51 Pegasi b: First Sun-Like Star Exoplanet
On October 6, 1995, Swiss astronomers Michel Mayor and Didier Queloz announced the detection of 51 Pegasi b (formally named Dimidium)—the first exoplanet discovered orbiting a main-sequence Sun-like star. Using the ELODIE fiber-fed echelle spectrograph at Haute-Provence Observatory, they measured a 56 m/s radial velocity wobble repeating every 4.2308 days, revealing a half-Jupiter-mass planet orbiting 20× closer to its star than Earth orbits the Sun.
Physical Mechanism & Dynamics
Before 1995, planet searches modeled on our Solar System expected giant planets to form only beyond the water-ice snow line (~3–5 AU) with orbital periods of 10+ years. While Aleksander Wolszczan and Dale Frail had discovered terrestrial-mass planets around the millisecond pulsar PSR B1257+12 in 1992 via radio pulse timing, no planet was known around a normal hydrogen-burning star.
Mayor and Queloz achieved ~13 m/s Doppler precision with ELODIE using simultaneous thorium-argon reference spectra and cross-correlation pipelines. Finding a ≥0.46 M_Jup planet at a = 0.0527 AU (equilibrium temperature ~1,280 K) stunned astronomers, Was rapidly confirmed within days by Geoffrey Marcy and Paul Butler at Lick Observatory, and established that protoplanetary disk migration reshapes planetary architectures.
Quantitatively, this mechanism is governed by 51 Pegasi b Orbital Semi-Major Axis and Minimum Mass (a = [ G M_★ P² / (4π²) ]^(1/3) = 0.0527 AU ; M_p sin i = 0.46 M_Jup (K_★ = 55.9 m/s)). With P = 4.230785 days around a 1.06 M_☉ G2V star, 51 Pegasi b orbits seven times closer to its host star than Mercury does to the Sun.
51 Pegasi b Orbital Semi-Major Axis and Minimum Mass
a = [ G M_★ P² / (4π²) ]^(1/3) = 0.0527 AU ; M_p sin i = 0.46 M_Jup (K_★ = 55.9 m/s)With P = 4.230785 days around a 1.06 M_☉ G2V star, 51 Pegasi b orbits seven times closer to its host star than Mercury does to the Sun.
Quantitative Benchmarks & Parameters
| Parameter / Observable | Measured Value / Scale | Method & Context |
|---|---|---|
| Orbital period P | 4.230785 days (101.5 hours) | ELODIE radial velocity periodicity (1995) |
| Semi-major axis a | 0.0527 AU (7.9 million km) | Kepler's third law around 51 Pegasi (d = 15.5 pc) |
| Stellar reflex velocity K_★ | 55.9 ± 0.7 m/s (circular orbit) | 2019 Nobel Prize in Physics (Mayor & Queloz) |
| Minimum mass M_p sin i | 0.46 M_Jup (~150 M_⊕) | Archetype of the 'hot Jupiter' exoplanet class |
Observational Evidence & How Scientists Know
High-resolution spectroscopy later detected Doppler-shifted CO and H₂O absorption lines directly from 51 Pegasi b's dayside atmosphere, breaking the sin i inclination degeneracy (i ≈ 70°–80°) and confirming a true mass near ~0.5 M_Jup.
Across reference datasets (NASA Exoplanet Archive, SIMBAD Astronomical Database (CDS)), quantitative benchmarks for 51 pegasi b: first sun-like star exoplanet are anchored by: Orbital period P (4.230785 days (101.5 hours), via ELODIE radial velocity periodicity (1995)); Semi-major axis a (0.0527 AU (7.9 million km), via Kepler's third law around 51 Pegasi (d = 15.5 pc)); Stellar reflex velocity K_★ (55.9 ± 0.7 m/s (circular orbit), via 2019 Nobel Prize in Physics (Mayor & Queloz)); Minimum mass M_p sin i (0.46 M_Jup (~150 M_⊕), via Archetype of the 'hot Jupiter' exoplanet class).
Uncertainties, Systematics & Open Questions
Determining the exact fraction of hot Jupiters formed via smooth Type II gas-disk migration versus late high-eccentricity tidal circularization remains an active observational test using stellar obliquity (Rossiter-McLaughlin) measurements.
Real Astronomical Benchmarks & Cross-Domain Links
TRAPPIST-1 e
Contrast between 51 Peg b (hot Jupiter) and compact multi-planet rocky M-dwarf systems
Radial Velocity Doppler Wobble
Spectroscopic method used by Mayor and Queloz
Planetary Migration & Architectures
How giant planets migrate inward to 0.05 AU
Guided Learning Path
Related Concepts, Methods & Explorers
Scientific Sources & Provenance
- NASA Exoplanet Archive (2026-10-01 snapshot) — Akeson, R. L., et al. 2013, PASP, 125, 989; NASA Exoplanet Archive (IPAC/Caltech). Archive / Reference ↗
- SIMBAD Astronomical Database (CDS) (2026-10 release) — Wenger, M., et al. 2000, A&AS, 143, 9 (The SIMBAD astronomical database). Archive / Reference ↗
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