51 Pegasi

Star

51 Pegasi ('Helvetios') — a Sun-like G5 star 50 light-years away in Pegasus, a little older than the Sun. Ordinary in itself, it made history in 1995 as the first normal star found to host a planet — the discovery that opened the age of exoplanets.

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

Key facts

Mass

2.13 × 10³⁰ kg

1.07 M☉

Radius

827,883 km

1.19× wider than the Sun

Surface temperature

5,761 K (5,488 °C)

Spectral type

G5V (a Sun-like yellow dwarf, a little older than the Sun)

Science

Physical

Mass: 2.13 × 10³⁰ kg · 1.07 M☉
Radius: 827,883 km · 1.19× wider than the Sun
Density: 895 kg/m³ · 0.9× water; Computed from mass and radius
Spectral type: G5V (a Sun-like yellow dwarf, a little older than the Sun)

Motion through space

Proper motion: 0.216 arcseconds per year · its drift across our sky; SIMBAD — 2020yCat.1350....0G, 2026A&A...710A.345C
Radial velocity: -33.1 km/s · approaching the Sun; SIMBAD — 2020yCat.1350....0G, 2026A&A...710A.345C
Speed across our line of sight: 15.9 km/s; Computed from proper motion and parallax
Speed through space, relative to the Sun: 36.8 km/s; Computed from proper motion, parallax and radial velocity

Composition

By mass: A near solar-twin — hydrogen/helium, slightly metal-rich, about 1.1 solar masses. Unremarkable in itself; its fame is entirely its planet, the first ever found around a Sun-like star.

Temperature

Surface: 5,761 K (5,488 °C)

Radiation & temperature

Luminosity: 5.38 × 10²⁶ W · 1.41 L☉
Surface temperature: 5,761 K (5,488 °C)
Peak wavelength: 503 nm · green-yellow visible light; Computed (Wien's law) from the surface temperature

Discovery & history

Method: Telescope
Name: A Flamsteed number (51 Pegasi); its IAU proper name is 'Helvetios' (Latin for the Helvetii, honouring its Swiss discoverers).

Catalogued long ago, it drew the world's attention in October 1995 when Michel Mayor and Didier Queloz measured its tiny wobble and announced a planet — the first around a Sun-like star, a result that won the 2019 Nobel Prize in Physics.

Images

Where to find it — in Pegasus, the Winged Horse (IAU constellation chart). To the eye it is a single point of light; no telescope has imaged its surface. The first star known to host an exoplanet. — IAU / Sky & Telescope, R. Sinnott & R. Fienberg (CC BY 3.0) · Wikimedia Commons

Images from open archives · credit shown per image