Hot Jupiter · HD 209458

HD 209458 b

ExoplanetsDiscovered 1999

Scientific overview

HD 209458 b is a hot jupiter orbiting HD 209458, completing an orbit every 4 days, about 158 light-years from Earth. It was discovered in 1999 using the radial velocity method.

Host star

HD 209458

Spectral type

G0 V

Temperature

6,091 K

Mass

1.23 M☉

Discovery and detection

Detection method

Radial Velocity Method

Discovery year

1999

Discovered at

W. M. Keck Observatory

Knowledge connections

Child ofHD 209458
OrbitsHD 209458
Discovered by methodRadial Velocity Method
Discovered atW. M. Keck Observatory
Catalogued inNASA Exoplanet Archive
Similar toGas giant

Quick facts

Class: Hot Jupiter
Host star: HD 209458
Discovery method: Radial Velocity Method
Discovery year: 1999
Orbital period: 3.5247 d
Semi-major axis: 0.0471 AU
Eccentricity: 0
Radius: 15.58 R⊕
Mass: 232.02 M⊕
Density: 0.34 g/cm³ · derived
Equilibrium temp.: 1459 K
Insolation: 587.1 S⊕
Distance: 158 ly

Density is derived from the archive mass and radius via ρ = M / (4⁄3·π·r³); all other values are catalogued measurements.

Derived values

Computed from source-backed inputs via standard formulae — not observations. Every input and constant is traced below.

Bulk density0.34 g/cm³derived
Formula & inputs
Formula: ρ = M / (4⁄3·π·r³) (v1.0.0, impl v1.0.0)
Inputs:
- massEarth = 232.02 M⊕ · NASA
- radiusEarth = 15.58 R⊕ · NASA
Assumptions: M⊕ = 5.972e+24 kg, R⊕ = 6371000 m (IAU 2015 B3 nominal); Assumes a uniform sphere.
Limitations: Withheld above 30 g/cm³ (inconsistent mass/radius inputs).; Archive mass/radius carry no propagated uncertainty here.
Uncertainty: not propagated (inputs carry no catalogued uncertainty)
Computed as of 2026-06-29.

Provenance & sources

review 2026.1

Asteria Scientific Review Process · verification: sourced · accurate

High evidenceconfirmed
HD 209458 b was the first exoplanet observed to transit its host star (2000) and the first exoplanet with an atmosphere detected.
Source: NASA Exoplanet Science Institute (Caltech/IPAC) — NASA Exoplanet Archive · Freely available; acknowledgement requested
- NASA Exoplanet Archive, NASA Exoplanet Archive

Reviewed by the internal Asteria Scientific Review Process — not an external institutional review. See the evidence framework and authority dashboard.

Scientific citations

Real, source-backed references — primary papers first, then datasets and institutional sources. Formatted through the citation engine; nothing is fabricated.

Peer-reviewed paperDetection of Planetary Transits Across a Sun-like Star

David Charbonneau et al. · The Astrophysical Journal 529, L45 · 2000

The Astrophysical Journal · doi:10.1086/312457

Cite (APA / BibTeX)

David Charbonneau, Timothy M. Brown, David W. Latham, & Michel Mayor (2000). Detection of Planetary Transits Across a Sun-like Star. The Astrophysical Journal 529, L45. The Astrophysical Journal. https://doi.org/10.1086/312457

@misc{cite:charbonneau-2000,
  title        = {Detection of Planetary Transits Across a Sun-like Star},
  author       = {David Charbonneau and Timothy M. Brown and David W. Latham and Michel Mayor},
  organization = {The Astrophysical Journal},
  howpublished = {The Astrophysical Journal 529, L45},
  year         = {2000},
  doi          = {10.1086/312457},
  url          = {https://doi.org/10.1086/312457},
  note         = {First detection of an exoplanet transit (HD 209458 b).}
}

Archive pageHD 209458 b — NASA Exoplanet Archive

NASA Exoplanet Science Institute (Caltech/IPAC)

Cite (APA / BibTeX)

NASA Exoplanet Science Institute (Caltech/IPAC) (n.d.). HD 209458 b — NASA Exoplanet Archive. NASA Exoplanet Science Institute (Caltech/IPAC). https://exoplanetarchive.ipac.caltech.edu/

@misc{cite:exoarchive-exoplanet-hd-209458-b,
  title        = {HD 209458 b — NASA Exoplanet Archive},
  organization = {NASA Exoplanet Science Institute (Caltech/IPAC)},
  year         = {n.d.},
  url          = {https://exoplanetarchive.ipac.caltech.edu/},
  note         = {Confirmed parameters and discovery references for HD 209458 b.}
}

Quality & authority

Complete

Reviewed

Description: Complete
Source coverage: Complete
Relationship coverage: Complete
Review coverage: Complete
Citation coverage: Complete
Image coverage: Not applicable

Data from the NASA Exoplanet Archive. See source quality.