HD 216520 c
Astrophysical Synthesis & Habitability Characterization
HD 216520 c is a confirmed sub-neptune exoplanet orbiting the star HD 216520, located approximately 63.8 light-years (19.55 parsecs) from the Solar System. First announced in 2020 by Multiple Observatories using the Radial Velocity detection technique (measuring periodic Doppler reflex motion in the host star's spectrum), this world measures 3.03 Earth radii (0.27 Jupiter radii) and has a measured mass of 9.44 Earth masses. Combining its mass and radius yields a mean bulk density of 1.87 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 1.03 g⊕ (10.1 m/s²), and an escape velocity of 19.7 km/s.
In its orbital architecture around HD 216520, HD 216520 c completes one full revolution every 154.43 Earth days at a semi-major axis of 0.528 AU (situated at an orbital separation between Mercury's 0.39 AU and Venus's 0.72 AU). Its mean orbital velocity is approximately 37.2 km/s (compared to Earth's 29.8 km/s). Its moderate orbital eccentricity of e = 0.12 carries the world between 0.4646 AU at periapsis and 0.5914 AU at apoapsis, driving measurable seasonal insolation cycles.
☁️ Atmospheric Regime (Volatile / Gas Envelope): With a radius of 3.030 R⊕, HD 216520 c exceeds the empirical ~1.8–2.2 R⊕ Fulton radius valley threshold, indicating a substantial hydrogen-helium or supercritical volatile envelope without a conventional solid terrestrial surface.
The host star HD 216520 is classified as an orange K-type star of spectral type K2 with an effective photospheric temperature of 5,103 K, a radius of 0.76 R☉, a mass of 0.82 M☉, an estimated bolometric luminosity of 0.351 L☉. Stellar evolution modeling indicates an estimated stellar age of 6 Gyr and a metallicity of [Fe/H] = -0.16 dex (metal-poor relative to the Sun). From Earth, HD 216520 shines at apparent V-band magnitude 7.53 (accessible to binoculars and small amateur telescopes) in the Northern celestial hemisphere. The HD 216520 system is a multi-planet architecture hosting 2 confirmed worlds, with HD 216520 c sharing its stellar neighborhood alongside HD 216520 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: Multiple Observatories · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | HD 216520 |
| Spectral Type | K2 |
| Effective Temp (Teff) | 5,103 K |
| Stellar Radius | 0.7600 R☉ |
| Stellar Mass | 0.8200 M☉ |
| Bolometric Luminosity | 0.351 L☉ |
| Metallicity [Fe/H] | -0.160 dex |
| Stellar Age | 6.000 Gyr |
| Apparent Magnitude | V: 7.530 · G: 7.279 |
| System Architecture | 1 Star, 2 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.528000 AU |
| Orbital Period | 154.43 days |
| Mean Orbital Velocity | 37.2 km/s |
| Orbital Eccentricity (e) | 0.12000 |
| Orbital Inclination (i) | — |
| Mean Bulk Density | 1.87 g/cm³ (19.7 km/s vesc) |
| Insolation Flux | 1.2669 S⊕ (~1,724 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 63.800 LY (19.552 pc) |