XO-2 S b
Astrophysical Synthesis & Habitability Characterization
XO-2 S b is a confirmed gas giant exoplanet orbiting the star XO-2 S, located approximately 493.8 light-years (151.4 parsecs) from the Solar System. First announced in 2014 by Roque de los Muchachos Observatory using the Radial Velocity detection technique (measuring periodic Doppler reflex motion in the host star's spectrum), this world measures 10.9 Earth radii (0.97 Jupiter radii) and has a measured mass of 82.635 Earth masses (0.26 Jupiter masses). Combining its mass and radius yields a mean bulk density of 0.35 g/cm³ (a low bulk density dominated by an extended hydrogen-helium gaseous envelope), a surface gravity of 0.7 g⊕ (6.86 m/s²), and an escape velocity of 30.8 km/s.
In its orbital architecture around XO-2 S, XO-2 S b completes one full revolution every 18.22 Earth days at a semi-major axis of 0.1347 AU (orbiting interior to Mercury's 0.387 AU benchmark in the Solar System). Its mean orbital velocity is approximately 80.4 km/s (compared to Earth's 29.8 km/s). Its moderate orbital eccentricity of e = 0.15 carries the world between 0.1145 AU at periapsis and 0.1549 AU at apoapsis, driving measurable seasonal insolation cycles.
⚠️ Thermal Regime (Irradiated / Hot Zone): At an equilibrium temperature of 707 K (+434°C / +813°F) and receiving 41.52 S⊕ (~56,507 W/m²), XO-2 S b orbits interior to the runaway greenhouse boundary where surface liquid water cannot persist.
The host star XO-2 S is classified as a solar-type yellow star of spectral type G8 V with an effective photospheric temperature of 5,325 K, a radius of 1.02 R☉, a mass of 0.98 M☉, an estimated bolometric luminosity of 0.751 L☉. Stellar evolution modeling indicates an estimated stellar age of 7.1 Gyr and a metallicity of [Fe/H] = 0.32 dex (metal-rich relative to the Sun). From Earth, XO-2 S shines at apparent V-band magnitude 11.196 (faint and requiring research-grade or CCD telescopic photometry) in the Northern celestial hemisphere. The XO-2 S system is a multi-planet architecture hosting 3 confirmed worlds, with XO-2 S b sharing its stellar neighborhood alongside XO-2 S c, XO-2 S d.
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: Roque de los Muchachos Observatory · Catalogue Source: NASA Exoplanet Archive (IPAC / Caltech).
☀️ Host Star Telemetry
| Host Star Name | XO-2 S |
| Spectral Type | G8 V |
| Effective Temp (Teff) | 5,325 K |
| Stellar Radius | 1.0200 R☉ |
| Stellar Mass | 0.9800 M☉ |
| Bolometric Luminosity | 0.751 L☉ |
| Metallicity [Fe/H] | 0.320 dex |
| Stellar Age | 7.100 Gyr |
| Apparent Magnitude | V: 11.196 · G: 10.928 |
| System Architecture | 2 Stars, 4 Planets |
🪐 Orbital & Physical Dynamics
| Semi-Major Axis (a) | 0.134700 AU |
| Orbital Period | 18.22 days |
| Mean Orbital Velocity | 80.4 km/s |
| Orbital Eccentricity (e) | 0.15000 |
| Orbital Inclination (i) | — |
| Mean Bulk Density | 0.35 g/cm³ (30.8 km/s vesc) |
| Insolation Flux | 41.5190 S⊕ (~56,507 W/m²) |
| Habitable Zone Status | Exterior / Non-HZ |
| Celestial Distance | 493.800 LY (151.398 pc) |