An illustration of a rocky, Earth-sized planet outside our solar system NASA / JPL-Caltech

Updated on April 16 at 4:19 p.m. ET

For millennia, the only planets we knew of were the ones in our own solar system. That changed in October 1995, when a pair of Swiss astrophysicists discovered a planet orbiting a sun-like star about 50 light-years from Earth, in the constellation Pegasus. For decades, scientists had suspected that other planets existed in the cosmos, and they finally had the proof.

The discovery of 51 Pegasi b, as it was called, was just the beginning. The astronomy community was witnessing “A Parade of New Planets,” declared a headline in Scientific American in 1996. In the months since the exoplanet discovery had been announced, the publication reported, astronomers had reported finding at least four more planets.

More than two decades later, the parade is still going. Today, there are 3,717 known exoplanets, and nearly 4,500 other suspected exoplanets waiting to be verified. More than 900 of them are thought to have a rocky surface like Earth’s.

On Wednesday, NASA will launch a new spacecraft designed to discover still more exoplanets. (The launch was scheduled for Monday but was postponed to conduct additional pre-flight analysis, SpaceX said.) The Transiting Exoplanet Survey Satellite, or TESS, will spend two years surveilling more than 200,000 stars, watching for evidence of planets around them.

TESS will employ a method different than the one that was used to discover 51 Pegasi b, but the spacecraft owes a great deal to the first known exoplanet. Without 51 Pegasi b, astrophysicists may not have seriously considered the technology that should allow TESS to find thousands of new planets in the Milky Way.

In 1995, Michel Mayor and Didier Queloz, both at the University of Geneva, were trying to find exoplanets with a technique called the radial-velocity method. Sometimes, when a planet orbits a star, the planet’s gravity causes the star to wobble ever so slightly. The wobbling motion produces shifts in the star’s light, which can be detected with special instruments from Earth. By studying these shifts, astrophysicists can figure out the mass of a planet and how long it takes to complete one orbit around its star.

Back then, some were skeptical that the radial-velocity method would work, and a number of searches using the technique had proved unsuccessful since the late 1980s. The discovery of 51 Peg, as Mayor and Queloz like to call the exoplanet, was the breakthrough everyone was waiting for.

But no one was prepared for the planet they found.

“When we discovered 51 Peg, it was quite an unusual object,” says Mayor, now a professor emeritus at the university. “It was absolutely not expected from theory.”

51 Pegasi b was about half the mass of Jupiter and orbited extremely close to its star. One trip all the way around took just four days. Astrophysicists didn’t think a planet that size could orbit so closely to a star. The innermost planet in our solar system, Mercury, is thousands of times less massive than Jupiter and takes 88 days to orbit the sun. When Mayor and Queloz announced their find at a scientific conference in Italy, some members of the field were skeptical, but a different team soon confirmed the discovery.

“The shock was so profound that 51 Peg completely changed our perspective of how we could look for planets,” says Queloz, now a physics professor at both Geneva and the University of Cambridge.

The discovery of 51 Pegasi b meant that astrophysicists could look much closer to a star to search for exoplanets. This led them to more seriously consider another technique for detecting planets, known as the transit method. Like the radial-velocity method, the transit method relies on a star’s light. Astronomers train their telescopes on a star and watch for any dimming in its brightness, which would occur when a planet passes in front of it and blocks out the light. The closer the planet is to its parent star, the easier it is for telescopes to spot it.

The first detection of an exoplanet using this method was announced in 2003 by astronomers from the Harvard-Smithsonian Center for Astrophysics. OGLE-TR-56b, located around 5,000 light-years away, is about the size of Jupiter and orbits 14 times closer to its star than Mercury does to the sun.

In 2009, the Kepler mission took the transit method to space. Kepler, a NASA spacecraft, launched into an orbit around the sun equipped with instruments to detect dips in the brightness of thousands of stars in its field of view. The mission has discovered thousands of confirmed and potential exoplanets since. NASA now announces the verification of new exoplanets so often—about every few months or so—that these discoveries are no longer headline-making news.

Today, the planets in our solar system seem like the weird ones. Kepler has found rocky planets 10 times the mass of Earth, gas giants the size of Jupiter with scorching temperatures, and even rogue planets floating around the galaxy without a star to call home. At least 30 exoplanets are about the size of Earth and orbit in the habitable zone of their star systems, that cosmic sweet spot where water exists as a liquid on the surface.

“I still think that today, without 51 Peg, Kepler would never have flown,” Queloz said. And if Kepler hadn’t flown, TESS probably wouldn’t have, either. Like Kepler, TESS will search for exoplanets using the transit method.

TESS’s timing couldn’t be better. Kepler is expected to run out of fuel and will cease operations sometime in the coming months. Engineers didn’t give Kepler a gas gauge, so they just have to watch for warning signs of low fuel and wait.

TESS will launch from Cape Canaveral in Florida on a SpaceX Falcon 9 rocket. Once in space, the spacecraft will fire its engines several times to position itself for an encounter with the moon’s gravity, which will push it into its final orbit.

From its vantage point in high-Earth orbit, above where satellites normally operate, TESS will have an unobstructed view of the sky. The telescope will spend two years staring into the cosmos, turning back only to beam home data, covering more area than Kepler did. Kepler could stare only at specific patches of sky at a time, but TESS will be able to see about 90 percent of it.

TESS will focus on exoplanets around the brightest, closest stars in the galaxy, the kind of target perfect for follow-up observations by other space observatories and ground-based telescopes. Astronomers predict TESS will discover more than 1,600 new exoplanets, including about 70 Earth-sized exoplanets.

Where Kepler and TESS leave off, powerful telescopes currently under construction pick up. In the next decade, observatories like the Extremely Large Telescope in Chile will take the study of exoplanets a step further. They will examine the atmospheres of other planets, looking for the molecules that we know can, under the right conditions, create a world suitable for life.

They may even tell us something about 51 Pegasi b. Last year, astrophysicists using an instrument on the Very Large Telescope in Chile said they detected traces of water in the atmosphere of the exoplanet. The object that kicked off the parade of planets more than two decades ago may hold more surprises for us still.

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