HD 181720 b
Astrophysical Synthesis & Habitability Characterization
HD 181720 b is a confirmed gas giant exoplanet orbiting the star HD 181720, located approximately 196.4 light-years (60.21 parsecs) from the Solar System. First announced in 2009 by La Silla Observatory using the Radial Velocity detection technique (measuring periodic Doppler reflex motion in the host star's spectrum), this world measures 14 Earth radii (1.25 Jupiter radii) and has a measured mass of 127.132 Earth masses (0.4 Jupiter masses). Combining its mass and radius yields a mean bulk density of 0.26 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 0.65 g⊕ (6.37 m/s²), and an escape velocity of 33.7 km/s.
In its orbital architecture around HD 181720, HD 181720 b completes one full revolution every 956 Earth days (2.62Julian years) at a semi-major axis of 1.85 AU (corresponding to the middle-system regime between Mars at 1.52 AU and Jupiter at 5.20 AU). Its mean orbital velocity is approximately 21.1 km/s (compared to Earth's 29.8 km/s). The planet follows a highly eccentric trajectory (e = 0.26, swinging from 1.369 AU at periapsis out to 2.331 AU at apoapsis), pointing to past dynamical scattering or secular gravitational perturbations.
☁️ Atmospheric Regime (Volatile / Gas Envelope): With a radius of 14.000 R⊕, HD 181720 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 181720 is classified as a solar-type yellow star of spectral type G1 V with an effective photospheric temperature of 5,736 K, a radius of 1.5 R☉, a mass of 1.03 M☉, an estimated bolometric luminosity of 2.185 L☉. Stellar evolution modeling indicates a metallicity of [Fe/H] = -0.62 dex (metal-poor relative to the Sun). From Earth, HD 181720 shines at apparent V-band magnitude 7.84 (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: La Silla Observatory · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | HD 181720 |
| Spectral Type | G1 V |
| Effective Temp (Teff) | 5,736 K |
| Stellar Radius | 1.5000 R☉ |
| Stellar Mass | 1.0300 M☉ |
| Bolometric Luminosity | 2.185 L☉ |
| Metallicity [Fe/H] | -0.620 dex |
| Stellar Age | — |
| Apparent Magnitude | V: 7.840 · G: 7.675 |
| System Architecture | 1 Star, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 1.850000 AU |
| Orbital Period | 956 days |
| Mean Orbital Velocity | 21.1 km/s |
| Orbital Eccentricity (e) | 0.26000 |
| Orbital Inclination (i) | — |
| Mean Bulk Density | 0.26 g/cm³ (33.7 km/s vesc) |
| Insolation Flux | 0.5671 S⊕ (~772 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 196.400 LY (60.205 pc) |