October 6, 1995. On this date – 20 years ago today – astronomers Michel Mayor and Didier Queloz announced the discovery of the first planet in orbit around a distant sunlike star. They made their announcement in the journal Nature.
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 will 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 another name. They unofficially call it Bellerophon, from ancient Greek mythology. Bellerophon was said to be the Greek hero who tamed Pegasus the Flying Horse.
Any detailed picture you see of an exoplanet, such as the one above, is an artist’s concept. We cannot see planets orbiting distant suns in anything like this amount of detail.
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 years 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 about the radial velocity method for finding extrasolar planets.
Now we know hundreds of exoplanets. As of October 6, 2013, the Extrasolar Planets Encyclopedia is listing 756 planetary systems with 992 planets; 168 are multiple planet systems.
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, so secure in its place in astronomical history? 51 Pegasi b is 0.6 times the size of our solar system’s largest planet, Jupiter. It orbits very close to its parent star, requiring only 4 days to orbit its star, in contrast to 365 days for our Earth to orbit the sun. It’s also known that the planet is tidally locked to its star, much as our moon is tidally locked to Earth, always presenting the same face to it. It is what’s known today as a hot Jupiter.
Bottom line: On October 6, 1995, astronomers Michel Mayor and Didier Queloz announced in the journal Nature the discovery of the first planet in orbit around a distant sunlike star. This planet is designated as 51 Pegasi b.
Deborah Byrd created the EarthSky radio series in 1991 and 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.