Record-holder
GJ 436
A Neptune-sized planet leaving a 1-million-km hydrogen tail behind it.
- Planets
- 1
- Distance
- 31.8 ly
- Host
- M-type
About GJ 436
Discovery
GJ 436 b was discovered in 2004 by Butler and colleagues using radial velocities at Keck — a Neptune-mass planet around a nearby M-dwarf, later found to also transit, making it the first transiting Neptune-class exoplanet.
Why it matters
In 2015, Ehrenreich and colleagues used Hubble UV transit observations to discover that GJ 436 b is shedding a gigantic comet-like cloud of hydrogen — a roughly million-kilometre tail of escaping atmosphere — the most dramatic atmospheric-escape signature seen in any exoplanet.
Current research
Modeling of the hydrogen exosphere and its sculpting by stellar wind continues; recent work is also probing whether GJ 436 b's atmosphere is helium-rich, as has been seen on similar warm Neptunes.
Comparable to
A warm Neptune trailing a hydrogen tail bigger than the star itself — a planet halfway between exoplanet and comet.
System geometry
At a glance
- Hostname
- GJ 436
- Spectral type
- M2.5 V
- Distance
- 31.8 ly · 9.75 pc
- Stellar mass
- 0.47 M☉
- Stellar radius
- 0.46 R☉
- Luminosity
- 0.026 L☉
- Effective temp
- 3586 K
- Confirmed planets
- 1
- Habitable zone
- 0.153 – 0.221 AU
Top-down orbital diagram
Orbits to scale within this system. Dashed green = habitable-zone edges.
Planet positions are illustrative (evenly spaced in phase). For live motion see the 3D scene.
The planets
1 confirmed.
Gas giant
GJ 436 b
- Orbit
- 0.029 AU
- Period
- 2.64 days
- Radius
- 4.17 R⊕
- Mass
- 22.10 M⊕
- Eq. temperature
- 686 K
- Eccentricity
- 0.138
- Discovered
- 2004 · Radial Velocity
Compared to our Solar System
Each row shows the closest Solar-System analog by radius (log-space). Earth is pinned at the bottom as the constant frame of reference.
| Planet | Radius (R⊕) | Mass (M⊕) | Orbit (AU) | Period (days) | Eq temp (K) | Solar analog |
|---|---|---|---|---|---|---|
| GJ 436 b | 4.17 | 22.10 | 0.029 | 2.64 | 686 | Uranus |
| Earth (reference) | 1.00 | 1.00 | 1.000 | 365.25 | 255 | — |
Research status
◇ JWST observation status
1 planet has confirmed JWST observation time across Cycles 1–3.
- GJ 436 b
JWST programs include transit spectroscopy, thermal phase curves, and direct imaging coronagraph observations depending on planet class.
Discovery timeline
- 2004
GJ 436 b
via Radial Velocity
If you liked this
Other systems in the same theme:
PSR B1257+12
3 planets · 1956.9 ly · G2V
First exoplanets ever confirmed (1992) — orbiting a pulsar, not a normal star.
Kepler-16
1 planets · 244.9 ly · G2V
A real-world Tatooine — a planet orbiting two stars at once.
GJ 1132
2 planets · 41.1 ly · M4.5 V
A Venus analog 41 ly away — terrestrial world losing its atmosphere.
Experience it
See GJ 436 in interactive 3D
Fly through the system, click any planet, watch orbits play out at 100× speed.
▶ Launch 3D scene