XO-2 S b
XO-2 S b is a cold gas giant orbiting the G8 V star XO-2 S in the constellation Lynx. It lies about 494 light-years from Earth and was discovered in 2014 using the radial velocity method.
Is XO-2 S b in the Habitable Zone?
XO-2 S b orbits inside the inner edge of the habitable zone of XO-2 S. So close to its star, surface conditions are far too hot for liquid water.
Habitable zone of XO-2 S: 0.667–1.594 AU (conservative: 0.845–1.511 AU), per Kopparapu et al. (2014). Earth orbits the Sun at 1 AU.
See the full interactive habitable-zone view in the Exoplanet Explorer app ›
Orbit and Year Length
A year on XO-2 S b — one full orbit around XO-2 S — lasts 18.2 Earth days, shorter than Mercury's 88-day year. It orbits at an average distance of 0.135 AU — closer to its star than Mercury is to the Sun. Its orbit is mildly elliptical (eccentricity 0.15).
How Was XO-2 S b Discovered?
XO-2 S b was discovered in 2014 using the radial velocity method, with observations from Roque de los Muchachos Observatory.
The radial velocity method measures the subtle wobble of a star caused by the gravitational pull of an orbiting planet, visible as periodic shifts in the star's light spectrum. The size of the wobble reveals the planet's minimum mass.
How Far Away Is XO-2 S b?
XO-2 S b is 493.8 light-years (151.4 parsecs) from Earth. Light arriving here tonight left the planet around the year 1533. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 8,690,880 years to make the journey.
Earth Similarity Index
The Earth Similarity Index (ESI) scores how physically similar a planet is to Earth, from 0 to 1, based on radius, density, escape velocity and surface temperature. XO-2 S b scores 0.10, ranking #4,414 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: XO-2 S
XO-2 S b belongs to a system of 2 stars; it orbits XO-2 S.
XO-2 S
- Spectral type
- G8 V
- Surface temperature
- 5,325 K (Sun: 5,772 K)
- Mass
- 0.98 M☉
- Radius
- 1.02 R☉
The XO-2 S Planetary System
XO-2 S b is one of 3 known planets in the XO-2 S system. Its siblings:
XO-2 S b — Complete Data
| Mass (best estimate) | 82.64 Earth masses |
|---|---|
| Orbital period | 18.22 days |
| Orbital distance | 0.135 AU |
| Eccentricity | 0.150 |
| Earth Similarity Index | 0.10 |
| Distance from Earth | 493.8 light-years (151.4 parsecs) |
| Constellation | Lynx |
| Discovery method | Radial Velocity |
| Discovery facility | Roque de los Muchachos Observatory |
| Discovery year | 2014 |
Data: NASA Exoplanet Archive, last updated 2026-05-21. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About XO-2 S b
Is XO-2 S b habitable?
No — XO-2 S b orbits too close to its star and is too hot for liquid water to exist on its surface.
How far away is XO-2 S b?
XO-2 S b is about 494 light-years from Earth in the constellation Lynx. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 8,690,880 years to get there.
How long is a year on XO-2 S b?
One orbit around XO-2 S takes 18.2 Earth days — short enough that 20 of its years would fit into one Earth year.
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