HD 20781 c
Astrophysical Synthesis & Habitability Characterization
HD 20781 c is a confirmed sub-neptune exoplanet orbiting the star HD 20781, located approximately 117.3 light-years (35.97 parsecs) from the Solar System. First announced in 2019 by La Silla Observatory using the Radial Velocity detection technique (measuring periodic Doppler reflex motion in the host star's spectrum), this world measures 2.17 Earth radii and has a measured mass of 5.33 Earth masses. Combining its mass and radius yields a mean bulk density of 2.88 g/cm³ (an intermediate density indicative of a deep water-ice mantle or dense volatile envelope), a surface gravity of 1.13 g⊕ (11.08 m/s²), and an escape velocity of 17.5 km/s.
In its orbital architecture around HD 20781, HD 20781 c completes one full revolution every 13.891 Earth days at a semi-major axis of 0.1004 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 78.6 km/s (compared to Earth's 29.8 km/s). Its moderate orbital eccentricity of e = 0.09 carries the world between 0.0914 AU at periapsis and 0.1094 AU at apoapsis, driving measurable seasonal insolation cycles.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 730 K (+457°C / +855°F) and receiving 48.61 S⊕ (~66,159 W/m²), HD 20781 c orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star HD 20781 is classified as an orange K-type star of spectral type K0 V with an effective photospheric temperature of 5,256 K, a radius of 0.8336 R☉, a mass of 0.7 M☉, an estimated bolometric luminosity of 0.476 L☉. Stellar evolution modeling indicates a metallicity of [Fe/H] = -0.11 dex (metal-poor relative to the Sun). From Earth, HD 20781 shines at apparent V-band magnitude 8.48 (accessible to binoculars and small amateur telescopes) in the Southern celestial hemisphere. The HD 20781 system is a multi-planet architecture hosting 4 confirmed worlds, with HD 20781 c sharing its stellar neighborhood alongside HD 20781 b, HD 20781 d, HD 20781 e.
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: La Silla Observatory · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | HD 20781 |
| Spectral Type | K0 V |
| Effective Temp (Teff) | 5,256 K |
| Stellar Radius | 0.8336 R☉ |
| Stellar Mass | 0.7000 M☉ |
| Bolometric Luminosity | 0.476 L☉ |
| Metallicity [Fe/H] | -0.110 dex |
| Stellar Age | — |
| Apparent Magnitude | V: 8.480 · G: 8.237 |
| System Architecture | 2 Stars, 5 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.100400 AU |
| Orbital Period | 13.891 days |
| Mean Orbital Velocity | 78.6 km/s |
| Orbital Eccentricity (e) | 0.09000 |
| Orbital Inclination (i) | — |
| Mean Bulk Density | 2.88 g/cm³ (17.5 km/s vesc) |
| Insolation Flux | 48.6103 S⊕ (~66,159 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 117.300 LY (35.972 pc) |