HD 5278 c
Astrophysical Synthesis & Habitability Characterization
HD 5278 c is a confirmed sub-neptune exoplanet orbiting the star HD 5278, located approximately 187.8 light-years (57.59 parsecs) from the Solar System. First announced in 2021 by Paranal Observatory using the Radial Velocity detection technique (measuring periodic Doppler reflex motion in the host star's spectrum), this world measures 4.49 Earth radii (0.4 Jupiter radii) and has a measured mass of 18.4 Earth masses. Combining its mass and radius yields a mean bulk density of 1.12 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 0.91 g⊕ (8.92 m/s²), and an escape velocity of 22.6 km/s.
In its orbital architecture around HD 5278, HD 5278 c completes one full revolution every 40.87 Earth days at a semi-major axis of 0.2416 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 64.3 km/s (compared to Earth's 29.8 km/s). Its moderate orbital eccentricity of e = 0.07 carries the world between 0.2247 AU at periapsis and 0.2585 AU at apoapsis, driving measurable seasonal insolation cycles.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 665 K (+392°C / +738°F) and receiving 32.67 S⊕ (~44,465 W/m²), HD 5278 c orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star HD 5278 is classified as a warm F-type star of spectral type F V with an effective photospheric temperature of 6,203 K, a radius of 1.194 R☉, a mass of 1.126 M☉, an estimated bolometric luminosity of 1.894 L☉. Stellar evolution modeling indicates an estimated stellar age of 3 Gyr and a metallicity of [Fe/H] = -0.12 dex (metal-poor relative to the Sun). From Earth, HD 5278 shines at apparent V-band magnitude 7.94 (accessible to binoculars and small amateur telescopes) in the Southern celestial hemisphere. The HD 5278 system is a multi-planet architecture hosting 2 confirmed worlds, with HD 5278 c sharing its stellar neighborhood alongside HD 5278 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: Paranal Observatory · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | HD 5278 |
| Spectral Type | F V |
| Effective Temp (Teff) | 6,203 K |
| Stellar Radius | 1.1940 R☉ |
| Stellar Mass | 1.1260 M☉ |
| Bolometric Luminosity | 1.894 L☉ |
| Metallicity [Fe/H] | -0.120 dex |
| Stellar Age | 3.000 Gyr |
| Apparent Magnitude | V: 7.940 · G: 7.804 |
| System Architecture | 1 Star, 2 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.241600 AU |
| Orbital Period | 40.87 days |
| Mean Orbital Velocity | 64.3 km/s |
| Orbital Eccentricity (e) | 0.07000 |
| Orbital Inclination (i) | — |
| Mean Bulk Density | 1.12 g/cm³ (22.6 km/s vesc) |
| Insolation Flux | 32.6706 S⊕ (~44,465 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 187.800 LY (57.585 pc) |