GJ 338 B b
GJ 338 B b is a Neptune-like planet orbiting the M0 V star GJ 338 B in the constellation Ursa Major. It lies about 21 light-years from Earth and was discovered in 2020 using the radial velocity method.
Is GJ 338 B b in the Habitable Zone?
GJ 338 B b orbits inside the inner edge of the habitable zone of GJ 338 B. So close to its star, surface conditions are far too hot for liquid water.
Habitable zone of GJ 338 B: 0.228–0.577 AU (conservative: 0.289–0.547 AU), per Kopparapu et al. (2014). Earth orbits the Sun at 1 AU.
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Orbit and Year Length
A year on GJ 338 B b — one full orbit around GJ 338 B — lasts 24.4 Earth days, shorter than Mercury's 88-day year. It orbits at an average distance of 0.142 AU — closer to its star than Mercury is to the Sun. Its orbit is mildly elliptical (eccentricity 0.11).
How Was GJ 338 B b Discovered?
GJ 338 B b was discovered in 2020 using the radial velocity method, with observations from Calar Alto 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 GJ 338 B b?
GJ 338 B b is 20.7 light-years (6.3 parsecs) from Earth. Light arriving here tonight left the planet around the year 2006. A probe traveling at the speed of Voyager 1 — about 17 km/s, the fastest outbound spacecraft ever launched — would need roughly 364,320 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. GJ 338 B b scores 0.44, ranking #372 of 5,568 planets with a known ESI. For reference, Mars scores about 0.70.
The Host Star: GJ 338 B
GJ 338 B b belongs to a system of 2 stars; it orbits GJ 338 B.
GJ 338 B
- Spectral type
- M0 V
- Surface temperature
- 4,014 K (Sun: 5,772 K)
- Mass
- 0.64 M☉
- Radius
- 0.58 R☉
- Luminosity
- 0.0792 L☉
- Age
- 4.0 billion years (Sun: 4.6)
Planetary System
GJ 338 B b is the only planet known to orbit GJ 338 B so far.
GJ 338 B b — Complete Data
| Mass (best estimate) | 10.60 Earth masses |
|---|---|
| Orbital period | 24.42 days |
| Orbital distance | 0.142 AU |
| Eccentricity | 0.109 |
| Earth Similarity Index | 0.44 |
| Distance from Earth | 20.7 light-years (6.3 parsecs) |
| Constellation | Ursa Major |
| Discovery method | Radial Velocity |
| Discovery facility | Calar Alto Observatory |
| Discovery year | 2020 |
Data: NASA Exoplanet Archive, last updated 2023-01-18. Earth Similarity Index: PHL @ UPR Arecibo.
Frequently Asked Questions About GJ 338 B b
Is GJ 338 B b habitable?
No — GJ 338 B b orbits too close to its star and is too hot for liquid water to exist on its surface.
How far away is GJ 338 B b?
GJ 338 B b is about 21 light-years from Earth in the constellation Ursa Major. A spacecraft traveling as fast as Voyager 1 (about 17 km/s) would need roughly 364,320 years to get there.
How long is a year on GJ 338 B b?
One orbit around GJ 338 B takes 24.4 Earth days — short enough that 15 of its years would fit into one Earth year.
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