An Interstellar Visitor Had a Sad Story to Tell

A hint of warmth from our sun helped reveal a mysterious comet’s secrets.

A comet traveling so fast the the background stars appeared as streaks of light
The interstellar comet Borisov, captured zooming through our solar system in late 2019 (O. Hainaut / ESO)

In 2019, Gennady Borisov, an amateur astronomer in Crimea, discovered his seventh comet. This icy object wasn’t like the others Borisov had found, or like any of the other comets in the solar system. This one wasn’t orbiting the sun.

Instead, it had been drifting alone in interstellar space, following its own path, until one day, it entered our solar system and grazed past the sun. Warmed by the heat of a star, for the first time in who knows how long, the icy comet thawed just a little bit.

Some of Earth’s most powerful telescopes captured the cosmic interloper as it went by. Astronomers could see the comet enveloped in a fuzzy glow of once-frozen dust particles loosed by the sun.

By analyzing these particles from afar, researchers have managed to learn about the comet’s composition, its origins, and its long journey here. One recent finding demonstrates something rather melancholy.

Of the comets astronomers have observed, this one—named Borisov, after its discoverer—is one of the most pristine. “Think of the wind erosion of the mountains, or even the suntan on our skin when we go to the beach,” Stefano Bagnulo, an astronomer at Armagh Observatory, in the United Kingdom, who had studied Borisov, told me. Borisov shows very few signs of another sunny encounter in its journey through space. For a comet to be as unblemished as this one means it has been extremely alone.

Astronomers had already assumed that, given the vast distances between stars, objects like Borisov can travel for eons without running into another one. The last time Borisov felt the warmth of a star was probably in the bounds of its own system. Astronomers can retrace Borisov’s journey only so far, and we’ll likely never know where the comet came from. But the shimmery cloud of particles surrounding Borisov, known as a coma, can tell us something. “Dust carries rich information about the planetary system,” Bin Yang, an astronomer at the European Southern Observatory, in Chile, told me.

In another recent analysis of Borisov’s dust particles, Yang and her team found evidence that suggests the comet formed close to its parent star before spinning into the outer parts of its system, gathering up different kinds of cosmic material as it went along. Yang says Borisov might owe its composition to the presence of giant planets, which are known for stirring things up with their gravity. Perhaps Borisov once shared a home with its own versions of Jupiter and Saturn.

Though Borisov’s arrival was a surprise, the comet is less mysterious than ‘Oumuamua, the first interstellar visitor ever detected, seen in 2017. At the time, astronomers had been expecting something that resembled Borisov; current theories suggest that icy comets near the edges of a planetary system can be jostled by big planets and flung out into an untethered existence in the space between stars. Our solar system, in its early, more chaotic days, probably kicked out a few comets of its own. But ‘Oumuamua looked more like an asteroid, and astronomers are still debating its exact nature, including its shape. Given two very different interstellar visitors, the astronomy community is eager to see what’s next, and it won’t have to wait long. A new observatory in Chile expected to excel at spotting interstellar objects will begin operations in 2022, and the European Space Agency plans to launch a set of spacecraft in 2029 that will idle in space until they’re commanded to chase after a newly found interstellar object.

Borisov has now left us behind, traveling beyond the view of any telescopes. It will not leave our solar system in the condition it arrived. Last spring, as Borisov neared Jupiter, the Hubble space telescope captured imagery that showed a piece of the comet breaking off. “It is not pristine anymore,” Ludmilla Kolokolova, an astronomer at the University of Maryland who worked with Bagnulo, told me. Borisov now bears a mark of its visit through the solar system. Which presents an interesting question: Hypothetically, if Borisov were to pass by another star, where a set of alien astronomers could observe it, would they notice any evidence of its encounter with our sun?

Bagnulo said it’s difficult to say whether Borisov’s trip through the solar system noticeably altered the cometary properties he studied, which, presumably, his hypothetical alien counterparts might also investigate. But when a comet moves away from the sun and cools off, Kolokolova said, some of the particles in its coma can return to the surface and harden into a crust. “If this comet goes to another system where astronomers look at it, they will see it was heated,” Kolokolova said. “They wouldn’t know if it was the sun or any other star, but they could see the comet was heated.”

But Borisov is unlikely to skim by another star. More than one astronomer told me that the chances are nearly zero. The distances between stars are simply too big. “If you had a collision between the Milky Way and another Milky Way, you could collide the galaxies and no two stars would ever hit,” David Jewitt, an astronomer at UCLA who studies comets, told me. Astronomers believe Borisov coasted alone for hundreds of millions of years, even billions, through space before reaching us. “In that amount of time, you might pass by one star,” Jewitt said. “So for Borisov, maybe this is it.”

For us, the fleeting experience was illuminating. Our interstellar guest gave us evidence of its own home. There, too, the cosmos had struck a match, igniting a star into existence and leaving just enough kindling behind to make planets and moons. The process has unfolded countless times across the universe, creating islands of clustered matter, isolated far from one another. Through a chance encounter with a comet like Borisov, we can glimpse some of the ways these alien places might resemble our own.