---
title: "GJ 338 B b"
url: "https://spacecatalog.org/object/gj-338-b-b"
release: "DR1"
licence: "No licence stated; acknowledgement requested"
citation: "SpaceCatalog. GJ 338 B b [catalogue entry]. Data release DR1 (2026-08-21). https://spacecatalog.org/object/gj-338-b-b"
---

# GJ 338 B b

GJ 338 B b is a Neptune-like [planet](https://spacecatalog.org/planets) in the constellation [Ursa Major](https://spacecatalog.org/constellation/ursa-major), 20.65 ly from Earth. It orbits GJ 338 B at a mean distance of 0.142 AU. It has a minimum mass of 10.60 M⊕ and an estimated radius of 3.25 R⊕ · 20 729 km. The radius is an estimate from the mass rather than a measurement. It was discovered in 2020.

## At a glance

- **WHAT IT IS:** Neptune-like planet
- **HOW FAR:** 20.65 ly — The light arriving now left it that long ago.
- **HOW BIG:** 3.25 R⊕ · 20 729 km — Its radius: half the width across.
- **WHERE TO LOOK:** [Ursa Major](https://spacecatalog.org/constellation/ursa-major) — The patch of sky it sits in, as seen from Earth.
- **ONE ORBIT TAKES:** 24.4 days — One trip round GJ 338 B.

## Properties

| Quantity | Value |
| --- | --- |
| CLASS | Neptune-like planet |
| MASS | 10.60 M⊕ ±1.2 — a minimum mass (M sin i): radial velocities cannot see the orbit's tilt |
| RADIUS | 3.25 R⊕ · 20 729 km — estimated from the mass, not measured |
| DISTANCE FROM EARTH | 20.65 ly +0.0054 / −0.0055 — by NASA Exoplanet Archive sy\_dist (TICv8, 2019AJ....158..138S) |
| ORBITS | GJ 338 B |
| SEMI-MAJOR AXIS | 0.142 AU ±0.005 |
| CONSTELLATION | Ursa Major |
| RIGHT ASCENSION (J2000) | 09h 14m 22.0s |
| DECLINATION (J2000) | +52° 41′ 00.7″ |
| DISCOVERED | 2020 |

## Catalogue data

### WHAT KIND OF THING IT IS

What the object is in itself, rather than how it happens to look from here.

| Field | Value |
| --- | --- |
| Equilibrium temperature | 391 K |
| Host star spectral type | M0 V |
| Host star temperature | 4014 ±100 K |
| Host star mass | 0.64 ±0.07 M☉ |
| Host star radius | 0.58 ±0.03 R☉ |

### HOW BRIGHT IT IS

What the light adds up to, measured through one filter at a time. A magnitude is a brightness on a backwards scale — the smaller the number, the brighter the object.

| Field | Value |
| --- | --- |
| Host star apparent magnitude (V) | 7.70 |

### THE ORBIT

The shape of the path, where the object sits on it, and the instant those figures were true of. An orbit is a fit to observations and drifts away from the real one either side of its epoch.

| Field | Value |
| --- | --- |
| Host star | GJ 338 B |
| Orbital period | 24.422 days ±0.014 |
| Eccentricity | 0.109 +0.1 / −0.075 |
| Argument of periastron | 204.3 +94 / −70° |
| Periastron epoch | 2457517.0600 BJD |
| Transit midpoint | 2457521.4300 BJD |
| Stars in system | 2 |
| Planets in system | 1 |

### WHEN IT WAS FOUND

Who first saw it, from where, and by what method.

| Field | Value |
| --- | --- |
| Discovery method | Radial Velocity |
| Discovered at | Calar Alto Observatory |

## Also known as

- Exoplanet: GJ 338 B b
- Host star: GJ 338 B

## Source

[NASA Exoplanet Archive (Planetary Systems Composite Parameters)](https://exoplanetarchive.ipac.caltech.edu/cgi-bin/TblView/nph-tblView?app=ExoTbls&config=PSCompPars) — No licence stated; acknowledgement requested

The producer asks: This research has made use of the NASA Exoplanet Archive, which is operated by the California Institute of Technology, under contract with the National Aeronautics and Space Administration under the Exoplanet Exploration Program. Cite Christiansen et al. (2025), Planetary Science Journal (doi:10.3847/PSJ/ade3c2), and the literature reference given with each value.

How this was made: https://spacecatalog.org/methods#source-exoplanet-archive

## Cite this page

SpaceCatalog. GJ 338 B b \[catalogue entry\]. Data release DR1 (2026-08-21). https://spacecatalog.org/object/gj-338-b-b

```bibtex
@misc{spacecatalog_gj-338-b-b,
  author       = {{SpaceCatalog}},
  title        = {{GJ 338 B b}},
  howpublished = {SpaceCatalog, data release DR1 (2026-08-21)},
  year         = {2026},
  url          = {https://spacecatalog.org/object/gj-338-b-b},
  note         = {Catalogue entry, data release DR1 (2026-08-21). Values from NASA Exoplanet Archive (Planetary Systems Composite Parameters).}
}
```

NASA Exoplanet Archive (Planetary Systems Composite Parameters) — No licence stated; acknowledgement requested

Acknowledgement the producer asks for: This research has made use of the NASA Exoplanet Archive, which is operated by the California Institute of Technology, under contract with the National Aeronautics and Space Administration under the Exoplanet Exploration Program. Cite Christiansen et al. (2025), Planetary Science Journal (doi:10.3847/PSJ/ade3c2), and the literature reference given with each value.

The page itself, with its pictures, the sky map and the observing tools: https://spacecatalog.org/object/gj-338-b-b