HD 28109 d
Astrophysical Synthesis & Habitability Characterization
HD 28109 d is a confirmed sub-neptune exoplanet orbiting the star HD 28109, located approximately 455.3 light-years (139.61 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 0.04% across the stellar disk), this world measures 3.11 Earth radii (0.28 Jupiter radii) and has a measured mass of 5.8 Earth masses. Combining its mass and radius yields a mean bulk density of 1.06 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 0.6 g⊕ (5.88 m/s²), and an escape velocity of 15.3 km/s.
In its orbital architecture around HD 28109, HD 28109 d completes one full revolution every 84.29 Earth days at a semi-major axis of 0.411 AU (situated at an orbital separation between Mercury's 0.39 AU and Venus's 0.72 AU). Its mean orbital velocity is approximately 53 km/s (compared to Earth's 29.8 km/s). With a near-circular eccentricity of e = 0.0054 (ranging from 0.4088 AU at periapsis to 0.4132 AU at apoapsis), the planet experiences minimal orbital distance variation.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 561 K (+288°C / +550°F) and receiving 17.37 S⊕ (~23,647 W/m²), HD 28109 d orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star HD 28109 is classified as a warm F-type star of spectral type F8 with an effective photospheric temperature of 6,189 K, a radius of 1.425 R☉, a mass of 1.227 M☉, an estimated bolometric luminosity of 2.673 L☉. Stellar evolution modeling indicates an estimated stellar age of 3.2 Gyr and a metallicity of [Fe/H] = 0.1 dex (metal-rich relative to the Sun). From Earth, HD 28109 shines at apparent V-band magnitude 9.42 (accessible to binoculars and small amateur telescopes) in the Southern celestial hemisphere. The HD 28109 system is a multi-planet architecture hosting 3 confirmed worlds, with HD 28109 d sharing its stellar neighborhood alongside HD 28109 b, HD 28109 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: Transiting Exoplanet Survey Satellite (TESS) · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | HD 28109 |
| Spectral Type | F8 |
| Effective Temp (Teff) | 6,189 K |
| Stellar Radius | 1.4250 R☉ |
| Stellar Mass | 1.2270 M☉ |
| Bolometric Luminosity | 2.673 L☉ |
| Metallicity [Fe/H] | 0.100 dex |
| Stellar Age | 3.200 Gyr |
| Apparent Magnitude | V: 9.420 · G: 9.300 |
| System Architecture | 1 Star, 3 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.411000 AU |
| Orbital Period | 84.29 days |
| Mean Orbital Velocity | 53 km/s |
| Orbital Eccentricity (e) | 0.00540 |
| Orbital Inclination (i) | 89.7700° |
| Mean Bulk Density | 1.06 g/cm³ (15.3 km/s vesc) |
| Insolation Flux | 17.3745 S⊕ (~23,647 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 455.300 LY (139.611 pc) |