1st planet orbiting a sunlike star discovered 27 years ago

Planet orbiting sunlike star: Comparison of Jupiter to exoplanet Pegasi 51 b and Pegasi 51 to the sun.
Astronomers discovered the 1st planet orbiting a sunlike star in 1995. Here is a comparison showing an artists’ conception of 51 Pegasi b to Jupiter and 51 Pegasi to the sun. Image via NASA Exoplanet Exploration.

51 Pegasi b: 1st planet found orbiting a sunlike star

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 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.

Other names for the planet 51 Pegasi b

Astronomers call 51 Pegasi b by other names. The astronomer Geoffrey Marcy dubbed it Bellerophon. Marcy helped confirm its existence and was following the convention of naming planets after Greek and Roman mythological figures. In fact, 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. Which is Latin for half, referring to its mass of at least half the mass of Jupiter.

Also, astronomers consider 51 Pegasi b the prototype for the class of planets astronomers call hot Jupiters.

51 Pegasi b was the first of thousands of exoplanets

While 51 Pegasi b was the first, we now know there are thousands of exoplanets. As of September 2022, astronomers have confirmed more than 5,100 exoplanets.

But 51 Pegasi b will always be the first known exoplanet to orbit a star like our sun.

What we know about 51 Pegasi b

What do we know today of 51 Pegasi b, a world whose place in astronomical history is so secure? Its mass is about half that of Jupiter. It’s thought to have a larger 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. As you know, the Earth orbits the sun in 365 days. It takes Jupiter 12 years to complete one orbit. 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. So, the planet is always presenting the same face to Pegasi 51. Plus, it’s what’s known today as a hot Jupiter.

By the way, 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. In fact, the Webb telescope has already imaged an exoplanet.

Searching for exoplanets is challenging

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, the light of their parent stars hides any exoplanets orbiting them. So, astronomers had to develop clever technologies to discover them. As with many extrasolar planets, astronomers used the radial velocity method to locate Pegasi 51 b. Click here to learn more about how astronomers find exoplanets.

Infographic showing facts about 51 Pegasi b including its size relative to Jupiter and its sun's size.
View larger. | The momentous discovery of the 1st exoplanet around a sunlike star – 51 Pegasi b – caused astronomers to question what they knew of our universe. It launched further searches for new worlds. Infographic via NASA/ JPL-Caltech.

Bottom line: On October 6, 1995, astronomers Michel Mayor and Didier Queloz announced the discovery of the first planet orbiting around a distant sunlike star. This planet is 51 Pegasi b.

October 6, 2022

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