81 Cet b
81 Cet b is a cold gas giant orbiting the G5 III star 81 Cet in the constellation Cetus. It lies about 330 light-years from Earth and was discovered in 2008 using the radial velocity method.
Is 81 Cet b in the Habitable Zone?
81 Cet b orbits inside the inner edge of the habitable zone of 81 Cet. So close to its star, surface conditions are far too hot for liquid water.
Habitable zone of 81 Cet: 6.014–14.703 AU (conservative: 7.618–13.940 AU), per Kopparapu et al. (2014). Earth orbits the Sun at 1 AU.
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Orbit and Year Length
A year on 81 Cet b — one full orbit around 81 Cet — lasts 1,005.6 Earth days, longer than an Earth year. It orbits at an average distance of 2.104 AU. Its orbit is nearly circular (eccentricity 0.037).
How Was 81 Cet b Discovered?
81 Cet b was discovered in 2008 using the radial velocity method, with observations from Okayama Astrophysical 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 81 Cet b?
81 Cet b is 330.3 light-years (101.3 parsecs) from Earth. Light arriving here tonight left the planet around the year 1696. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 5,813,280 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. 81 Cet b scores 0.14, ranking #4,245 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: 81 Cet
81 Cet
- Spectral type
- G5 III
- Surface temperature
- 4,734 K (Sun: 5,772 K)
- Mass
- 1.23 M☉
- Radius
- 11.21 R☉
- Luminosity
- 57.5798 L☉
Planetary System
81 Cet b is the only planet known to orbit 81 Cet so far.
81 Cet b — Complete Data
| Mass (best estimate) | 1,051.06 Earth masses |
|---|---|
| Orbital period | 1,005.57 days |
| Orbital distance | 2.104 AU |
| Eccentricity | 0.037 |
| Earth Similarity Index | 0.14 |
| Distance from Earth | 330.3 light-years (101.3 parsecs) |
| Constellation | Cetus |
| Discovery method | Radial Velocity |
| Discovery facility | Okayama Astrophysical Observatory |
| Discovery year | 2008 |
Data: NASA Exoplanet Archive, last updated 2023-09-19. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About 81 Cet b
Is 81 Cet b habitable?
No — 81 Cet b orbits too close to its star and is too hot for liquid water to exist on its surface.
How far away is 81 Cet b?
81 Cet b is about 330 light-years from Earth in the constellation Cetus. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 5,813,280 years to get there.
How long is a year on 81 Cet b?
One orbit around 81 Cet takes 1,005.6 Earth days.
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