K2-4 b
Astrophysical Synthesis & Habitability Characterization
K2-4 b is a confirmed sub-neptune exoplanet orbiting the star K2-4, located approximately 756.1 light-years (231.83 parsecs) from the Solar System. First announced in 2015 by K2 using the Transit detection technique (producing an estimated primary transit depth of 0.1143% across the stellar disk), this world measures 2.1 Earth radii and has a measured mass of 5.06 Earth masses. Combining its mass and radius yields a mean bulk density of 3.01 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 17.4 km/s.
In its orbital architecture around K2-4, K2-4 b completes one full revolution every 10.003 Earth days at a semi-major axis of 0.078 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 84.8 km/s (compared to Earth's 29.8 km/s). Its moderate orbital eccentricity of e = 0.08 carries the world between 0.0718 AU at periapsis and 0.0842 AU at apoapsis, driving measurable seasonal insolation cycles.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 563 K (+290°C / +554°F) and receiving 15.60 S⊕ (~21,232 W/m²), K2-4 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star K2-4 is classified as a stellar host with an effective photospheric temperature of 4,015 K, a radius of 0.5694 R☉, a mass of 0.635 M☉, an estimated bolometric luminosity of 0.076 L☉. Stellar evolution modeling indicates a metallicity of [Fe/H] = -0.12 dex (metal-poor relative to the Sun). From Earth, K2-4 shines at apparent V-band magnitude 14.855 (faint and requiring research-grade or CCD telescopic photometry) in the Southern celestial hemisphere.
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 | K2-4 |
| Spectral Type | Unclassified |
| Effective Temp (Teff) | 4,015 K |
| Stellar Radius | 0.5694 R☉ |
| Stellar Mass | 0.6350 M☉ |
| Bolometric Luminosity | 0.076 L☉ |
| Metallicity [Fe/H] | -0.120 dex |
| Stellar Age | — |
| Apparent Magnitude | V: 14.855 · G: 14.340 |
| System Architecture | 1 Star, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.078000 AU |
| Orbital Period | 10.003 days |
| Mean Orbital Velocity | 84.8 km/s |
| Orbital Eccentricity (e) | 0.08000 |
| Orbital Inclination (i) | 89.5400° |
| Mean Bulk Density | 3.01 g/cm³ (17.4 km/s vesc) |
| Insolation Flux | 15.6000 S⊕ (~21,232 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 756.100 LY (231.828 pc) |