For the past three decades, this has been science fact, not fiction. In September 2025, astronomers working at NASA’s Exoplanet Archive at Caltech announced that the number of known planets around stars other than our Sun — extra-solar planets, or exoplanets — had passed the 6,000 mark.

It’s been a truly staggering rate of discovery since the first planet orbiting a Sun-like star, 51 Pegasi b, was announced thirty years ago on October 6th, 1995.

The man who discovered that planet was Didier Queloz, who is now the Jacksonian Professor of Natural Philosophy at the University of Cambridge, but at the time of his historic discovery he was in the final year of his PhD and working with his supervisor, Michel Mayor, at the University of Geneva. 

Queloz is affable, humorous and incredibly passionate about exoplanet research and passing his knowledge on to others. Those 6,000 exoplanets (actually 6,022 as of the beginning of October 2025) are his legacy, one that he is only too well aware of, and he lauds the work and dedication that has gone into discovering each and every one.

“If you put it into perspective, 6,000 planets is nothing,” he tells Supercluster, yet this is not arrogance speaking, but rather humility in front of the vastness of the cosmos. 

“There are 100 billion stars in the galaxy,” he continues, “And the statistics are just amazing, because when you run the math you realize that the vast majority of stars must have planets.”

A hundred billion planets, likely many more, inhabiting our Milky Way galaxy is a mind-boggling number, especially when you consider that prior to the 1990s there were no exoplanets known.

The shock announcement of the discovery of 51 Pegasi b, which is a ‘hot Jupiter’ — a gas giant so close to its star that it orbits in a matter of days and reaches temperatures in excess of a thousand degrees Celsius — changed all that. Back in 1995, the concept of hot jupiters wasn’t even on the minds of astronomers, who thought such worlds to be impossible. And, Queloz included, they certainly were not looking for this kind of planet.

“Actually, we were expecting to find [a planet like] Jupiter,” says Queloz, highlighting how astronomers had been expecting spitting images of our own Solar System, with the smaller, rocky planets closer to the Sun and the large gas giants like Jupiter farther away.

To hunt for exoplanets, Queloz and Mayor were using a spectrograph called ELODIE on the 1.9-meter telescope at the Observatoire de Haute–Provence in France. It worked by detecting a property of stars called ‘radial velocity’. Picture a system with a planet orbiting a star. The center of mass of the system is somewhere between the planet and star. In most cases it is still very close to the star, often inside it, but crucially not at the center of the star’s rotational axis. Therefore, a star will appear to wobble around this offset center of mass, and this can be detected in the form of a Doppler shift in the wobbling star’s light. This Doppler shift represents a change in apparent radial velocity towards and away from us. ELODIE was set up to measure changes in radial velocity as low as 10 meters per second. Modern spectrographs can detect radial velocities of less than one meter per second.

Queloz and Mayor weren’t the only astronomers partaking in the hunt. A team of Canadian astronomers led by Bruce Campbell (not the Bruce Campbell of Evil Dead fame!) and Gordon Walker were using a similar method to the Swiss pair, while in the United States a group headed up by Paul Butler and Geoff Marcy had adopted a similar but more computationally complex technique. 

So the race was one, but when Queloz won it, he didn’t initially believe it.

It was expected that for giant planets on orbits similar to Jupiter, which takes 12 years to orbit the Sun, years’ worth of data would need to be accumulated to spot the radial velocity signal.

“So you can imagine my surprise when I first observed the star 51 Pegasi and I started to see the change in radial velocity straight away,” Queloz says. “I thought there was something really bad happening, bad in the sense that I had made a mistake. I battled with the data, wanting to fix the problem, until I realized that it was real and must be a planet.”

Queloz recalls how Mayor, who was on sabbatical in Hawaii at the time, was skeptical, but on his return to Geneva he became quickly convinced. Eager not to get scooped on their once-in-a-lifetime discovery, they kept their finding secret and hurried to publication in Nature, on 6 October 1995.

You’d be forgiven for maybe thinking that there would be celebrations among astronomers the world over, yet Queloz and Mayor found themselves presented with a tough crowd.

“It was really hard to convince the community,” recalls Queloz. “After all, people had tried to find planets before and there had been fake announcements, false alarms and publications describing no detections at all.”

Certainly, nobody had expected a gas giant just 7.9 million kilometers from its star and this only bolstered the skeptics’ incredulity. For comparison, Jupiter’s mean distance from our Sun is 778 million kilometers; even our innermost planet, diminutive Mercury, only gets as close as 46 million kilometers. 

Searching for an explanation, the community dug up a little known model from two decades earlier that focused on the formation of the moons of Jupiter and Saturn and how they may have formed farther out and then migrated in towards their parent planets. In similar fashion, perhaps this new exoplanet, 51 Pegasi b, had also somehow migrated in towards its star. “Of course,” says Queloz, “Nobody had any idea how to stop the migration of a planet.”

