Exoplanet Anniversary: 30 Years Since the Discovery of 51 Pegasi b

Article October 11, 2025

This month marks the 30^th anniversary of a pretty major event in astronomical history: the first discovery of a planet orbiting a main-sequence star. Considering that astronomy as a science is tens if not hundreds of thousands of years old, the fact that we’ve only known about other solar systems for 30 years is kind of astonishing.

In honor of this anniversary I thought I’d take a stroll back to the heady days of 1995 when this discovery rocked the astronomical world. We’ll explore what was discovered, how it was discovered, why it was discovered, and the reason why this was such a big deal even though this was not actually the first exoplanet to be found! Come wander into the Wayback Machine with me!

Not the First

An artist’s impression of the planetary system around the pulsar PSR B1257+12. Credit: NASA/JPL-Caltech

Okay, first the big technicality: the thing that was discovered in 1995, the gas giant world 51 Pegasi b, was not the first exoplanet to be discovered. Heck, it wasn’t even the second or third. Those discoveries were actually announced years earlier, the first two in 1992 and the third in 1994. All three were found by astronomer Alex Wolszczan with the assistance of Dale Fail for the 1992 discoveries and Maciej Konacki for the 1994 one.

The thing is, these planets are not orbiting a star—technically. What they are orbiting is a pulsar, the remnant of a star, or in this case probably the remnants of remnants of stars. The pulsar, PSR B1257+12, is suspected to have formed when two white dwarfs (themselves the remains of mid-to-lower-mass stars) merged.

The planets in this case are suspected to have formed after that merging, from material left behind. In other words, these planets never orbited a healthy, living star—this system was born zombified. These discoveries were certainly exciting. Mind-blowing even! But this was a dead system. Not quite the stuff that exoplanet dreams were being made of at the time.

A Bright Star

Didier Queloz and Michel Mayor, the discoverers of the exoplanet 51 Pegasi b. Credit: ESO

It’s a little tough to tell from the research I’ve done whether Swiss astronomer Michel Mayor and his graduate student Didier Queloz were hoping for signs of exoplanets when they helped develop the ELODIE spectrograph, which was installed on a telescope at the Observatoire de Haute-Provence in 1993.

Some retrospectives on their discovery of 51 Peg b seem to imply that was the goal, while other sources just say they were interested in seeking out suspected binary star systems that actually consisted of a single star and a substellar object which, yes, theoretically could be a planet, but could also be something like a brown dwarf.

Diagram displaying out the lines in a star’s spectrum can change position, or wobble, due to the tug of an orbiting planet. While detecting planets this way is officially known as the radial velocity method, many astronomers simply call it the wobble method. Credit: NASA/JPL-Caltech

Remember, at the time we suspected planets existed around other stars, but had zero evidence for it, dead pulsar planets notwithstanding. Think of it like we think of life on other worlds: we highly suspect life exists elsewhere, but have absolutely no surefire evidence for it, and someone wanting to make the claim that they had found the first life elsewhere would need to have some very solid evidence to back that claim up.

This was the position Mayor and Queloz found themselves in in the summer of 1995 as they examined ELODIE’s spectral data of the Sun-like star 51 Pegasi. The lines in the spectra were wobbling, and they were wobbling with a very distinct pattern.

Because gravity is a two-way street, as a star’s gravity pulls on its planet and moves the planet in orbit around itself, the planet also pulls on the star. It’s a much smaller pull, so the star doesn’t orbit so much as wobble in place as the planet moves around it. The bigger and/or closer the planet, the more the planet can tug on the star, and the bigger the wobble. That wobble is what Mayor and Queloz were seeing—51 Pegasi had a planet, the first ever seen around a healthy star.

Infographic detailing the 51 Pegasi system and how it compares to objects in our own solar system. Credit: NASA/JPL-Caltech

Some Like It Hot

It’s fun listening to Queloz recall that summer and the uncertainty he and Mayor felt about the discovery. He talks about how seeing the pattern once was intriguing but of course they’d need to see it repeat to be certain. And then when it repeated they decided they really needed to see it a third time to be sure. Then when it showed up again they decided they really ought to wait to see it a fourth time before coming to any conclusions.

