The First Exoplanet Was Discovered 25 Years Ago

This artist’s view shows the hot Jupiter exoplanet 51 Pegasi b, sometimes referred to as Bellerophon, which orbits a star about 50 light-years from Earth in the northern constellation of Pegasus (The Winged Horse). This was the first exoplanet around a normal star to be found in 1995. Twenty years later this object was also the first exoplanet to be be directly detected spectroscopically in visible light.

There was a time when humanity believed Earth was the entire universe. As our place in the cosmos became clear, it was natural to wonder if there were planets around other stars. Today, we know the answer to that question is a resounding “yes,” and it all started exactly 25 years ago. That’s when astronomers announced the discovery of the first exoplanet orbiting a sun-like star. It was known as 51 Pegasi b, and it forever changed how we study the universe. 

Astronomers announced the discovery of 51 Pegasi b on October 6, 1995. I vividly recall watching a news report on this when I was a nerdy 11-year-old. Within a week of the announcement, another team had confirmed the observation, cementing 51 Pegasi b’s place in history. It was a big deal at the time, and the importance of the discovery has only become clearer now that we’re in the golden age of exoplanet detection. 51 Pegasi b even earned its discoverers a Nobel Prize in Physics in 2019. 

51 Pegasi b orbits the star 51 Pegasi about 50 light-years away from Earth. Michel Mayor and Didier Queloz of the University of Geneva spotted 51 Pegasi b using what we now know as the radial velocity method. A very sensitive spectroscope pointed at the star showed small velocity changes on the order of 70 meters per second. The pair confirmed that wobble was due to the gravity of an exoplanet—51 Pegasi b. We still discover some exoplanets using this same basic method, although most detections are made with the transit method. That’s what the Kepler telescope and the newer TESS satellite use. 

The researchers determined that 51 Pegasi b was a gas giant about half the size of Jupiter, and that later served as the basis for the planet’s new name. In 2014, the International Astronomical Union (IAU) launched a project to give important stars and exoplanets proper names instead of the awkward lettering scheme we use for most of them. It settled on Helvetios for the star and Dimidium for the planet, the latter of which comes from Latin for “half.”

The radial velocity method to detect an exoplanet.

While 51 Pegasi b was not the very first planetoid discovered outside the solar system, it was the first that we would think of as a “real” planet. The first exoplanet detections were made in 1992 when astronomers saw a pair of (probably wrecked) worlds orbiting a pulsar known as PSR B1257+12. However, 51 Pegasi is a “main sequence” star similar to the sun, making its planet a much more interesting object. The planet also orbits very close to the star with a temperature in excess of 1,000 degrees Fahrenheit. That was not compatible with our theories on solar system formation in 1995, but we now know “hot Jupiters” are quite common. 

Dimidium set us on a path to better understand the universe, and think about all the progress made in the ensuing 25 years. In 1995, we knew about one exoplanet. Now, there are more than 4,000 in the books. There’s even one right next door in Proxima Centauri. Where will we be in another 25 years? Extragalactic planets? Planets that support alien life? Your guess is as good as ours. 

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