51 Pegasi b
Hot Jupiter · First planet around a Sun-like star
51 Pegasi b was the first planet found orbiting a star like our Sun. It shocked astronomers: a giant planet like Jupiter, but so close to its star that it races around it in just over four days.
- 50.6 light-years awayEven light – the fastest thing there is – needs 50.6 years to get here.
- 1995Discovered
- 4.2 daysYear
Take a closer look
A birthday every 4 days
51 Pegasi b goes all the way round its star in just over four days, so a ten-year-old there would have had hundreds of birthdays. It orbits about seven times closer to its star than Mercury is to our Sun, so it is roasted by starlight all the time.
A wandering giant
Giant planets like Jupiter are thought to form far from their star, where it is cold enough for ice to gather. So most scientists think 51 Pegasi b was born much farther out and slowly drifted inwards. Finding it showed that planets can move around after they form.
The start of a flood
The star 51 Pegasi sways back and forth at about 200 kilometres per hour – the speed of a racing car – and astronomers measured that from 50 light-years away. Since this first find in 1995, more than 6,000 planets around other stars have been confirmed.
How we found out
- 1995
Michel Mayor and Didier Queloz detect the star wobbling back and forth, pulled by an unseen planet.
- 2015
In a worldwide public naming contest, the planet becomes “Dimidium” and its star “Helvetios”.
- 2019
Mayor and Queloz win the Nobel Prize in Physics for the discovery.
How can you find a planet by watching its star wobble?
As the planet orbits, it tugs the star towards and away from us. That makes the starlight very slightly bluer, then redder – a shift that sensitive instruments can measure.
See it yourself
The star 51 Pegasi is just visible to the eye from a dark site, in the constellation Pegasus.
From the encyclopedia
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