Interstellar visitors open new window to the cosmos

All eyes are on Comet Borisov. But this isn’t just any comet.

The cosmic block of ice, rock, and gas hurtling toward our sun came from another solar system. It’s only the second of its kind ever spotted. 

The first interstellar visitor, a peculiar object dubbed ‘Oumuamua, flashed through the night sky in October 2017. Scientists didn’t expect to see another until after 2022. But Borisov is here now, burning bright in a cloud of dust and cyanogen gas that makes it closely resemble our solar system’s comets. 

Together, these visitors herald a new frontier in astronomy – one where the universe is coming to us, rock by rock, providing clues about our neighbors in the Milky Way. The presence of these interstellar travelers in our solar system also adds a new dimension to our understanding of the cosmos. 

“It shows right away that these objects are probably a lot more common than people had thought they were,” says Robert Weryk, an astronomer at the University of Hawaii at Manoa, who discovered ‘Oumuamua. “And now that we know they’re there, there’s going to be a much more active search to find them and study them.”

What happened when we stopped looking

These interstellar visitors are generating a lot of buzz among 21st-century scientists. But if you went back about 400 years, “no one would have batted an eye,” says Sara Schechner, a historian of astronomy at Harvard University. 

Philosophers at the time conceptualized comets and asteroids as objects that hurtled around the galaxy, not necessarily bounded by fixed star systems. But as modern notions of a vast and orderly universe emerged, interstellar traffic fell out of fashion.

In the 20th century, a new barrier for where a comet could go was introduced: the Oort cloud, a theoretical shell of icy objects that surrounds our solar system. A collision within the cloud could send an object flying deeper into the solar system, but it’s much harder to push an object outward, against the sun’s gravity. If the same structure exists around other star systems, scientists surmised, it would be very difficult for a comet to escape a star system, let alone travel the unfathomable distances and survive a trip through our Oort cloud. 

In short, says Dr. Schechner, interstellar comets became so unlikely, we stopped looking.

Scientists have probably seen objects like Borisov or ‘Oumuamua before but dismissed them as observational errors, says Javier Licandro, a planetary scientist at the Instituto de Astrofísica de Canarias in the Canary Islands. 

But Borisov was no mistake. 

A new window into the cosmos 

An amateur astronomer in Crimea first spotted Borisov on Aug. 30. Other astronomers around the world quickly began tracking its trajectory, eager to determine if it was a second interstellar visitor. After several days, the International Astronomical Union confirmed that the comet did indeed come from another star system.

While ‘Oumuamua hung around for about a week, Borisov is projected to be observable well into 2020. Its path is projected to take it closest to our sun on Dec. 8, though comets can behave unpredictably in heat as gases inside it burn, creating high-pressure jet streams that can push the comet off course. The sun’s heat could also disintegrate Borisov altogether. 

Whatever happens, scientists are tracking the comet closely. Dr. Licandro is particularly focused on capturing Borisov’s journey. His lab was one of the first to capture high-resolution images of our guest, and they will continue to observe as it exits our neighborhood. “This is a unique opportunity to see how an object forms around another star,” he says. 

The first two interstellar visitors present many puzzles: Where do they come from? How frequently do such objects fly through our solar system? And, how many have we missed? 

The differences between Borisov and ‘Oumuamua make things even more interesting. While Borisov has an aura and tail familiar to comet scientists, ‘Oumuamua appeared as a dim, oblong rock that tumbled end over end. “Being able to contrast the two really tells you something about these objects,” Dr. Weryk says – and these objects are one of the few ways we can connect with the world beyond our solar system. 

Some scientists suggest the interstellar objects – and this is completely serious – might be alien sent. Even without aliens aboard or sending them as envoys, space rocks flying between solar systems offer another possibility for exchange: Could comets carry cosmic material from one side of the galaxy to another, like dandelion seeds? Scientists have pondered whether these interstellar visitors might influence the development of solar systems, define the chemistry of young planets, or even seed life across the universe. 

Fascinated by these mysteries, and inspired by Borisov’s early detection, many scientists are already thinking about future guests. One tool in development that could help comb the starscape for interstellar objects is the Large Synoptic Survey Telescope in Chile, which, if all goes according to plan, will take nightly wide-angle snapshots of the sky starting in the next few years. Other organizations, like the European Space Agency, are working on comet interceptors that may one day visit an interstellar guest. 

For Dr. Weryk, future visitors are definitely on the mind. ‘Oumuamua caught him by surprise when he discovered it in 2017. But now, every time he spots a new space rock, he wonders: Is this from interstellar space, too? 

“That’s something I never would have considered before we knew they existed for sure,” he says. “Pretty much every candidate we get now, it’s like, oh, maybe this could be the next one.”