HD 224018 d
Astrophysical Synthesis & Habitability Characterization
HD 224018 d is a confirmed sub-neptune exoplanet orbiting the star HD 224018, located approximately 344 light-years (105.48 parsecs) from the Solar System. First announced in 2025 by K2 using the Transit detection technique (producing an estimated primary transit depth of 0.0368% across the stellar disk), this world measures 2.4 Earth radii and has a measured mass of 4.2 Earth masses. Combining its mass and radius yields a mean bulk density of 1.68 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 0.73 g⊕ (7.16 m/s²), and an escape velocity of 14.8 km/s.
In its orbital architecture around HD 224018, HD 224018 d completes one full revolution every 138.073 Earth days at a semi-major axis of 0.53 AU (situated at an orbital separation between Mercury's 0.39 AU and Venus's 0.72 AU). Its mean orbital velocity is approximately 41.8 km/s (compared to Earth's 29.8 km/s). With a near-circular eccentricity of e = 0.04 (ranging from 0.5088 AU at periapsis to 0.5512 AU at apoapsis), the planet experiences minimal orbital distance variation.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 411 K (+138°C / +280°F) and receiving 4.73 S⊕ (~6,444 W/m²), HD 224018 d orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star HD 224018 is classified as a solar-type yellow star of spectral type G V with an effective photospheric temperature of 5,784 K, a radius of 1.147 R☉, a mass of 1.013 M☉, an estimated bolometric luminosity of 1.321 L☉. Stellar evolution modeling indicates an estimated stellar age of 7 Gyr and a metallicity of [Fe/H] = 0.05 dex (near-solar heavy-element abundance). From Earth, HD 224018 shines at apparent V-band magnitude 9.715 (accessible to binoculars and small amateur telescopes) in the Southern celestial hemisphere. The HD 224018 system is a multi-planet architecture hosting 3 confirmed worlds, with HD 224018 d sharing its stellar neighborhood alongside HD 224018 b, HD 224018 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: K2 · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | HD 224018 |
| Spectral Type | G V |
| Effective Temp (Teff) | 5,784 K |
| Stellar Radius | 1.1470 R☉ |
| Stellar Mass | 1.0130 M☉ |
| Bolometric Luminosity | 1.321 L☉ |
| Metallicity [Fe/H] | 0.050 dex |
| Stellar Age | 7.000 Gyr |
| Apparent Magnitude | V: 9.715 · G: 9.521 |
| System Architecture | 1 Star, 3 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.530000 AU |
| Orbital Period | 138.073 days |
| Mean Orbital Velocity | 41.8 km/s |
| Orbital Eccentricity (e) | 0.04000 |
| Orbital Inclination (i) | 89.9000° |
| Mean Bulk Density | 1.68 g/cm³ (14.8 km/s vesc) |
| Insolation Flux | 4.7348 S⊕ (~6,444 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 344.000 LY (105.480 pc) |