius yields a mean bulk density of 3.09 g/cm³ (an intermediate density indicative of a deep water-ice mantle or dense volatile envelope), a surface gravity of 1.15 g⊕ (11.28 m/s²), and an escape velocity of 17.3 km/s.
In its orbital architecture around K2-117, K2-117 c completes one full revolution every 5.444 Earth days at a semi-major axis of 0.0492 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 98.3 km/s (compared to Earth's 29.8 km/s). Its moderate orbital eccentricity of e = 0.15 carries the world between 0.0418 AU at periapsis and 0.0566 AU at apoapsis, driving measurable seasonal insolation cycles.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 537 K (+264°C / +507°F) and receiving 21.40 S⊕ (~29,125 W/m²), K2-117 c orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star K2-117 is classified as a low-mass red dwarf of spectral type M1.0 V with an effective photospheric temperature of 3,778 K, a radius of 0.51 R☉, a mass of 0.54 M☉, an estimated bolometric luminosity of 0.048 L☉. Stellar evolution modeling indicates a metallicity of [Fe/H] = -0.05 dex (near-solar heavy-element abundance). From Earth, K2-117 shines at apparent V-band magnitude 14.639 (faint and requiring research-grade or CCD telescopic photometry) in the Northern celestial hemisphere. The K2-117 system is a multi-planet architecture hosting 2 confirmed worlds, with K2-117 c sharing its stellar neighborhood alongside K2-117 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 | K2-117 |
| Spectral Type | M1.0 V |
| Effective Temp (Teff) | 3,778 K |
| Stellar Radius | 0.5100 R☉ |
| Stellar Mass | 0.5400 M☉ |
| Bolometric Luminosity | 0.048 L☉ |
| Metallicity [Fe/H] | -0.050 dex |
| Stellar Age | — |
| Apparent Magnitude | V: 14.639 · G: 13.801 |
| System Architecture | 1 Star, 2 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.049200 AU |
| Orbital Period | 5.444 days |
| Mean Orbital Velocity | 98.3 km/s |
| Orbital Eccentricity (e) | 0.15000 |
| Orbital Inclination (i) | 89.4900° |
| Mean Bulk Density | 3.09 g/cm³ (17.3 km/s vesc) |
| Insolation Flux | 21.4000 S⊕ (~29,125 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 331.500 LY (101.632 pc) |