HD 209458 b (Osiris)
Coordinates, Gaia DR3 distance (48.3 pc), 3.5247-day transit period, 1.359 R♃ inflated radius, and atmospheric escape data for HD 209458 b.
Provenance-Backed Scientific Measurements
Every measurement below preserves its unit, uncertainty interval, observational method, reference epoch, quality status, and primary catalog source. See Uncertainty & Provenance for how competing measurements are handled.
| Parameter | Value | Uncertainty | Method & Epoch | Quality Status | Source Archive |
|---|---|---|---|---|---|
| Transit orbital period | 3.5247486 d | ± 4e-7 d | HST, Spitzer, TESS & JWST transit timing | definitive | NASA Exoplanet Archive |
| Planetary radius | 15.23 R⊕ | ± 0.2 R⊕ | 1.46% (14,600 ppm) optical/IR transit depth (1.359 ± 0.018 R♃) | definitive | NASA Exoplanet Archive |
| Planetary mass | 217.7 M⊕ | ± 4.8 M⊕ | Radial velocity + known 86.71° transit inclination (0.685 ± 0.015 M♃) | definitive | NASA Exoplanet Archive |
| Equilibrium temperature | 1449 K | ± 12 K | Secondary eclipse thermal emission & insolation | definitive | NASA Exoplanet Archive |
| Host star distance (Gaia DR3) | 48.3 pc | ± 0.12 pc | Gaia DR3 astrometric parallax (20.70 ± 0.05 mas, 157.5 ly) (J2016.0) | definitive | ESA Gaia Archive — Data Release 3 |
| Host star visual magnitude (V) | 7.65 mag | ± 0.01 mag | Johnson UBV photometry (dims by 0.016 mag during 3-hour transit) | definitive | Hipparcos New Reduction Catalog |
J2000 Equatorial Sky Chart & Culmination Altitude
The chart below plots HD 209458 b (Osiris First Transiting Planet) at RA 22h 03m 10.77s, Dec +18° 53′ 03.5″ within Pegasus, alongside peak meridian transit altitudes for 40°N and 30°S observers. Use the RA/Dec Coordinate Converter to compute exact culmination altitude for your latitude.
Astrophysical Context & Structure
HD 209458 b (informally nicknamed 'Osiris') is one of the most historic worlds in exoplanet astronomy: in November 1999, teams led by David Charbonneau and Gregory Henry detected a 1.5% (0.016-magnitude) drop in the brightness of the 7.65-magnitude G0 V star HD 209458 every 3.5247 days—the very first planetary transit ever observed outside the Solar System.
That first transit proved beyond all doubt that Hot Jupiters are genuine low-density gas giants rather than high-inclination brown dwarfs or stellar pulsations. Weighing 0.685 Jupiter masses (218 Earth masses) yet inflated to 1.359 Jupiter radii (15.23 Earth radii), HD 209458 b has a mean bulk density of only 0.34 g/cm³—less than half the density of Saturn and roughly a third the density of liquid water.
In 2001 Charbonneau and colleagues used the Hubble Space Telescope to detect atomic sodium absorption in starlight filtering through HD 209458 b's limb during transit—the first atmosphere ever detected around an exoplanet. Subsequent UV, HST, and JWST NIRCam observations revealed a hydrodynamically escaping comet-like tail of hydrogen, carbon, oxygen, water vapor, and carbon dioxide boiling off the 1,449 K planet.
Practical Visual Observing Guide
- Shining at visual magnitude 7.65 in Pegasus (about 9° east-northeast of Enif, ε Pegasi), the host star HD 209458 (also given the variable-star designation V376 Pegasi) is easily visible in 10×50 binoculars or any small finder scope.
- Its 3-hour transit dims the star by 1.46% (16 millimagnitudes)—deep enough that amateur astronomers with a 100–200 mm telescope and a standard CMOS/CCD camera routinely measure the transit light curve from backyard observatories.
Catalog Cross-Identifications & Aliases
Common Aliases: Osiris · V376 Pegasi b · HIP 108859 b
| Catalog Authority | Identifier |
|---|---|
| NASAEXOPLANET | HD 209458 b |
| HIP | HIP 108859 |
| HD | HD 209458 |
| SIMBAD | HD 209458 b |
| WIKIDATA | Q271601 |
Related Objects, Comparisons & Guides
51 Pegasi b (Dimidium First Hot Jupiter)
Coordinates, Gaia DR3 distance (15.46 pc), 4.2308-day orbital period, Nobel-winning radial-velocity discovery, and observing guide for 51 Pegasi b.
WASP-39 b (JWST Benchmark Saturn Exoplanet)
Coordinates, Gaia DR3 distance (213 pc), 4.055-day period, 0.28 M♃ mass, 1.27 R♃ radius, and JWST CO2/SO2 photochemical spectrum of WASP-39 b.
K2-18 b (Temperate Sub-Neptune in Leo)
Coordinates, Gaia DR3 distance (38.07 pc), 32.94-day habitable-zone orbit, 8.63 M⊕ mass, 2.61 R⊕ radius, and JWST CH4/CO2 spectrum of K2-18 b.
TRAPPIST-1 e (Temperate Terrestrial World)
Coordinates, Gaia distance (12.47 pc), 6.10-day orbital period, mass 0.692 M⊕, radius 0.920 R⊕, and JWST transit spectroscopy for TRAPPIST-1 e.
Apparent vs Absolute Magnitude Explained
Understand the logarithmic Pogson astronomical magnitude scale, apparent visual magnitude (m), absolute magnitude (M at 10 parsecs), and bolometric corrections.
Proper Motion, Radial Velocity & Redshift
How astronomers combine tangential proper motion (mas/yr), Doppler radial velocity (km/s), and cosmological redshift (z) to reconstruct 3D space motion.
Confirmed Exoplanets & Host Systems Catalog
Curated NASA Exoplanet Archive reference catalog of benchmark exoplanets: Proxima Centauri b, TRAPPIST-1 e, 51 Pegasi b, HD 209458 b, K2-18 b, and WASP-39 b.
Scientific Sources & Dataset Provenance
- NASA Exoplanet Archive (2026-10-01 snapshot): Akeson, R. L., et al. 2013, PASP, 125, 989; NASA Exoplanet Archive (IPAC/Caltech) — NASA Exoplanet Science Institute (NExScI / Caltech / IPAC)
- ESA Gaia Archive — Data Release 3 (DR3 (2022-06-13)): Gaia Collaboration, Vallenari, A., et al. 2023, A&A, 674, A1 (Gaia Data Release 3) — European Space Agency / Gaia DPAC
- JWST / MAST Science Archive & Mission Documentation (Cycle 1–4 Public Releases (2022–2026)): Rigby, J., et al. 2023, PASP, 135, 048001 (The Science Performance of JWST as Characterized in Commissioning) — NASA / ESA / CSA / STScI