HD 17156 b
HD 17156 b is a cold gas giant orbiting HD 17156 in the constellation Cassiopeia. It lies about 255 light-years from Earth and was discovered in 2007 using the radial velocity method.
How Big Is HD 17156 b?
HD 17156 b has a radius of 12.26 times that of Earth, or 1.09 times the radius of Jupiter. Its mass is 1,036 times that of Earth, giving it a density of 3.08 g/cm³ — between that of rocky and gaseous planets.
Is HD 17156 b in the Habitable Zone?
HD 17156 b orbits inside the inner edge of the habitable zone of HD 17156. So close to its star, surface conditions are far too hot for liquid water.
Habitable zone of HD 17156: 1.227–2.871 AU (conservative: 1.554–2.722 AU), per Kopparapu et al. (2014). Earth orbits the Sun at 1 AU.
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Temperature on HD 17156 b
The equilibrium temperature of HD 17156 b is about 888 K (615 °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. It receives 68.80 times the stellar energy that Earth gets from the Sun.
Orbit and Year Length
A year on HD 17156 b — one full orbit around HD 17156 — lasts 21.2 Earth days, shorter than Mercury's 88-day year. It orbits at an average distance of 0.163 AU — closer to its star than Mercury is to the Sun. Its orbit is highly elliptical (eccentricity 0.68), swinging dramatically closer to and farther from its star.
How Was HD 17156 b Discovered?
HD 17156 b was discovered in 2007 using the radial velocity method, with observations from Subaru Telescope.
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 HD 17156 b?
HD 17156 b is 254.7 light-years (78.1 parsecs) from Earth. Light arriving here tonight left the planet around the year 1772. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 4,482,720 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. HD 17156 b scores 0.08, ranking #4,656 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: HD 17156
HD 17156
- Surface temperature
- 6,046 K (Sun: 5,772 K)
- Mass
- 1.29 M☉
- Radius
- 1.52 R☉
- Luminosity
- 2.7600 L☉
- Age
- 3.3 billion years (Sun: 4.6)
Planetary System
HD 17156 b is the only planet known to orbit HD 17156 so far.
HD 17156 b — Complete Data
| Radius | 12.263 Earth radii (1.094 Jupiter radii) |
|---|---|
| Mass | 1,036.12 Earth masses (3.260 Jupiter masses) |
| Density | 3.08 g/cm³ (Earth: 5.51) |
| Orbital period | 21.22 days |
| Orbital distance | 0.163 AU |
| Eccentricity | 0.677 |
| Equilibrium temperature | 888 K (615 °C) |
| Stellar irradiation | 68.80× Earth |
| Earth Similarity Index | 0.08 |
| Distance from Earth | 254.7 light-years (78.1 parsecs) |
| Constellation | Cassiopeia |
| Discovery method | Radial Velocity |
| Discovery facility | Subaru Telescope |
| Discovery year | 2007 |
Data: NASA Exoplanet Archive, last updated 2025-12-11. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About HD 17156 b
Is HD 17156 b habitable?
No — HD 17156 b orbits too close to its star and is too hot for liquid water to exist on its surface.
How far away is HD 17156 b?
HD 17156 b is about 255 light-years from Earth in the constellation Cassiopeia. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 4,482,720 years to get there.
How big is HD 17156 b compared to Earth?
HD 17156 b has 12.26 times the radius of Earth and about 1,036 times its mass.
How long is a year on HD 17156 b?
One orbit around HD 17156 takes 21.2 Earth days — short enough that 17 of its years would fit into one Earth year.
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