51 Peg b
51 Peg b is a hot Jupiter orbiting 51 Peg in the constellation Pegasus. It lies about 50 light-years from Earth and was discovered in 1995 using the radial velocity method.
Is 51 Peg b in the Habitable Zone?
51 Peg b orbits inside the inner edge of the habitable zone of 51 Peg. So close to its star, surface conditions are far too hot for liquid water.
Habitable zone of 51 Peg: 0.879–2.072 AU (conservative: 1.113–1.965 AU), per Kopparapu et al. (2014). Earth orbits the Sun at 1 AU.
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Orbit and Year Length
A year on 51 Peg b — one full orbit around 51 Peg — lasts 4.23 Earth days, shorter than Mercury's 88-day year. It orbits at an average distance of 0.053 AU — closer to its star than Mercury is to the Sun. Its orbit is nearly circular (eccentricity 0.013).
How Was 51 Peg b Discovered?
51 Peg b was discovered in 1995 using the radial velocity method, with observations from Haute-Provence 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 51 Peg b?
51 Peg b is 50.4 light-years (15.5 parsecs) from Earth. Light arriving here tonight left the planet around the year 1976. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 887,040 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. 51 Peg b scores 0.07, ranking #5,049 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: 51 Peg
51 Peg
- Surface temperature
- 5,758 K (Sun: 5,772 K)
- Mass
- 1.03 M☉
- Radius
- 1.18 R☉
- Luminosity
- 1.3687 L☉
Planetary System
51 Peg b is the only planet known to orbit 51 Peg so far.
51 Peg b — Complete Data
| Mass (best estimate) | 150.01 Earth masses |
|---|---|
| Orbital period | 4.23 days |
| Orbital distance | 0.053 AU |
| Eccentricity | 0.013 |
| Earth Similarity Index | 0.07 |
| Distance from Earth | 50.4 light-years (15.5 parsecs) |
| Constellation | Pegasus |
| Discovery method | Radial Velocity |
| Discovery facility | Haute-Provence Observatory |
| Discovery year | 1995 |
Data: NASA Exoplanet Archive, last updated 2014-05-14. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About 51 Peg b
Is 51 Peg b habitable?
No — 51 Peg b orbits too close to its star and is too hot for liquid water to exist on its surface.
How far away is 51 Peg b?
51 Peg b is about 50 light-years from Earth in the constellation Pegasus. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 887,040 years to get there.
How long is a year on 51 Peg b?
One orbit around 51 Peg takes 4.2 Earth days — short enough that 86 of its years would fit into one Earth year.
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