Hot Jupiter · 51 Peg

51 Peg b

ExoplanetsDiscovered 1995

Scientific overview

51 Peg b is a hot jupiter orbiting 51 Peg, completing an orbit every 4 days, about 50 light-years from Earth. It was discovered in 1995 using the radial velocity method.

Host star

Helvetios

Spectral type

G5V

Temperature

5,758 K

Mass

1.03 M☉

Discovery and detection

Detection method

Radial Velocity Method

Discovery year

1995

Knowledge connections

Child ofHelvetios
OrbitsHelvetios
Discovered by methodRadial Velocity Method
Catalogued inNASA Exoplanet Archive
Similar toHot Jupiter
Related toThe First Exoplanet Around a Sun-like Star

Quick facts

Class: Hot Jupiter
Host star: Helvetios
Discovery method: Radial Velocity Method
Discovery year: 1995
Orbital period: 4.2308 d
Semi-major axis: 0.0527 AU
Eccentricity: 0.013
Radius: 14.3 R⊕
Mass: 146.2 M⊕
Density: 0.28 g/cm³ · derived
Equilibrium temp.: 1311 K
Insolation: 492.82 S⊕
Distance: 50 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 density0.28 g/cm³derived
Formula & inputs
Formula: ρ = M / (4⁄3·π·r³) (v1.0.0, impl v1.0.0)
Inputs:
- massEarth = 146.2 M⊕ · NASA
- radiusEarth = 14.3 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
51 Pegasi b was the first confirmed exoplanet discovered orbiting a Sun-like star (1995); it is a 'hot Jupiter' in a very short-period orbit, a discovery recognised with the 2019 Nobel Prize in Physics.
Source: NASA Exoplanet Science Institute (Caltech/IPAC) — NASA Exoplanet Archive · Freely available; acknowledgement requested
- A Jupiter-mass companion to a solar-type star, Nature 378, 355–359 (1995)doi:10.1038/378355a0
- 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 paperA Jupiter-mass companion to a solar-type star

Michel Mayor & Didier Queloz · Nature 378, 355–359 · 1995

Nature · doi:10.1038/378355a0

Cite (APA / BibTeX)

Michel Mayor & Didier Queloz (1995). A Jupiter-mass companion to a solar-type star. Nature 378, 355–359. Nature. https://doi.org/10.1038/378355a0

@misc{cite:mayor-queloz-1995,
  title        = {A Jupiter-mass companion to a solar-type star},
  author       = {Michel Mayor and Didier Queloz},
  organization = {Nature},
  howpublished = {Nature 378, 355–359},
  year         = {1995},
  doi          = {10.1038/378355a0},
  url          = {https://doi.org/10.1038/378355a0},
  note         = {Discovery of 51 Pegasi b, the first confirmed exoplanet around a Sun-like star.}
}

Archive page51 Pegasi b — NASA Exoplanet Archive

NASA Exoplanet Science Institute (Caltech/IPAC)

Cite (APA / BibTeX)

NASA Exoplanet Science Institute (Caltech/IPAC) (n.d.). 51 Pegasi b — NASA Exoplanet Archive. NASA Exoplanet Science Institute (Caltech/IPAC). https://exoplanetarchive.ipac.caltech.edu/

@misc{cite:exoarchive-exoplanet-51-pegasi-b,
  title        = {51 Pegasi 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 51 Pegasi b.}
}

Historical referenceThe Nobel Prize in Physics 2019

2019

The Nobel Prize

Cite (APA / BibTeX)

The Nobel Prize (2019). The Nobel Prize in Physics 2019. The Nobel Prize. https://www.nobelprize.org/prizes/physics/2019/

@misc{cite:nobel-physics-2019,
  title        = {The Nobel Prize in Physics 2019},
  organization = {The Nobel Prize},
  year         = {2019},
  url          = {https://www.nobelprize.org/prizes/physics/2019/},
  note         = {Peebles (physical cosmology); Mayor & Queloz (first exoplanet around a Sun-like star).}
}

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.