A Flash of Recognition in a Lonely Universe

Beyond our solar system, the universe is full of planets—but also asteroids, auroras, and other familiar cosmic wonders.

An animation of a ring of colorful planets, blinking in the style of a circular loading icon, set against the star-speckled darkness of space
Illustration by Paul Spella / The Atlantic. Sources: Getty; NASA.

Several centuries ago, as scientists began to embrace the startling idea that Earth was not the center of the universe, they also began to ponder its startling implication: that the stars in the night sky might be suns in their own right, orbited by their own worlds. Until the 1990s, that idea was no more than a hypothesis. By then, telescopes had become sufficiently advanced to reveal the hard evidence: A star about 50 light-years away was wobbling, a sign that a small world was tugging on it. This world was called an exoplanet.

Astronomers have since discovered more than 5,300 exoplanets and counting, and they’re studying the atmosphere of these worlds to determine if the molecules suspended in their clouds could sustain life. They’ve even recently captured evidence of an exoplanet getting swallowed up by its dying star. (Truly, telescopes have gotten really good.) Although most people probably can’t name an exoplanet—something like “HD 108236 b” doesn’t exactly roll off the tongue—the fact that the cosmos is full of them is now well known.

The exo- prefix extends beyond the realm of planets. Astronomers have found exoauroras, exoasteroid belts, and even exorings, such as the ones that surround Saturn. Exomoons haven’t been proved to exist, but astronomers believe that they are more numerous than exoplanets themselves. The search for such celestial objects and structures has intensified in recent years. Just this week, astronomers announced that they have found an exoradiation belt, an invisible cocoon of charged particles, held in place by a planet’s magnetic field. We are firmly in the era of exothings.

There is plenty of scientific possibility in the discovery of exothings, and also comfort. Not to be dramatic, but Earth is a small island in an endless sea. Ours is a cosmically lonely existence. So it’s nice to look out across that sea, dark and unknowable, and occasionally spot other islands, other familiar shores. The study of exothings is a way of conversing with the rest of the cosmos and saying, “Oh, you’ve got one of those too?” And because any hypothetical life must live somewhere, exothings could someday answer one of our most existential questions: Are we alone?

When Melodie Kao, an astronomer at UC Santa Cruz, went looking for exothings, she set her sights 20 light-years away, on a brown dwarf—a rather unusual object that is neither a star nor a planet, with a mass somewhere between the two. Kao had previously studied this particular brown dwarf and found that its intense radio emissions produced auroras similar to our own northern lights. She couldn’t account for what was causing the aurora, so she turned to our own solar system for inspiration, and realized that planets with auroras also have radiation belts. No one had ever found one outside the solar system before. But when Kao went looking for one around her brown dwarf, voilà, there it was: a radiation belt, invisible to the human eye but billowing in radio wavelengths, in all its exoglory.

Exoasteroid belts are downright stunning, particularly in infrared wavelengths. Astronomers recently used the most powerful observatory in operation, the James Webb Space Telescope, to observe a star about 25 light-years away with a known asteroid belt. They were surprised to find not one single ring of material, but three. The dusty region was likely shaped by the same forces that produced the versions in our own solar system: the movements of planets, hidden somewhere in those concentric rings. “It’s sort of comforting that the same processes likely play out, to some extent, very similarly,” Andras Gaspar, an astronomer at the University of Arizona who led the observations, told me. Perhaps, he added, we’re “not the only ones who are deciphering how our universe works.”

An infrared view of an asteroid belt around a nearby star
An exoasteroid belt (NASA / ESA / CSA / A. Gaspar (University of Arizona) / A. Pagan (STScI))

Although the universe is full of repeats, each iteration brings its own little quirks. The exoradiation belt that Kao discovered, for example, is “almost 10 million times more intense” than Earth’s, Kao said. The exoasteroid belt is far more complex than our main asteroid belt, between Mars and Jupiter, as well as the Kuiper Belt farther out, a region beyond Neptune of icy objects and dust. And the only known set of exorings is 200 times larger than Saturn’s—possibly because it sits around a young sunlike star, not a planet.

Some exothings have proved elusive, even though astronomers would bet all the money in the universe that they exist. Exomoons are one of them: Our solar system is full of moons, so others must be too. But no one’s been able to prove they exist. In 2018, a pair of astronomers said they had found evidence of what could be the first-known exomoon, orbiting a planet around a star 8,000 light-years from here. But when other teams took up the same data and conducted their own analyses, they got mixed results. The search continues.

It continues as well for that most elusive exothing: a truly Earthlike planet, nice and rocky, with a chemically rich atmosphere and temperatures that would allow water to lap on the surface. Mansi Kasliwal, an astronomy professor at Caltech who was involved in the discovery of the unfortunate exoplanet consumed by its star, told me she thinks a lot about the idea of an exohome. Not another place in the universe where humans might relocate (though she hopes future generations take a crack at that before our sun swallows our own planet in a few billion years), but some other speck out there where life managed to emerge and thrive. “It doesn’t even have to be identical to Earth,” she said. “It just needs to be habitable and hospitable, a planet that is home to some species.”

Our collection of exothings large and small is growing every year. “Until recently, I had a lot of exoplanet envy,” says Jackie Villadsen, a physics and astronomy professor at Bucknell University who worked with Kao. “Now we can do exoradiation belts.” Compared with exoplanets—which can have oceans of lava and raindrops made of glass—radiation belts might seem a little boring. But exothings, whatever they are, lend themselves to daydreaming. Consider the exoasteroid belt that Gaspar studies. The star at its center is about 440 million years old, much younger than our approximately 4.6-billion-year-old sun. When our sun was that age, Earth was still lifeless. “It’s very doubtful that any life could have emerged, let alone sophisticated, intelligent life forms,” Gaspar said.

But maybe sometime in the cosmic future, a spark will flash. Someday, perhaps, a group of beings on a planet inside that system will dispatch little robots into the depths, to the other planets and moons nearby. They will sprinkle signs of their existence beyond their cozy atmosphere, all the way out to their asteroid belt and beyond. After all, it happened here.