Over the years the migration model has become the accepted model, and to a degree has even been applied to the early movements of Jupiter and Saturn under the guise of the ‘Grand Tack’ hypothesis, even if it is not yet fully understood. Meanwhile the discovery of 51 Pegasi b was verified when more hot jupiters were discovered in subsequent years. As those years passed by, smaller exoplanets began to turn up, and on increasingly wider orbits. In 1999 the first transiting exoplanet, another hot jupiter by the designation HD 209458b, was seen, and this heralded another revolution, one in which thousands of exoplanets would swiftly be added to the roster thanks to NASA’s Kepler mission.

And so, thirty years later, we find ourselves with 6,000 exoplanets and counting on our hands. What we’ll do with them we’ll come onto, but first let’s take a brief side-step and give a mention to the forgotten exoplanets of history.

Some of you reading this might be thinking, hang on — how can it be only 30 years since the discovery of exoplanets? Weren’t the first exoplanets discovered in 1992?

Three years before Didier Queloz and Michel Mayor found 51 Pegasi b, radio astronomers Dale Frail and Aleksander Wolszczan found a puzzling blip in the timing of radio pulses from the pulsar PSR B1257+12. Pulsars are spinning neutron stars, which are the core remnants of massive stars that have exploded as a supernova. The detonation of the star is expected to take out any orbiting planets, yet the blip in the timing of PSR B1257+12 regular pulses was attributed to three orbiting planets that must have formed in the aftermath of the supernova from the debris of the exploded star. Frail and Wolszczan identified two of the planets around PSR B1257+12 in 1992, and a third in 1994. One of the planets is the lowest mass planet ever found, with a mass of just two per cent that of Earth.

In all, less than 20 pulsar planets are known, and they are frequently forgotten about in the exoplanet discussion. One reason is that, because they orbit pulsars and are bathed in deadly radiation rather than warm light, they will be dead worlds incapable of supporting life. The second reason is that scientists don’t think the pulsar planets formed like typical planets — one prominent exoplanet astronomer whom I spoke to described the pulsar planets as “freaks”. 

And so, while they are part of the Exoplanet Archive, there is a general sense among the astronomical community that, rightly or wrongly, pulsar planets are not ‘real’ planets worthy of much study. Its the ones around Sun-like stars, like 51 Pegasi b, that astronomers are really interested in. The pulsar planets have ended up becoming an inconvenient truth.

Radial velocity measurements are essential for providing the mass of a planet — the more massive the planet, the stronger the tug on the star — but they are hard work, relying on many careful, precise, measurements. Transit surveys, on the other hand, provide an opportunity to simultaneously observe and detect exoplanets around thousands of stars at once, just looking for the characteristic dip in light as a planet moves across its star. It’s primarily thanks to transits that we jumped from a few hundred known exoplanets to thousands.

Ten years after the first transit observation, NASA launched the Kepler Space Telescope to stare at 150,000 stars in the direction of the Milky Way in the constellation of Cygnus, the Swan.

For Caltech’s Aurora Kesseli, who works on the Exoplanet Archive, the Kepler mission was her moment of inspiration. 

“I was four years old in 1995, so for me the big thing that caught my attention when I was in college in the early 2010s was Kepler’s discoveries,” she recalls. “I thought that was so cool.”

During the Kepler mission, NASA scientists would routinely announce huge batches of exoplanets, hundreds at a time. Kepler’s mission expanded over the years, morphing into the K2 mission that began surveying stars beyond that patch in Cygnus, and in total it has discovered over 3,300 confirmed exoplanets and left a legacy of nearly 3,000 more unconfirmed candidates.

“We have graphs in the Exoplanet Archive that show the pace of discovery,” says Kesseli. “You can see that before Kepler it is a few tens of planets per year, and then all of a sudden with Kepler it jumps to an order of magnitude more.”

“Strictly speaking, on its primary goal Kepler completely failed because it didn’t find any planets like Earth,” says Queloz. 

In light of all its discoveries, labelling Kepler as a failure does seem rather mean-spirited, and Queloz agrees. 

“We don’t care that Kepler didn’t find any like Earth, because instead Kepler discovered another type of world that we had never dreamed about,” says Queloz.

This new kind of world is best described as a sub-neptune, at the upper end of the super-earth regime (rocky worlds more massive than Earth) and the lower end of the Neptune-class of planet. And unlike Neptune, which is on the cold outer fringes of our Solar System, these sub-neptunes are often hot neptunes close to their star, and their composition and internal structure isn’t clear; many have densities indicating them to be vast ocean worlds.

“We started finding them with HARPS and we could sense they were an interesting planet, but Kepler discovered thousands of them [confirmed and unconfirmed] and I remember the excitement of my colleagues when they would find not one, but two or three of these planets around the same star,” says Queloz.

