51 Pegasi b
Planet· orbits 51 Pegasi
51 Pegasi b ('Dimidium') — the first exoplanet ever found around a Sun-like star (1995). A 'hot Jupiter' of at least ~0.46 Jupiter masses whipping around its star every 4.2 days, far closer than Mercury — a type of world no one expected. Found by radial velocity, so its mass is a minimum and its size is a model estimate; no density is quoted. Never imaged.
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Key facts
Minimum mass
8.72 × 10²⁶ kg
0.459 M♃ (146 M⊕)
Year
4.23 Earth days
Distance from 51 Pegasi
7,883,808 km
Science
Physical
- Minimum mass (m·sin i)
- 8.72 × 10²⁶ kg · 0.459 M♃ (146 M⊕)
- Radial-velocity detection measures m·sin i; the true mass depends on the unknown orbital inclination.
- Radius
- Not measured
- Found by radial velocity, never transited: the mass is a minimum (m·sin i, ≥ ~0.46 Jupiter) and the ~1.9 Jupiter-radii size is a model estimate for a hot Jupiter, so no density is quoted.
Orbital
- Semi-major axis
- 7,883,808 km
- From the NASA Exoplanet Archive (radial-velocity orbit)
- Eccentricity
- 0.0063
- From the NASA Exoplanet Archive (radial-velocity orbit)
- Inclination
- 0.00 °
- From the NASA Exoplanet Archive (radial-velocity orbit)
- Orbital period
- 4.2 days
- From the NASA Exoplanet Archive (radial-velocity orbit)
- Perihelion
- 7,834,140 km
- Computed from the orbital elements
- Aphelion
- 7,933,476 km
- Computed from the orbital elements
- Mean orbital velocity
- 136 km/s
- Computed from the orbital elements
Composition
- Bulk composition
- A 'hot Jupiter' — a gas giant of at least about half Jupiter's mass orbiting its star every 4.2 days, far closer than Mercury. Its very existence overturned expectations that giant planets must form far from their stars, and launched exoplanet science. Never imaged.
- Cloud appearance
- Appearance based on NASA/JPL-Caltech's artist's concept. No spacecraft has imaged this world, so its real appearance is unknown — The banded clouds shown are a representation of its class, shown only in Visual mode; Realism shows a plain spectral colour.
Radiation & temperature
- Insolation
- 506 S⊕ · 506× the sunlight Earth receives
- Computed from 51 Pegasi's luminosity and the orbital distance (inverse-square; Earth's sunlight = 1).
- Equilibrium temperature
- 1,284 K (1,011 °C)
- Computed model value for its scorching 4-day orbit — not measured.
Discovery & history
- Method
- Radial-velocity detection (stellar wobble)
- Name
- Its IAU proper name is 'Dimidium' (Latin for 'half', for its ~half-Jupiter minimum mass) — the first exoplanet ever given a proper name (2015).
Announced in October 1995 by Mayor & Queloz — the discovery that launched exoplanet science and shared the 2019 Nobel Prize in Physics. In 2015 it became the first exoplanet to receive an IAU proper name, 'Dimidium'.
Images
Images from open archives · credit shown per image