51 Pegasi b (Dimidium)
Coordinates, Gaia DR3 distance (15.46 pc), 4.2308-day orbital period, Nobel-winning radial-velocity discovery, and observing guide for 51 Pegasi 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 |
|---|---|---|---|---|---|
| Orbital period | 4.230785 d | ± 0.000036 d | ELODIE, Lick, HIRES & HARPS radial-velocity spectroscopy | definitive | NASA Exoplanet Archive |
| Minimum planetary mass (M sin i) | 146.2 M⊕ | ± 6.4 M⊕ | Radial-velocity semi-amplitude K = 55.94 ± 0.69 m/s (0.460 ± 0.020 M♃) | definitive | NASA Exoplanet Archive |
| Orbital semi-major axis | 0.0527 AU | ± 0.003 AU | Keplerian orbit around 1.12 M☉ host star | definitive | NASA Exoplanet Archive |
| Dayside equilibrium temperature | 1284 K | ± 45 K | High-resolution CRIRES/HARPS dayside molecular spectroscopy | high-confidence | NASA Exoplanet Archive |
| Host star distance (Gaia DR3) | 15.461 pc | ± 0.014 pc | Gaia DR3 astrometric parallax (64.68 ± 0.06 mas, 50.43 ly) (J2016.0) | definitive | ESA Gaia Archive — Data Release 3 |
| Host star apparent magnitude (V) | 5.49 mag | ± 0.01 mag | Johnson UBV photoelectric photometry | definitive | Hipparcos New Reduction Catalog |
J2000 Equatorial Sky Chart & Culmination Altitude
The chart below plots 51 Pegasi b (Dimidium First Hot Jupiter) at RA 22h 57m 27.98s, Dec +20° 46′ 07.8″ 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
51 Pegasi b (officially named Dimidium by the IAU, orbiting the star Helvetios) revolutionized modern astronomy on October 6, 1995, when Michel Mayor and Didier Queloz announced its discovery using the ELODIE fiber-fed spectrograph at the Observatoire de Haute-Provence—a breakthrough recognized with the 2019 Nobel Prize in Physics. It was the first exoplanet ever discovered orbiting a normal, hydrogen-fusing main-sequence star (51 Pegasi, spectral type G2 IV–V).
Before 1995, planet formation models assumed gas giants could only reside beyond a star's icy 'snow line' (>3–5 AU), where Jupiter and Saturn orbit. Instead, 51 Pegasi b is a 0.47-Jupiter-mass gas giant whipsawing around its sun every 4.2308 days at a scorching distance of just 0.0527 AU (7.9 million kilometers)—one-seventh of Mercury's distance from the Sun! Its discovery forced astrophysicists to develop the theory of planetary disk migration, showing that giant planets can form far out and spiral inward through gravitational torques with the protoplanetary gas disk.
Even though 51 Pegasi b does not transit its host star, astronomers using the HARPS and VLT/CRIRES spectrographs detected the Doppler-shifted spectral lines of water vapor and reflected starlight directly from the planet's 1,284 K dayside as it whips around its orbit at 133 km/s.
Practical Visual Observing Guide
- Unlike almost all transiting exoplanet hosts, the star 51 Pegasi shines at magnitude 5.49—bright enough to see with the naked eye under a dark country sky and effortless in any pair of 7×50 binoculars!
- To find 51 Pegasi, look along the western edge of the Great Square of Pegasus, almost exactly halfway between Scheat (β Pegasi) and Markab (α Pegasi), shifted 2.5° west.
Catalog Cross-Identifications & Aliases
Common Aliases: Dimidium · HD 217014 b · Helvetios b
| Catalog Authority | Identifier |
|---|---|
| NASAEXOPLANET | 51 Peg b |
| HIP | HIP 113357 |
| HD | HD 217014 |
| SIMBAD | * 51 Peg b |
| WIKIDATA | Q244551 |
Related Objects, Comparisons & Guides
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.
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.
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.
Jupiter (Gas Giant & Galilean Moons)
JPL orbital parameters (5.2038 AU, 11.862 yr), opposition magnitude −2.94, 50.1″ angular disk, Galilean moons, and observing guide for Jupiter.
Autumn Sky Tour: Andromeda, Perseus & Cassiopeia
Autumn observing guide from Cassiopeia and Perseus (Double Cluster, Algol, Cas A) across Andromeda and Triangulum to Local Group galaxies M31 and M33.
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.
Stellar Spectral Classification (OBAFGKM)
Reference guide to Morgan-Keenan (MK) stellar classification (O B A F G K M), luminosity classes (I–V, WD), color index (B-V), and the H-R diagram.
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
- 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