51 Pegasi b

51 Pegasi b

Planet· orbits the Sun

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.

Open in 3D viewSee 51 Pegasi b in the live, interactive 3D map

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⊕)
Radius
Not measured

Orbital

Semi-major axis
7,883,808 km
Eccentricity
0.0063
Inclination
0.00 °
Orbital period
4.2 days
Perihelion
7,834,140 km
Aphelion
7,933,476 km
Mean orbital velocity
136 km/s

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 Vision mode; Realism shows a plain spectral colour.

Radiation & temperature

Insolation
506 S⊕ · 506× the sunlight Earth receives
Equilibrium temperature
1,284 K (1011 °C)

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

Artist's impression of 51 Pegasi b ('Dimidium'), the first exoplanet found around a Sun-like star — an artist's concept, not a photograph.
ESO / M. Kornmesser / Nick Risinger (CC BY 4.0) · Wikimedia Commons

Images from open archives · credit shown per image