Such worlds are, currently, the most common type of exoplanet known and they include some of the weirdest planets found to date with exciting potential, such as K2-18b, which is a planet with nearly nine times the mass of Earth orbiting a red dwarf star 124 light years away. Like many of its kind, its exact nature is a little bit mysterious. Cambridge University astronomy Nikku Madhusudhan has argued that it is what he calls a ‘hycean world’, with a dense atmosphere of hydrogen wrapped around a deep global ocean of water. Furthermore, Madhusudhan’s team claims to have detected dimethyl sulphide in K2-18b’s atmosphere with the James Webb Space Telescope. Dimethyl sulphide is a biosignature gas on Earth, produced by life, but the chemical signature in K2-18b’s atmosphere has proven highly controversial, with many exoplanet researchers doubting that the signal is real.

Whether it’s real or not is perhaps missing the point, says Queloz. “We don’t really know exactly what these planets like K2-18b are, and while the claim [of dimethyl sulphide] may be an optimistic claim, it at least demonstrates that we can detect such a signal, and the more we observe the more we will clarify this signal, or maybe find another one.”

K2-18b demonstrates how, in thirty years, we’ve gone from a blip on a radial velocity chart to being able to ask whether there are signs of life on any of the planets that we have discovered. Yet Kepler’s number one science goal still hasn’t been achieved — we don’t know of any other habitable exoplanets. We can even take things a step further by pointing out that we haven’t even found any systems like our own Solar System yet, while the most common type of known exoplanet, those between the size of Earth and Neptune, are completely absent from our Solar System, nor do we have a hot Jupiter. This is leading planetary scientists to face an uncomfortable and so far unanswered question — why is the Solar System so different?

Kesseli describes how the discovery of exoplanets has become a turning point that has flipped the situation on its head.

“Before 1995, our theories for how planets form and what a typical planetary system looks like was just based on our Solar System,” she says. “Then, when we started finding exoplanets we realized that there’s so many different ways that planetary systems can form, so if we want to learn about what a typical planetary system is like, we have to look at a large number of exoplanetary systems.”

This is where the value of having so many exoplanets in the archive comes in, because we’re no longer limited to a sample of just one planetary system. 

The main purpose of studying exoplanets, besides looking for a second Earth, “is something very profound, which is to try and make sense of our Solar System,” says Queloz.

One of the reasons why Earth and our Solar System currently seem so unique is observation bias. Finding systems and planets like ours is hard. Smaller planets are harder to detect, and planets farther away from their star are harder to detect, so the basic architecture of having small terrestrial planets closer to a star and gas giants farther out, like in our Solar System, is for the time being just really hard to find. This could change in the future, as the imminent launch of NASA’s Nancy Grace Roman Space Telescope in 2027, assuming it isn’t canceled as a result of the current US administration’s scorched earth approach to slashing government funding, could see up to 2,000 new planets discovered via a microlensing survey. Microlensing is an innovative use of gravitational lensing, with the telescope watching for unseen planets briefly magnifying the light of background stars. Microlensing is far better at detecting planetary systems like our own compared to radial velocity and transit measurements.

“The cool thing about this is that it actually will be sensitive to Earth-like planets around Sun-like stars, and this will give us our first look at whether such planets are actually common or not,” says Kesseli. Of course, Earth-like here really means that they are Earth-sized and in the habitable zone. Because microlensing planets go unseen, and are only visible for a very short period before their gravitational lens moves out of alignment with the background star, it is impossible to follow-up on them and characterize them.

We can, though, characterize many of the 6,000 worlds found so far, exactly because their vital statistics fall within the abilities of our telescopes and spectroscopic instruments to measure. 

“Nature has provided these planets that are easy to detect and observe with our really primitive approach of transits and radial velocity measurements. We don’t need complicated quantum mechanics to make these techniques work, they are very simple yet nobody thought they would find so many planets, because nobody thought that there would be so many planets orbiting with short periods.”

The Universe just keeps on giving and Queloz believes that it has brought us to the cusp of something extraordinary.

“The process has started,” he says. “We are exploring the planets in the galaxy right now and we try to put into perspective the diversity of the planets in our Solar System relative to a broader context, which is this huge diversity of planets in our galaxy. It is progress on this, giving us a better understanding of Earth and the Solar System, which is what I think is going to happen over the next 30 years.”

If Queloz is correct, then the legacy of his discovery 30 years ago isn’t just in the 6,000 or more planets catalogued in the Exoplanet Archive. Ultimately, it will also be in a greater appreciation of our home, the Earth and our Solar System. Perhaps this realization can help guide us to a better and more informed future where we become more responsible for our planet and its environment. If so, then discovering exoplanets when we did will prove rather auspicious, for it will have been just in the nick of time.