XO-3 b
XO-3 b is a hot Jupiter orbiting XO-3 in the constellation Camelopardalis. It lies about 695 light-years from Earth and was discovered in 2007 using the transit method.
How Big Is XO-3 b?
XO-3 b has a radius of 14.80 times that of Earth, or 1.32 times the radius of Jupiter. Its mass is 4,218 times that of Earth, giving it a density of 7.16 g/cm³ — comparable to rocky planets like Earth (5.51 g/cm³).
Is XO-3 b in the Habitable Zone?
XO-3 b orbits inside the inner edge of the habitable zone of XO-3. So close to its star, surface conditions are far too hot for liquid water.
Habitable zone of XO-3: 1.554–3.587 AU (conservative: 1.968–3.401 AU), per Kopparapu et al. (2014). Earth orbits the Sun at 1 AU.
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Temperature on XO-3 b
The equilibrium temperature of XO-3 b is about 1,890 K (1,617 °C) — hot enough to melt many metals. This estimate ignores any atmosphere, which could change surface temperatures dramatically — Earth's greenhouse effect adds about 33 °C.
Orbit and Year Length
A year on XO-3 b — one full orbit around XO-3 — lasts 3.19 Earth days, shorter than Mercury's 88-day year. It orbits at an average distance of 0.048 AU — closer to its star than Mercury is to the Sun. Its orbit is mildly elliptical (eccentricity 0.28).
How Was XO-3 b Discovered?
XO-3 b was discovered in 2007 using the transit method, with observations from XO.
The transit method watches a star for the tiny, regular dip in brightness that occurs when a planet crosses in front of it. The depth and timing of these dips reveal the planet's size and orbital period.
How Far Away Is XO-3 b?
XO-3 b is 694.9 light-years (213.1 parsecs) from Earth. Light arriving here tonight left the planet around the year 1332. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 12,230,240 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-3 b scores 0.06, ranking #5,182 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: XO-3
XO-3
- Surface temperature
- 6,970 K (Sun: 5,772 K)
- Mass
- 1.45 M☉
- Radius
- 1.52 R☉
- Luminosity
- 4.9201 L☉
- Age
- 1.0 billion years (Sun: 4.6)
Planetary System
XO-3 b is the only planet known to orbit XO-3 so far.
XO-3 b — Complete Data
| Radius | 14.796 Earth radii (1.320 Jupiter radii) |
|---|---|
| Mass | 4,217.58 Earth masses (13.270 Jupiter masses) |
| Density | 7.16 g/cm³ (Earth: 5.51) |
| Orbital period | 3.19 days |
| Orbital distance | 0.048 AU |
| Eccentricity | 0.279 |
| Equilibrium temperature | 1,890 K (1,617 °C) |
| Earth Similarity Index | 0.06 |
| Distance from Earth | 694.9 light-years (213.1 parsecs) |
| Constellation | Camelopardalis |
| Discovery method | Transit |
| Discovery facility | XO |
| Discovery year | 2007 |
Data: NASA Exoplanet Archive, last updated 2026-02-26. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About XO-3 b
Is XO-3 b habitable?
No — XO-3 b orbits too close to its star and is too hot for liquid water to exist on its surface.
How far away is XO-3 b?
XO-3 b is about 695 light-years from Earth in the constellation Camelopardalis. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 12,230,240 years to get there.
How big is XO-3 b compared to Earth?
XO-3 b has 14.80 times the radius of Earth and about 4,218 times its mass.
How long is a year on XO-3 b?
One orbit around XO-3 takes 3.2 Earth days — short enough that 114 of its years would fit into one Earth year.
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