HD 39855 b
Astrophysical Synthesis & Habitability Characterization
HD 39855 b is a confirmed sub-neptune exoplanet orbiting the star HD 39855, located approximately 75.9 light-years (23.26 parsecs) from the Solar System. First announced in 2019 by Las Campanas Observatory using the Radial Velocity detection technique (measuring periodic Doppler reflex motion in the host star's spectrum), this world measures 2.85 Earth radii and has a measured mass of 8.5 Earth masses. Combining its mass and radius yields a mean bulk density of 2.02 g/cm³ (an intermediate density indicative of a deep water-ice mantle or dense volatile envelope), a surface gravity of 1.05 g⊕ (10.3 m/s²), and an escape velocity of 19.3 km/s.
In its orbital architecture around HD 39855, HD 39855 b completes one full revolution every 3.25 Earth days at a semi-major axis of 0.041 AU (an ultra-close orbit less than one-seventh of Mercury's 0.387 AU distance from the Sun, where strong stellar tidal forces typically induce synchronous rotation). Its mean orbital velocity is approximately 137.3 km/s (compared to Earth's 29.8 km/s). Its moderate orbital eccentricity of e = 0.14 carries the world between 0.0353 AU at periapsis and 0.0467 AU at apoapsis, driving measurable seasonal insolation cycles.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 1146 K (+873°C / +1603°F) and receiving 287.20 S⊕ (~390,879 W/m²), HD 39855 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star HD 39855 is classified as a solar-type yellow star of spectral type G8 V with an effective photospheric temperature of 5,576 K, a radius of 0.7447 R☉, a mass of 0.87 M☉, an estimated bolometric luminosity of 0.481 L☉. From Earth, HD 39855 shines at apparent V-band magnitude 7.51 (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: Las Campanas Observatory · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | HD 39855 |
| Spectral Type | G8 V |
| Effective Temp (Teff) | 5,576 K |
| Stellar Radius | 0.7447 R☉ |
| Stellar Mass | 0.8700 M☉ |
| Bolometric Luminosity | 0.481 L☉ |
| Metallicity [Fe/H] | — |
| Stellar Age | — |
| Apparent Magnitude | V: 7.510 · G: 7.321 |
| System Architecture | 1 Star, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.041000 AU |
| Orbital Period | 3.25 days |
| Mean Orbital Velocity | 137.3 km/s |
| Orbital Eccentricity (e) | 0.14000 |
| Orbital Inclination (i) | — |
| Mean Bulk Density | 2.02 g/cm³ (19.3 km/s vesc) |
| Insolation Flux | 287.2000 S⊕ (~390,879 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 75.900 LY (23.260 pc) |