Exoplanets · Space Codex
51 Pegasi b
The first planet found around a Sun-like star, in 1995: a giant on a 4.2-day orbit, a 'hot Jupiter' that nobody had predicted.
The planet that started it all
Artist's concept. No telescope has seen 51 Pegasi b as more than a point of light, and most exoplanets not even that. What we know comes from how a planet dims or tugs its star. Any picture of its surface, colour or sky, anywhere, is an illustration, not an observation.
At a glance
Key points
The discovery won the 2019 Nobel Prize in Physics.
- First planet around a Sun-like star
- Defined the 'hot Jupiter' class
- Nobel Prize 2019
How it was found
Michel Mayor and Didier Queloz of the University of Geneva were surveying Sun-like stars with the ELODIE spectrograph at the Haute-Provence Observatory in France. 51 Pegasi's light shifted back and forth every 4.23 days, the star moving towards and away from us by about 56 metres per second. They announced the find at a conference in Florence on 6 October 1995, and their Nature paper followed that November. Other teams soon confirmed the signal.
A surprise
Theory said giant planets form far from their stars. 51 Peg b orbits closer than Mercury does to the Sun, which forced astronomers to accept that planets can migrate.
Light from the planet itself
51 Peg b does not transit, so for two decades only its minimum mass was known. In 2017 astronomers using the CRIRES spectrograph on ESO's Very Large Telescope picked out water absorption in its dayside spectrum and tracked it moving at about 133 km/s around its orbit. That gave a true mass of about 0.48 Jupiters and an orbital tilt of 70 to 82 degrees from face-on.
Observing it
The star sits just west of the Great Square of Pegasus, faint but visible to the unaided eye under dark skies on northern autumn evenings. The planet itself cannot be seen; it is lost in the star's glare at a few thousandths of an arcsecond.
What we still don't know
Its radius and colour. Without transits there is no direct size measurement, and its clouds and haze are unobserved.
Related entries
- Exoplanets: the censusPlanets around other stars
- PSR B1257+12 planetsThe first exoplanets ever confirmed
Sources
- NASA Exoplanet Archive: 51 Peg overview (opens in a new tab)
- NobelPrize.org: Physics 2019 (opens in a new tab)
- Birkby et al. 2017, AJ (arXiv) (opens in a new tab)
- IAU: NameExoWorlds results (iau1514) (opens in a new tab)
Last reviewed against these sources. Built from public NASA, ESA and JPL data; not affiliated with or endorsed by any agency.