This is because they were sitting on a discovery that was startling in more than one way. Yes, the first planet around a living star, but that was something we were pretty sure existed, and they had good evidence to back up their claim. The unbelievable part was the conclusions they had to draw about the planet’s size and distance from its star.

One of the reasons they were able to get multiple repetitions of the wobble pattern in 51 Pegasi’s spectra is because it repeated every four days, indicating that’s how long it took the planet to complete one orbit. That meant this planet was very close to the star, less than 5 million miles (8 million km) away. Just for comparison, Mercury’s average distance from the Sun is about 36 million miles (58 million km). Obviously, we had never seen a planet so close to a star before.

And for the star to be wobbling as much as the spectra indicated, the planet had to be hefty. It turns out to be about half of Jupiter’s mass, which sounds smallish until you think of it as also being 1.5 Saturn masses, because Jupiter is, shall we say, robust. That would mean what we had was an enormous gas giant right up against its star. And that, in 1995, was crazy talk.

Planet Zoo

While we assumed that other solar systems probably existed, we also assumed they looked like our solar system. There was, after all, no reason to assume there was anything special about the way ours formed and we had a perfectly good formation theory for why you’d always find small rocky worlds close in and big gas giants farther out (it has to do with the way we think the young Sun blasted most of the gases and ices out of the inner solar system, leaving mostly rock for the inner planets to form from).

When the Jet Propulsion Laboratory created a series of fanciful travel posters to various celestial objects, 51 Pegasi b was one of the objects selected to be honored with a poster. Credit: NASA/JPL-Caltech

So not only did Mayor and Queloz have to get the astronomy world to accept the discovery of a planet around another star (not necessarily difficult though incredibly exciting), they had to upend all our ideas about how solar systems form. The data didn’t lie though. On October 6, 1995, they revealed the existence of 51 Pegasi b and a whole new realm opened in space science.

51 Peg b became the prototype of a sort of exoplanet we now know is quite common in the universe, the “hot Jupiter”, which of course we don’t have in our solar system. We’ve found that the most common size for an exoplanet is something we call a mini-Neptune, which we don’t have. We’ve found an abundance of worlds we call super-Earths, which we don’t have. We’ve found gas planets and rocky planets intermixed, which we don’t have. We’ve found systems where there’s nothing but rocky worlds, which we don’t have. We’ve found lava planets and steam planets and planets where it rains iron.

Shakespeare was right, if nobody minds me paraphrasing a little. There are more worlds than were dreamt of in our philosophy.

We’ve Come a Long Way

In September 2025 NASA’s official tally of confirmed exoplanets crossed the 6,000 mark. The Exoplanet Encyclopedia lists over 8,000 additional exoplanet candidates, which have not yet been confirmed. Some of those will turn out not to be planets, but many will.

As our telescopes and detection methods have improved, we can now sometimes know a lot more about these exo-worlds than just their mass and distance from their star. We’re starting to be able to detect weather patterns, rotation speeds, and the possibility of ring systems. We’re hoping to find an exomoon someday. And, of course, we’re hoping to find a world that can support life as we know it. That world, when we find it, might wind up looking nothing like Earth…but it would also be nice if it reminded us of home.

An artist’s impression of a small fraction of the 6,000 confirmed exoplanets we have discovered in the past 30 years. Credit: NASA Goddard Space Flight Center

As for Mayor and Queloz and the 51 Pegasi system, nobody has forgotten them. Mayor and Queloz won the Nobel Prize in physics for their paradigm-shifting discovery. And in December 2015 the International Astronomical Union gave both the star and the planet a rare distinction in the vast catalogs of objects we’ve studied out in the universe: an official name to go along with its unwieldy designation. While most sources still call them 51 Pegasi and 51 Pegasi b, you can also talk about the bright star Helvetios and its close companion, the world that rocked ours, the planet Dimidium.

It’s only been 30 years. Just imagine what the next 30 years will bring.