51 Pegasi b vs HD 209458 b Hot Jupiters
Compare the first radial-velocity Hot Jupiter (51 Pegasi b) and the first transiting Hot Jupiter (HD 209458 b / Osiris) in orbital and physical data.
Compared Celestial Objects
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.
Constellation: Pegasus · Coords: 22h 57m 27.98s, +20° 46′ 07.8″
HD 209458 b (Osiris First Transiting Planet)
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.
Constellation: Pegasus · Coords: 22h 03m 10.77s, +18° 53′ 03.5″
Normalized Shared Parameter Table
| Parameter | Unit / Scale | 51 Pegasi b (Dimidium First Hot Jupiter) | HD 209458 b (Osiris First Transiting Planet) |
|---|---|---|---|
| Host Star Apparent Magnitude (V) | mag | +5.46 (G2.5 IVa solar analog) | +7.65 (G0 V solar-type dwarf) |
| Gaia DR3 Heliocentric Distance | pc | 15.461 ± 0.015 pc (50.43 ly) | 48.37 ± 0.12 pc (157.8 ly) |
| Orbital Period & Semi-Major Axis | days / AU | 4.230785 d (0.0527 AU) | 3.524749 d (0.0475 AU) |
| Planetary Mass | MJup | 0.46 ± 0.02 M_Jup (M sin i) | 0.69 ± 0.02 M_Jup (true mass) |
| Planetary Radius | RJup | Non-transiting (~1.2 R_Jup model) | 1.359 ± 0.016 R_Jup (transiting) |
Astrophysical & Observational Synthesis
In October 1995, Michel Mayor and Didier Queloz announced a 55.9 m/s sinusoidal Doppler wobble in the 5.46-magnitude G2.5 IVa star 51 Pegasi every 4.2308 days—revealing a minimum-half-Jupiter-mass planet orbiting 7.5 times closer to its star than Mercury orbits the Sun and winning the 2019 Nobel Prize in Physics. Yet because 51 Pegasi b's orbital plane is inclined ~70°–80° rather than 90°, it never crosses the face of its star, leaving its physical radius unmeasured by transit photometry.
Four years later in November 1999, astronomers caught the 3.5247-day radial-velocity candidate HD 209458 b crossing directly in front of its 7.65-magnitude G0 V star in Pegasus (orbital inclination i = 86.71°), producing a 1.46% (0.016 mag) photometric dip lasting 3.0 hours.
Measuring HD 209458 b's 1.359 R_Jup radius alongside its 0.69 M_Jup true mass proved beyond all doubt that Hot Jupiters are inflated hydrogen-helium gas giants (with a low bulk density of just 0.34 g/cm³) and inaugurated exoplanet atmospheric transmission spectroscopy (detecting sodium, water vapor, carbon monoxide, and an escaping hydrogen tail).
Related Reference Guides
Exoplanet Transit & Radial Velocity Methods
How astronomers detect and characterize exoplanets using photometric transit depth ((Rp/R*)²), Doppler radial velocity (M sin i), and JWST transmission spectra.
Trigonometric Parallax, Parsecs & Gaia DR3
How trigonometric parallax (d_pc = 1 / p_arcsec), the distance modulus equation, Hipparcos, and Gaia DR3 microarcsecond astrometry measure cosmic distances.
Primary Catalogs & Scientific Sources
- 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
- SIMBAD Astronomical Database (CDS) (2026-10 release): Wenger, M., et al. 2000, A&AS, 143, 9 (The SIMBAD astronomical database) — Centre de Données astronomiques de Strasbourg
- Revised New General Catalogue and Index Catalogue (RNGC/IC) (2024/2026 edition): Steinicke, W. 2010, Observing and Cataloguing Nebulae and Star Clusters (Cambridge Univ. Press); Verga, M. OpenNGC (CC-BY-SA 4.0 / data tables) — Wolfgang Steinicke / OpenNGC