Extreme-class world
HD 209458
First exoplanet seen transiting its star (1999) — atmospheric studies pioneer.
- Planets
- 1
- Distance
- 157.5 ly
- Host
- G-type
About HD 209458
Discovery
HD 209458 b was first detected by radial velocity in 1999 (Henry et al.; Mazeh et al.), then in 2000 became the first exoplanet ever observed transiting its star — a watershed by Charbonneau, Brown, and colleagues that opened up direct atmospheric study.
Why it matters
Nicknamed 'Osiris,' HD 209458 b was the first exoplanet found to have an atmosphere (2002), the first with detected atmospheric escape (2003), and remains the most-studied hot Jupiter in history.
Current research
Recent JWST transmission spectroscopy refined the planet's metallicity and C/O ratio, continuing nearly two decades of atmospheric retrievals on this G0V host.
Comparable to
A puffy hot Jupiter slightly larger than Jupiter itself, blowing off hydrogen so fast it streams from the planet like a comet's tail.
System geometry
At a glance
- Hostname
- HD 209458
- Spectral type
- G0 V
- Distance
- 157.5 ly · 48.30 pc
- Stellar mass
- 1.23 M☉
- Stellar radius
- 1.19 R☉
- Luminosity
- 1.702 L☉
- Effective temp
- 6091 K
- Confirmed planets
- 1
- Habitable zone
- 1.239 – 1.787 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
HD 209458 b
- Orbit
- 0.047 AU
- Period
- 3.52 days
- Radius
- 15.58 R⊕
- Mass
- 232.02 M⊕
- Eq. temperature
- 1459 K
- Eccentricity
- 0.000
- Discovered
- 1999 · 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 |
|---|---|---|---|---|---|---|
| HD 209458 b | 15.58 | 232.02 | 0.047 | 3.52 | 1459 | Jupiter |
| 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.
- HD 209458 b
JWST programs include transit spectroscopy, thermal phase curves, and direct imaging coronagraph observations depending on planet class.
Discovery timeline
- 1999
HD 209458 b
via Radial Velocity
If you liked this
Other systems in the same theme:
51 Pegasi
1 planets · 50.4 ly · G5V
First exoplanet ever found around a sun-like star (1995, Nobel Prize 2019).
WASP-12
1 planets · 1393.5 ly · G2V
A hot Jupiter being literally devoured by its star — losing 6×10⁹ tonnes/sec.
KELT-9
1 planets · 666.8 ly · B9.5-A0
The hottest known exoplanet — daytime hotter than most stars (4,600 K).
Experience it
See HD 209458 in interactive 3D
Fly through the system, click any planet, watch orbits play out at 100× speed.
▶ Launch 3D scene