HD 18438 b
Astrophysical Synthesis & Habitability Characterization
HD 18438 b is a confirmed gas giant exoplanet orbiting the star HD 18438, located approximately 731.1 light-years (224.15 parsecs) from the Solar System. First announced in 2023 by Bohyunsan Optical Astronomical Observatory using the Radial Velocity detection technique (measuring periodic Doppler reflex motion in the host star's spectrum), this world measures 12.1 Earth radii (1.08 Jupiter radii) and has a measured mass of 6674.397 Earth masses (21 Jupiter masses). Combining its mass and radius yields a mean bulk density of 20.77 g/cm³ (an iron-enriched, high-compression rocky core), a surface gravity of 45.59 g⊕ (447.09 m/s²), and an escape velocity of 262.7 km/s.
In its orbital architecture around HD 18438, HD 18438 b completes one full revolution every 803 Earth days (2.2Julian years) at a semi-major axis of 2.1 AU (corresponding to the middle-system regime between Mars at 1.52 AU and Jupiter at 5.20 AU). Its mean orbital velocity is approximately 28.5 km/s (compared to Earth's 29.8 km/s). Its moderate orbital eccentricity of e = 0.1 carries the world between 1.89 AU at periapsis and 2.31 AU at apoapsis, driving measurable seasonal insolation cycles.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 1208 K (+935°C / +1715°F) and receiving 210.66 S⊕ (~286,705 W/m²), HD 18438 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star HD 18438 is classified as a low-mass red dwarf of spectral type M2.5 III with an effective photospheric temperature of 3,860 K, a radius of 88.475 R☉, a mass of 1.84 M☉, an estimated bolometric luminosity of 1559.122 L☉. Stellar evolution modeling indicates an estimated stellar age of 5.5 Gyr and a metallicity of [Fe/H] = -0.4 dex (metal-poor relative to the Sun). From Earth, HD 18438 shines at apparent V-band magnitude 5.505 (visible to the unaided human eye under dark skies) in the Northern 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: Bohyunsan Optical Astronomical Observatory · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | HD 18438 |
| Spectral Type | M2.5 III |
| Effective Temp (Teff) | 3,860 K |
| Stellar Radius | 88.4750 R☉ |
| Stellar Mass | 1.8400 M☉ |
| Bolometric Luminosity | 1559.122 L☉ |
| Metallicity [Fe/H] | -0.400 dex |
| Stellar Age | 5.500 Gyr |
| Apparent Magnitude | V: 5.505 · G: 4.732 |
| System Architecture | 1 Star, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 2.100000 AU |
| Orbital Period | 803 days |
| Mean Orbital Velocity | 28.5 km/s |
| Orbital Eccentricity (e) | 0.10000 |
| Orbital Inclination (i) | — |
| Mean Bulk Density | 20.77 g/cm³ (262.7 km/s vesc) |
| Insolation Flux | 210.6576 S⊕ (~286,705 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 731.100 LY (224.146 pc) |