Even at 4.6 billion years old, our solar system still has its fair share of baby photos.
We know them as comets: bundles of ice, rock, and dust that have been pristinely preserved since around the time the planets first came together. Each of these frozen space nuggets constitutes an ancient cosmic snapshot—one that can tell us a lot about the origins of our solar system, and how it’s changed since.
But the realm of our sun isn’t the only place with planets orbiting a star, and there’s still plenty to learn about the many other stellar systems that populate our galaxy and beyond. So when a comet from another part of the universe swings by for a visit, you better believe astronomers are going to pay attention.
That appears to be the case with an interstellar interloper now known as 2I/Borisov, a comet first spotted on August 30 by amateur astronomer Gennady Borisov, who made the discovery with a homemade telescope in Crimea. Borisov is only the second confirmed object that’s infiltrated our cosmic neighborhood from beyond.
“When it comes to comets...we’ve been waiting for the interstellar ones for decades,” says Piotr Guzik, an astronomer at Jagiellonian University in Poland. “This is a dream come true for most solar system astronomers.”
In the weeks since the discovery, researchers around the world have been scrambling to gather up as much intel as possible on their mysterious guest before it bids them adieu sometime in the fall of 2020. Zeroing in on the comet’s chemical composition and trajectory could tell us a lot about the place it hails from, scientists say—and even allow us Earthlings to compare that star system to our own.
“This comet represents another planetary system,” says Karen Meech, an astronomer at the University of Hawaii’s Institute for Astronomy. “The question is, is it similar or different? To me, that’s one of the most exciting things.”
Our track record of detection aside, interstellar objects probably come snooping around our sun all the time, says Malena Rice, an astronomer at Yale University. These bits of cosmic shrapnel are natural byproducts of the violent conceptions of planetary systems around the galaxy. It’s pretty natural, she says, for tiny objects to get chucked from one neighborhood into another—they’re just usually too small and dim to see with the telescopes we have.
But in 2017, scientists unexpectedly glimpsed the oblong ‘Oumuamua as it was hurtling away from our sun. The shape and speed of the space rock’s trajectory told astronomers it had definitely been flung out of another planetary system. But the timing was off: Squinting after ‘Oumuamua’s rapid departure didn’t yield much useful information, and its oddball features made several astronomers hesitate to designate it a true comet.
In contrast to its predecessor, though, Borisov has been noticed on the inbound leg of its solar system sojourn. Researchers estimate it’ll make its closest approach to both the sun and Earth this December, before putting both in its rearview mirror and disappearing sometime next year. “Now we get a bit longer to watch it,” says Michele Bannister, a planetary astronomer at Queen’s University Belfast.
With just a handful of observations on Borisov under their belt, many scientists remain cautious. But so far, the preliminary results seem to support an intriguing idea: The comet’s extrasolar roots may be the most exotic thing about it.
Unlike ‘Oumuamua, it’s unquestionably a comet—one that bears a striking resemblance to those that originate in our own solar system, says Guzik, who, together with his colleagues, published a report on Borisov today in Nature Astronomy. The wayfaring rock sports all the necessary accoutrement for the classification, including a bright tail and a fuzzy, luminous halo called a coma—both byproducts of ice sublimating into gas under the sun’s heat. And with its reddish hue, it would blend seamlessly into a typical cometary lineup.
Borisov may also be made up of familiar ingredients. Data collected by Meech and her colleagues suggest Borisov’s coma contains cyanogens—toxic compounds of carbon and nitrogen that are common fixtures of our comets, too. “That’s typically the first thing you see in any comet in our solar system,” Meech says. “Cyanogens just get dragged out.”
Another group has also picked up traces of diatomic carbon (two carbon atoms bonded together) in its gassy shroud. The signature is relatively faint, they report, and would put Borisov in the “carbon-depleted” category of comets, were it local to our sun.
Given how weird ‘Oumuamua was, Borisov’s pedestrian nature has thrown some for a loop, Guzik says. In a lot of ways, he says, “this is just a normal-looking comet.”
But a lot about Borisov remains mysterious. Unlike ‘Oumuamua, which lacked a coma and had an easily discernible shape and size, this newcomer is cloaked in a cloud of gas, making its true form hard to distinguish from afar. Guzik’s estimates put it at roughly a mile or so across, but those numbers could still change. The real scoop on Borisov, Bannister says, is “by no means a done deal.”
As the comet gets up close and personal with the heat of the sun, it may begin to reveal its own personal flair. It could start emitting new kinds of gas—or even shatter into pieces. “That would be really fun if it did,” Bannister says.
But if Borisov’s qualities continue to echo those of domestic rocks, that’s still a big deal, Bannister says. Like our own comets, Borisov is probably a pretty decent time capsule from the dawn of its planetary system. With the wild variety that’s known to be out there in the cosmos, she says, what are the odds that the second interstellar space rock to pay us a visit has done little more than hold up a mirror?
“This is fascinating,” Bannister says. “It means the processes that formed the bodies in our own system have a degree of being universal...that what happened here is like what happened elsewhere.”
Where exactly “elsewhere” is, however, is a different question. Pinpointing Borisov’s roots means meticulously reverse-engineering its path through the galaxy—a task that requires keeping close tabs on its movements over the next few months. That’s not easy, Meech says. Plenty of things can perturb a comet from a straightforward path, from the pull of gravity from bigger-bodied objects, to jets of gas spewing out of the rock itself, which can act like thrusters on a spacecraft.
The map of the galaxy also isn’t static. Borisov has been traveling for at least many millions of years, and stars have shifted in its wake. “As you trace it back further in time, you have to rewind the clock and put stars back to where they were when [the comet] passed,” Meech says.
Because of all this uncertainty, a lot of things would have to line up for astronomers to make a solid estimate, Meech says. It’s very possible we’ll never know where Borisov came from. “It might be possible if we get really lucky, and it was ejected recently by a nearby star that was measured with Gaia [an ongoing effort to map the Milky Way],” she says. “But I think the probability that all those things happen is negligible.”
That’s no reason to despair, though. Noticing two interstellar invaders in the past two years is a sign that there’s a lot more to come—especially now that astronomers know to look. And with Chile’s upcoming, ultra-sensitive Large Synoptic Survey Telescope slated to start surveying the skies in 2022, Rice says, “we should expect a goldmine of these objects, relative to what’s been seen in the past.”
In the end, it’s a matter of us humans cluing in to what the universe has to offer, Bannister says. “This is all a natural outcome of how planetary systems form and evolve,” she says. “These objects are just bobbing through. Occasionally, we get lucky...and one of them lands on our shores.”