HD 155918 b
Astrophysical Synthesis & Habitability Characterization
HD 155918 b is a confirmed sub-neptune exoplanet orbiting the star HD 155918, located approximately 89.9 light-years (27.56 parsecs) from the Solar System. First announced in 2022 by Multiple Observatories using the Radial Velocity detection technique (measuring periodic Doppler reflex motion in the host star's spectrum), this world measures 2.67 Earth radii and has a measured mass of 7.628 Earth masses. Combining its mass and radius yields a mean bulk density of 2.21 g/cm³ (an intermediate density indicative of a deep water-ice mantle or dense volatile envelope), a surface gravity of 1.07 g⊕ (10.49 m/s²), and an escape velocity of 18.9 km/s.
In its orbital architecture around HD 155918, HD 155918 b completes one full revolution every 1.876 Earth days at a semi-major axis of 0.03 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 174 km/s (compared to Earth's 29.8 km/s). Its moderate orbital eccentricity of e = 0.194 carries the world between 0.0242 AU at periapsis and 0.0358 AU at apoapsis, driving measurable seasonal insolation cycles.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 1622 K (+1349°C / +2460°F) and receiving 1,153.74 S⊕ (~1,570,239 W/m²), HD 155918 b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star HD 155918 is classified as a solar-type yellow star of spectral type G2V with an effective photospheric temperature of 5,772 K, a radius of 1.0191 R☉, a mass of 1.03 M☉, an estimated bolometric luminosity of 1.034 L☉. Stellar evolution modeling indicates an estimated stellar age of 12.3 Gyr and a metallicity of [Fe/H] = -0.662 dex (metal-poor relative to the Sun). From Earth, HD 155918 shines at apparent V-band magnitude 7 (accessible to binoculars and small amateur telescopes) in the Southern celestial hemisphere. The HD 155918 system is a multi-planet architecture hosting 2 confirmed worlds, with HD 155918 b sharing its stellar neighborhood alongside HD 155918 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: Multiple Observatories · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | HD 155918 |
| Spectral Type | G2V |
| Effective Temp (Teff) | 5,772 K |
| Stellar Radius | 1.0191 R☉ |
| Stellar Mass | 1.0300 M☉ |
| Bolometric Luminosity | 1.034 L☉ |
| Metallicity [Fe/H] | -0.662 dex |
| Stellar Age | 12.300 Gyr |
| Apparent Magnitude | V: 7.000 · G: 6.841 |
| System Architecture | 1 Star, 2 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.030000 AU |
| Orbital Period | 1.876 days |
| Mean Orbital Velocity | 174 km/s |
| Orbital Eccentricity (e) | 0.19400 |
| Orbital Inclination (i) | — |
| Mean Bulk Density | 2.21 g/cm³ (18.9 km/s vesc) |
| Insolation Flux | 1153.7389 S⊕ (~1,570,239 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 89.900 LY (27.561 pc) |