HD 18599 b
Astrophysical Synthesis & Habitability Characterization
HD 18599 b is a confirmed sub-neptune exoplanet orbiting the star HD 18599, located approximately 125.8 light-years (38.57 parsecs) from the Solar System. First announced in 2022 by Transiting Exoplanet Survey Satellite (TESS) using the Transit detection technique (producing an estimated primary transit depth of 0.0966% across the stellar disk), this world measures 2.6 Earth radii and has a measured mass of 24.1 Earth masses. Combining its mass and radius yields a mean bulk density of 7.56 g/cm³ (an iron-enriched, high-compression rocky core), a surface gravity of 3.57 g⊕ (35.01 m/s²), and an escape velocity of 34.1 km/s.
In its orbital architecture around HD 18599, HD 18599 b completes one full revolution every 4.137 Earth days at a semi-major axis of 0.048 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 126.2 km/s (compared to Earth's 29.8 km/s). The planet follows a highly eccentric trajectory (e = 0.34, swinging from 0.0317 AU at periapsis out to 0.0643 AU at apoapsis), pointing to past dynamical scattering or secular gravitational perturbations.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 863 K (+590°C / +1094°F) and receiving 145.00 S⊕ (~197,345 W/m²), HD 18599 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star HD 18599 is classified as an orange K-type star of spectral type K2 V with an effective photospheric temperature of 5,145 K, a radius of 0.767 R☉, a mass of 0.863 M☉, an estimated bolometric luminosity of 0.37 L☉. Stellar evolution modeling indicates an estimated stellar age of 0.4 Gyr and a metallicity of [Fe/H] = 0 dex (near-solar heavy-element abundance). From Earth, HD 18599 shines at apparent V-band magnitude 8.99 (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: Transiting Exoplanet Survey Satellite (TESS) · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | HD 18599 |
| Spectral Type | K2 V |
| Effective Temp (Teff) | 5,145 K |
| Stellar Radius | 0.7670 R☉ |
| Stellar Mass | 0.8630 M☉ |
| Bolometric Luminosity | 0.37 L☉ |
| Metallicity [Fe/H] | 0.000 dex |
| Stellar Age | 0.400 Gyr |
| Apparent Magnitude | V: 8.990 · G: 8.731 |
| System Architecture | 2 Stars, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.048000 AU |
| Orbital Period | 4.137 days |
| Mean Orbital Velocity | 126.2 km/s |
| Orbital Eccentricity (e) | 0.34000 |
| Orbital Inclination (i) | 87.6000° |
| Mean Bulk Density | 7.56 g/cm³ (34.1 km/s vesc) |
| Insolation Flux | 145.0000 S⊕ (~197,345 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 125.800 LY (38.565 pc) |