K2-128 b
Astrophysical Synthesis & Habitability Characterization
K2-128 b is a confirmed super-earth exoplanet orbiting the star K2-128, located approximately 373.9 light-years (114.64 parsecs) from the Solar System. First announced in 2017 by K2 using the Transit detection technique (producing an estimated primary transit depth of 0.028% across the stellar disk), this world measures 1.42 Earth radii and has a measured mass of 2.6 Earth masses. Combining its mass and radius yields a mean bulk density of 5.01 g/cm³ (a terrestrial silicate-iron composition comparable to Earth (5.51 g/cm³)), a surface gravity of 1.29 g⊕ (12.65 m/s²), and an escape velocity of 15.1 km/s.
In its orbital architecture around K2-128, K2-128 b completes one full revolution every 5.676 Earth days at a semi-major axis of 0.056 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 107.3 km/s (compared to Earth's 29.8 km/s). Its moderate orbital eccentricity of e = 0.23 carries the world between 0.0431 AU at periapsis and 0.0689 AU at apoapsis, driving measurable seasonal insolation cycles.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 803 K (+530°C / +986°F) and receiving 68.80 S⊕ (~93,637 W/m²), K2-128 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star K2-128 is classified as a stellar host with an effective photospheric temperature of 4,470 K, a radius of 0.7776 R☉, a mass of 0.7112 M☉, an estimated bolometric luminosity of 0.217 L☉. Stellar evolution modeling indicates a metallicity of [Fe/H] = -0.26 dex (metal-poor relative to the Sun). From Earth, K2-128 shines at apparent V-band magnitude 12.58 (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 | K2-128 |
| Spectral Type | Unclassified |
| Effective Temp (Teff) | 4,470 K |
| Stellar Radius | 0.7776 R☉ |
| Stellar Mass | 0.7112 M☉ |
| Bolometric Luminosity | 0.217 L☉ |
| Metallicity [Fe/H] | -0.260 dex |
| Stellar Age | — |
| Apparent Magnitude | V: 12.580 · G: 12.220 |
| System Architecture | 1 Star, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.056000 AU |
| Orbital Period | 5.676 days |
| Mean Orbital Velocity | 107.3 km/s |
| Orbital Eccentricity (e) | 0.23000 |
| Orbital Inclination (i) | 87.4000° |
| Mean Bulk Density | 5.01 g/cm³ (15.1 km/s vesc) |
| Insolation Flux | 68.8000 S⊕ (~93,637 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 373.900 LY (114.637 pc) |