HD 209458 b
HD 209458 b is a hot Jupiter orbiting HD 209458 in the constellation Pegasus. It lies about 158 light-years from Earth and was discovered in 1999 using the radial velocity method.
How Big Is HD 209458 b?
HD 209458 b has a radius of 15.58 times that of Earth, or 1.39 times the radius of Jupiter. Its mass is 232 times that of Earth.
Is HD 209458 b in the Habitable Zone?
The position of HD 209458 b relative to the habitable zone of HD 209458 cannot be precisely determined from the available orbital data.
Habitable zone of HD 209458: 0.974–2.277 AU (conservative: 1.233–2.159 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 HD 209458 b — one full orbit around HD 209458 — lasts 3.52 Earth days, shorter than Mercury's 88-day year.
How Was HD 209458 b Discovered?
HD 209458 b was discovered in 1999 using the radial velocity method, with observations from W. M. Keck 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 HD 209458 b?
HD 209458 b is 157.5 light-years (48.3 parsecs) from Earth. Light arriving here tonight left the planet around the year 1869. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 2,772,000 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 209458 b scores 0.06, ranking #5,149 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: HD 209458
HD 209458
- Surface temperature
- 6,091 K (Sun: 5,772 K)
- Mass
- 1.23 M☉
- Radius
- 1.19 R☉
Planetary System
HD 209458 b is the only planet known to orbit HD 209458 so far.
HD 209458 b — Complete Data
| Radius | 15.581 Earth radii (1.390 Jupiter radii) |
|---|---|
| Mass | 232.02 Earth masses (0.730 Jupiter masses) |
| Orbital period | 3.52 days |
| Earth Similarity Index | 0.06 |
| Distance from Earth | 157.5 light-years (48.3 parsecs) |
| Constellation | Pegasus |
| Discovery method | Radial Velocity |
| Discovery facility | W. M. Keck Observatory |
| Discovery year | 1999 |
Data: NASA Exoplanet Archive, last updated 2018-12-03. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About HD 209458 b
Is HD 209458 b habitable?
HD 209458 b is not known to orbit within the habitable zone of HD 209458, and as a hot Jupiter it is an unlikely candidate for life as we know it.
How far away is HD 209458 b?
HD 209458 b is about 158 light-years from Earth in the constellation Pegasus. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 2,772,000 years to get there.
How big is HD 209458 b compared to Earth?
HD 209458 b has 15.58 times the radius of Earth and about 232 times its mass.
How long is a year on HD 209458 b?
One orbit around HD 209458 takes 3.5 Earth days — short enough that 104 of its years would fit into one Earth year.
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