HD 180617 b
Astrophysical Synthesis & Habitability Characterization
HD 180617 b is a confirmed sub-neptune exoplanet orbiting the star HD 180617, located approximately 19.3 light-years (5.91 parsecs) from the Solar System. First announced in 2018 by Calar Alto Observatory using the Radial Velocity detection technique (measuring periodic Doppler reflex motion in the host star's spectrum), this world measures 3.53 Earth radii (0.31 Jupiter radii) and has a measured mass of 12.214 Earth masses. Combining its mass and radius yields a mean bulk density of 1.53 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 0.98 g⊕ (9.61 m/s²), and an escape velocity of 20.8 km/s.
In its orbital architecture around HD 180617, HD 180617 b completes one full revolution every 105.911 Earth days at a semi-major axis of 0.343 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 35.2 km/s (compared to Earth's 29.8 km/s). Its moderate orbital eccentricity of e = 0.101 carries the world between 0.3084 AU at periapsis and 0.3776 AU at apoapsis, driving measurable seasonal insolation cycles.
☁️ Atmospheric Regime (Volatile / Gas Envelope): With a radius of 3.530 R⊕, HD 180617 b exceeds the empirical ~1.8–2.2 R⊕ Fulton radius valley threshold, indicating a substantial hydrogen-helium or supercritical volatile envelope without a conventional solid terrestrial surface.
The host star HD 180617 is classified as a low-mass red dwarf of spectral type M2.5 V with an effective photospheric temperature of 3,534 K, a radius of 0.481 R☉, a mass of 0.484 M☉, an estimated bolometric luminosity of 0.032 L☉. Stellar evolution modeling indicates a metallicity of [Fe/H] = -0.04 dex (near-solar heavy-element abundance). From Earth, HD 180617 shines at apparent V-band magnitude 9.12 (accessible to binoculars and small amateur telescopes) 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: Calar Alto Observatory · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | HD 180617 |
| Spectral Type | M2.5 V |
| Effective Temp (Teff) | 3,534 K |
| Stellar Radius | 0.4810 R☉ |
| Stellar Mass | 0.4840 M☉ |
| Bolometric Luminosity | 0.032 L☉ |
| Metallicity [Fe/H] | -0.040 dex |
| Stellar Age | — |
| Apparent Magnitude | V: 9.120 · G: 8.098 |
| System Architecture | 2 Stars, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.343000 AU |
| Orbital Period | 105.911 days |
| Mean Orbital Velocity | 35.2 km/s |
| Orbital Eccentricity (e) | 0.10100 |
| Orbital Inclination (i) | — |
| Mean Bulk Density | 1.53 g/cm³ (20.8 km/s vesc) |
| Insolation Flux | 0.2751 S⊕ (~374 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 19.300 LY (5.911 pc) |