HD 137388 b
Astrophysical Synthesis & Habitability Characterization
HD 137388 b is a confirmed gas giant exoplanet orbiting the star HD 137388, located approximately 132 light-years (40.48 parsecs) from the Solar System. First announced in 2011 by La Silla Observatory using the Radial Velocity detection technique (measuring periodic Doppler reflex motion in the host star's spectrum), this world measures 9.32 Earth radii (0.83 Jupiter radii) and has a measured mass of 63.566 Earth masses (0.2 Jupiter masses). Combining its mass and radius yields a mean bulk density of 0.43 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 29.2 km/s.
In its orbital architecture around HD 137388, HD 137388 b completes one full revolution every 330 Earth days at a semi-major axis of 0.89 AU (comparable to the inner terrestrial zone between Venus, Earth at 1.00 AU, and Mars at 1.52 AU). Its mean orbital velocity is approximately 29.3 km/s (compared to Earth's 29.8 km/s). The planet follows a highly eccentric trajectory (e = 0.36, swinging from 0.5696 AU at periapsis out to 1.2104 AU at apoapsis), pointing to past dynamical scattering or secular gravitational perturbations.
☁️ Atmospheric Regime (Volatile / Gas Envelope): With a radius of 9.320 R⊕, HD 137388 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 137388 is classified as an orange K-type star of spectral type K2 IV with an effective photospheric temperature of 5,181 K, a radius of 0.88 R☉, a mass of 0.68 M☉, an estimated bolometric luminosity of 0.501 L☉. Stellar evolution modeling indicates an estimated stellar age of 6.45 Gyr and a metallicity of [Fe/H] = 0.19 dex (metal-rich relative to the Sun). From Earth, HD 137388 shines at apparent V-band magnitude 8.71 (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 137388 |
| Spectral Type | K2 IV |
| Effective Temp (Teff) | 5,181 K |
| Stellar Radius | 0.8800 R☉ |
| Stellar Mass | 0.6800 M☉ |
| Bolometric Luminosity | 0.501 L☉ |
| Metallicity [Fe/H] | 0.190 dex |
| Stellar Age | 6.450 Gyr |
| Apparent Magnitude | V: 8.710 · G: 8.480 |
| System Architecture | 1 Star, 1 Planet |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.890000 AU |
| Orbital Period | 330 days |
| Mean Orbital Velocity | 29.3 km/s |
| Orbital Eccentricity (e) | 0.36000 |
| Orbital Inclination (i) | — |
| Mean Bulk Density | 0.43 g/cm³ (29.2 km/s vesc) |
| Insolation Flux | 0.5807 S⊕ (~790 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 132.000 LY (40.484 pc) |