October 6, 1995. On this date, astronomers Michel Mayor and Didier Queloz announced the discovery of the first planet in orbit around a distant sunlike star. They later published their finding in the journal Nature, in a paper titled simply A Jupiter-Mass Companion to a Solar-type Star.
The star was 51 Pegasi, located about 50 light-years away in the direction of our constellation Pegasus the Flying Horse. Astronomers officially designated the new planet as 51 Pegasi b, in accordance with nomenclature already decided upon for extrasolar planets. The b means that this planet was the first discovered orbiting its parent star. If additional planets are ever found for the star 51 Pegasi, they’ll be designated c, d, e, f, and so on. So far, this planet is the only one known in this system.
Astronomers call 51 Pegasi b by other names. It was dubbed Bellerophon by astronomer Geoffrey Marcy, who helped confirm its existence and who was following the convention of naming planets after Greek and Roman mythological figures. Bellerophon was a figure from Greek mythology who rode the winged horse Pegasus. Later, in the course of its NameExoWorlds contest, the International Astronomical Union named this planet Dimidium – Latin for half, referring to its mass of at least half the mass of Jupiter.
It remains to be seen whether astronomers will accept the IAU’s name recommendation, or whether 51 Pegasi b, like so many objects in astronomy, will continue to have multiple names.
51 Pegasi b was the first, but now we know thousands of exoplanets. As of 2019, astronomers have discovered more than 4,000 exoplanets.
But 51 Pegasi b will always be the first known to orbit a star like our sun.
What do we know today of 51 Pegasi b, this world whose place in astronomical history is so secure? Its mass is about half that of Jupiter, and it’s thought to have a greater diameter than Jupiter (the biggest planet in our solar system), despite its smaller mass. 51 Pegasi b orbits very close to its parent star, requiring only four days to orbit its star, in contrast to 365 days for our Earth to orbit the sun and 12 years for Jupiter. In other words, 51 Pegasi b orbits very close to its star.
It’s also known that this planet is tidally locked to its star, much as our moon is tidally locked to Earth, always presenting the same face to it. It’s what’s known today as a hot Jupiter.
Detailed pictures you see of exoplanets, such as the one at the top of this post, are always artists’ concepts. Even the largest earthly telescopes can’t see planets orbiting distant suns in anything like this amount of detail. At best, through earthly telescopes, they look like dots. Still, analyzing exoplanets – their atmospheres, for example, and their potential for life – is a major priority for NASA and for many astronomers in the years ahead.
Consider that, before 51 Pegasi b, the search for exoplanets – worlds beyond our own solar system – was exceedingly difficult. Once astronomers began in earnest to search for them, they searched for decades before finding any. In nearly all cases, exoplanets cannot be seen in the light of their parent stars, and astronomers had to develop clever technologies in order to discover them. As with many extrasolar planets, 51 Pegasi b was found using the radial velocity method. Click here to learn more about how astronomers find exoplanets.
Bottom line: On October 6, 1995, astronomers Michel Mayor and Didier Queloz announced the discovery of the first planet in orbit around a distant sunlike star. This planet is designated 51 Pegasi b.
Deborah Byrd created the EarthSky radio series in 1991 and founded EarthSky.org in 1994. Today, she serves as Editor-in-Chief of this website. She has won a galaxy of awards from the broadcasting and science communities, including having an asteroid named 3505 Byrd in her honor. A science communicator and educator since 1976, Byrd believes in science as a force for good in the world and a vital tool for the 21st century. "Being an EarthSky editor is like hosting a big global party for cool nature-lovers," she says.