One of astronomy’s most exciting discoveries began, as did many things in the 1990s, with a fax.
Didier Queloz, then an astronomer at the University of Geneva, spent the summer of ’94 sorting through data from a new piece of telescope technology that measured the subtle movements of stars. Such movements, scientists had theorized, could potentially suggest the presence of planets outside our solar system, orbiting their own suns. The gravity of a faraway planet could tug at its star, making the star wobble ever so slightly. No one had ever discovered a so-called exoplanet in this way before, so when Queloz finally did find a wobbling star, he thought it might be an instrument error. But the mysterious quiver didn’t go away. So Queloz sent a fax to his adviser, Michel Mayor, who was in Hawaii on sabbatical: “I think I found a planet.”
The wobble had revealed a world half the size of Jupiter orbiting around a sunlike star some 50 light-years from Earth. Queloz and his team named it 51 Pegasi b, after the star it orbited. The planet’s existence was astonishing on its own, but it also suggested something extraordinary: There must be more.
And there are more—many, many more. In the years since, astronomers have confirmed the existence of more than 4,400 exoplanets in our Milky Way galaxy. They’ve found frigid planets and scorching planets, a planet with oceans of lava and a planet where it rains glass; planets with a density reminiscent of cotton candy and a planet that is literally evaporating into space. But these days, the astronomy community doesn’t get as jazzed about a single discovery as it once did. The field of “exoplanets, for the first two decades, was really in the stamp-collecting phase,” Jessie Christiansen, a NASA astrophysicist who studies exoplanets, told me. “We were like, Ooh, shiny new thing.”
Now there are enough planets to really dig into what they’re like. Researchers are stretching the limits of current technology and imagining what’s possible with more powerful tools in order to fill in the details, and to apply the tantalizing exo prefix to other realms: exotopography, exogeology, exoecology, exomoons. In the tiny wiggles of distant starlight, astronomers have moved well beyond simply detecting new planets to examining these distant worlds with more precision than ever before.
While 51 Pegasi b was found through the movement of a star, most exoplanets found since were detected in the glow of one. When a distant planet orbits in front of its star, it blocks a smidge of starlight, making the star appear temporarily fainter to us on Earth. When astronomers watch the star long enough, making note of the dimmed starlight, they can confirm not only the existence of a planet but also how long that world takes to orbit its sun, the composition of its atmosphere, and the temperature of its surface.
These are the exoplanet basics that are usually quite straightforward to discern. But the light of other suns contains a lot more information that we’re only starting to understand. One astronomer, for example, has examined how much starlight exoplanets reflect to investigate what their surfaces might be made of; ice, for example, is more reflective than water, and water is more reflective than dirt. One of the most intriguing approaches that I’ve recently come across puts a twist on the traditional method of finding exoplanets within the dips of starlight. “We know that rocky planets are going to have bumpy features, and if the planet is rotating in front of the star, those features come in and out of view and block more of the star’s light,” explains Moiya McTier, an astrophysicist who has studied exoplanets. Those tiny shifts could suggest the presence of mountains, volcanoes, and other towering terrain.
Astronomers are also getting better at understanding exoplanet skies, squeezing as much out of telescopes as they can to observe the characteristics of faraway atmospheres. A planet with an atmosphere absorbs some of its star’s glow, leaving imprints on the light that eventually reaches Earth. Scientists have examined those marks to find evidence of all kinds of molecular signatures in exoplanet atmospheres: oxygen, hydrogen, sodium, iron, and even water vapor. Some of these substances are quite common in the universe, so astronomers are now broadening their search to include some more unusual biosignatures—the kind that living organisms, not chemical processes, could produce—and working out how they might spot their distinct imprints in starlight.
Only recently have researchers started to poke around for even more advanced indications of life on exoplanets, namely the radio transmissions of a bustling society. Breakthrough Listen, an international effort to find radio signals that could be produced by intelligent civilizations, recently partnered with NASA’s TESS space telescope, which has found 144 confirmed exoplanets hidden in the glare of other stars. Radio emissions from Earth have been wafting into space for decades, carrying information about our existence. Perhaps the same has been true for other planets.
Ultimately, the animating force behind exoplanet research is less about finding new types of worlds than it is about finding one specific kind: Can we find another Earth? Most of the planets astronomers have found so far, including Pegasi 51b, the subject of the fax that changed astronomy, are unlivable. We are still looking for what Christiansen, the NASA astrophysicist, calls the “holy grail of exoplanets”: a rocky world about the size of Earth, orbiting at a comfortable distance from its star, where water would not always freeze or evaporate but lap across its surface. This is the kind of place—the only place, really—where we could confidently say life could arise.
Of the thousands of known exoplanets, only 165 are rocky worlds about the size of Earth, which are more difficult to detect than giant planets made of gas. Still, statistics are on astronomers’ side. Scientists estimate that every star in the Milky Way galaxy has at least one planet, and they believe that planets nestled in their star’s habitable zone are common. As the writer Jo Marchant writes in her book The Human Cosmos, “Even if life is vanishingly unlikely to arise on any particular planet, we know that in our galaxy alone, there are billions of chances for it to occur.”
But as is often the case in exoplanet research, technology is still catching up to theory. To try out McTier’s exotopography approach, for example, the astronomy community needs more powerful instruments than are currently in operation. And astronomers can only daydream about spotting something as wondrous as the twinkle of someone else’s city lights or as dramatic as blast shields constructed to protect against deadly supernovas. In spite of all the new research, we are still many years away from photographing the small, rocky planets at a resolution greater than a single pixel.
The study of exoplanets reminds me of the Apollo landings on the moon, the only other world that human beings have actually visited. When Armstrong, Aldrin, and Collins came home from their trip, they filled out a customs form, reporting “moon” as their departure location and declaring moon rock and dust samples as goods. The slip of paper, a small token of the historic mission, transformed the moon into a real place people could touch and walk on. Humankind may visit the moon again, and perhaps someday may make a real place out of Mars. And we can certainly try to make exoplanets into places in some ways—with the help of gorgeous illustrations, says Lisa Messeri, an anthropologist at Yale who has written about how scientists can help the public view scientific targets like exoplanets as real worlds. Perhaps that can give some of us—the nonscientists who don’t spend hours picking apart starlight—some fuzzy feeling. “Being able to identify likeness, even if it’s impossible for us to reach, somehow shrinks the size of the universe to make us feel more connected,” Messeri told me.
But exoplanets will, for the foreseeable future, remain impossibly distant, abstract. The Earth-sized planet around our nearest star would take many decades to reach, on technology capable of traveling at a fraction of the speed of light. The study of planets beyond our solar system always comes back to one melancholy but immutable truth: So much of astronomy is the pursuit of understanding everything out there from right here, and there will always be limits to our reach.