HD 219828 b
Astrophysical Synthesis & Habitability Characterization
HD 219828 b is a confirmed sub-neptune exoplanet orbiting the star HD 219828, located approximately 238.5 light-years (73.11 parsecs) from the Solar System. First announced in 2007 by La Silla Observatory using the Radial Velocity detection technique (measuring periodic Doppler reflex motion in the host star's spectrum), this world measures 4.59 Earth radii (0.41 Jupiter radii) and has a measured mass of 19.07 Earth masses. Combining its mass and radius yields a mean bulk density of 1.09 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 22.8 km/s.
In its orbital architecture around HD 219828, HD 219828 b completes one full revolution every 3.835 Earth days at a semi-major axis of 0.049 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 139 km/s (compared to Earth's 29.8 km/s). Its moderate orbital eccentricity of e = 0.091 carries the world between 0.0445 AU at periapsis and 0.0535 AU at apoapsis, driving measurable seasonal insolation cycles.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 1603 K (+1330°C / +2426°F) and receiving 1,095.49 S⊕ (~1,490,960 W/m²), HD 219828 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star HD 219828 is classified as a solar-type yellow star of spectral type G0 IV with an effective photospheric temperature of 5,891 K, a radius of 1.56 R☉, a mass of 1.04 M☉, an estimated bolometric luminosity of 2.63 L☉. Stellar evolution modeling indicates an estimated stellar age of 5.69 Gyr and a metallicity of [Fe/H] = 0.19 dex (metal-rich relative to the Sun). From Earth, HD 219828 shines at apparent V-band magnitude 8.04 (accessible to binoculars and small amateur telescopes) in the Northern celestial hemisphere. The HD 219828 system is a multi-planet architecture hosting 2 confirmed worlds, with HD 219828 b sharing its stellar neighborhood alongside HD 219828 c.
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 219828 |
| Spectral Type | G0 IV |
| Effective Temp (Teff) | 5,891 K |
| Stellar Radius | 1.5600 R☉ |
| Stellar Mass | 1.0400 M☉ |
| Bolometric Luminosity | 2.63 L☉ |
| Metallicity [Fe/H] | 0.190 dex |
| Stellar Age | 5.690 Gyr |
| Apparent Magnitude | V: 8.040 · G: 7.883 |
| System Architecture | 1 Star, 2 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.049000 AU |
| Orbital Period | 3.835 days |
| Mean Orbital Velocity | 139 km/s |
| Orbital Eccentricity (e) | 0.09100 |
| Orbital Inclination (i) | — |
| Mean Bulk Density | 1.09 g/cm³ (22.8 km/s vesc) |
| Insolation Flux | 1095.4885 S⊕ (~1,490,960 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 238.500 LY (73.114 pc) |