HD 20794 d
Astrophysical Synthesis & Habitability Characterization
HD 20794 d is a confirmed sub-neptune exoplanet orbiting the star HD 20794, located approximately 19.6 light-years (6 parsecs) from the Solar System. First announced in 2011 by La Silla Observatory using the Radial Velocity detection technique (measuring periodic Doppler reflex motion in the host star's spectrum), this world measures 2.04 Earth radii and has a measured mass of 4.8 Earth masses. Combining its mass and radius yields a mean bulk density of 3.12 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 17.2 km/s.
In its orbital architecture around HD 20794, HD 20794 d completes one full revolution every 89.68 Earth days at a semi-major axis of 0.3625 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 44 km/s (compared to Earth's 29.8 km/s). Its moderate orbital eccentricity of e = 0.077 carries the world between 0.3346 AU at periapsis and 0.3904 AU at apoapsis, driving measurable seasonal insolation cycles.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 388 K (+115°C / +239°F) and receiving 5.23 S⊕ (~7,114 W/m²), HD 20794 d orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star HD 20794 is classified as a solar-type yellow star of spectral type G6 V with an effective photospheric temperature of 5,368 K, a radius of 0.93 R☉, a mass of 0.79 M☉, an estimated bolometric luminosity of 0.644 L☉. Stellar evolution modeling indicates an estimated stellar age of 5.76 Gyr and a metallicity of [Fe/H] = -0.42 dex (metal-poor relative to the Sun). From Earth, HD 20794 shines at apparent V-band magnitude 4.26 (visible to the unaided human eye under dark skies) in the Southern celestial hemisphere. The HD 20794 system is a multi-planet architecture hosting 4 confirmed worlds, with HD 20794 d sharing its stellar neighborhood alongside HD 20794 b, HD 20794 e, HD 20794 f.
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 20794 |
| Spectral Type | G6 V |
| Effective Temp (Teff) | 5,368 K |
| Stellar Radius | 0.9300 R☉ |
| Stellar Mass | 0.7900 M☉ |
| Bolometric Luminosity | 0.644 L☉ |
| Metallicity [Fe/H] | -0.420 dex |
| Stellar Age | 5.760 Gyr |
| Apparent Magnitude | V: 4.260 · G: 4.066 |
| System Architecture | 1 Star, 4 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.362500 AU |
| Orbital Period | 89.68 days |
| Mean Orbital Velocity | 44 km/s |
| Orbital Eccentricity (e) | 0.07700 |
| Orbital Inclination (i) | 90.0000° |
| Mean Bulk Density | 3.12 g/cm³ (17.2 km/s vesc) |
| Insolation Flux | 5.2273 S⊕ (~7,114 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 19.600 LY (6.003 pc) |