Kapteyn c
Astrophysical Synthesis & Habitability Characterization
Kapteyn c is a confirmed sub-neptune exoplanet orbiting the star Kapteyn, located approximately 12.8 light-years (3.93 parsecs) from the Solar System. First announced in 2014 by Multiple Observatories using the Radial Velocity detection technique (measuring periodic Doppler reflex motion in the host star's spectrum), this world measures 2.54 Earth radii and has a measured mass of 7 Earth masses. Combining its mass and radius yields a mean bulk density of 2.36 g/cm³ (an intermediate density indicative of a deep water-ice mantle or dense volatile envelope), a surface gravity of 1.09 g⊕ (10.69 m/s²), and an escape velocity of 18.6 km/s.
In its orbital architecture around Kapteyn, Kapteyn c completes one full revolution every 121.54 Earth days at a semi-major axis of 0.311 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 27.8 km/s (compared to Earth's 29.8 km/s). Its moderate orbital eccentricity of e = 0.23 carries the world between 0.2395 AU at periapsis and 0.3825 AU at apoapsis, driving measurable seasonal insolation cycles.
❄️ Thermal Regime (Cryogenic Zone): With an equilibrium temperature of 166 K (-107°C / -161°F) and an incident flux of 0.17 S⊕ (~227 W/m²), surface water and volatile species on Kapteyn c would remain locked in cryogenic ice phases unless warmed by subsurface tidal or radiogenic heating.
The host star Kapteyn is classified as a low-mass red dwarf of spectral type M2.0V with an effective photospheric temperature of 3,550 K, a radius of 0.291 R☉, a mass of 0.281 M☉, an estimated bolometric luminosity of 0.012 L☉. Stellar evolution modeling indicates a metallicity of [Fe/H] = -0.89 dex (metal-poor relative to the Sun). From Earth, Kapteyn shines at apparent V-band magnitude 8.86 (accessible to binoculars and small amateur telescopes) 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: Multiple Observatories · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | Kapteyn |
| Spectral Type | M2.0V |
| Effective Temp (Teff) | 3,550 K |
| Stellar Radius | 0.2910 R☉ |
| Stellar Mass | 0.2810 M☉ |
| Bolometric Luminosity | 0.012 L☉ |
| Metallicity [Fe/H] | -0.890 dex |
| Stellar Age | — |
| Apparent Magnitude | V: 8.860 · G: 8.062 |
| System Architecture | 1 Star, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.311000 AU |
| Orbital Period | 121.54 days |
| Mean Orbital Velocity | 27.8 km/s |
| Orbital Eccentricity (e) | 0.23000 |
| Orbital Inclination (i) | — |
| Mean Bulk Density | 2.36 g/cm³ (18.6 km/s vesc) |
| Insolation Flux | 0.1666 S⊕ (~227 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 12.800 LY (3.933 pc) |