When ‘Oumuamua passed through our solar system in 2017, no one knew where it came from. Astronomers, on the other hand, think they know how Comet 2I/Borisov got here.

Astronomers have found an object from outside our solar system flying through it for the second time in history. But scientists think they know where it came from this time.

The interstellar comet was first seen by Gennady Borisov, an amateur astronomer in Crimea who was using his own telescope to look at the sky. When he found it, the object was the first interstellar visitor found since 2017, when the long ‘Oumuamua flashed by our solar neighborhood. In a publication, a group of Polish astronomers figured out how this new comet, called Comet 2I/Borisov or (in earlier descriptions) C/2019 Q4, got to our sun’s gravity well. And that path goes back to Kruger 60, which is a system of two red dwarf stars 13.15 light-years away.

Researchers found that Comet Borisov passed just 5.7 light-years from the center of Kruger 60 1 million years ago. This means that it was moving at just 2.13 miles per second (3.43 kilometers per second).

In human terms, that’s fast—about as fast as an X-43A Scramjet can go, which is one of the fastest planes ever made. But because of the sun’s gravity, an X-43A Scramjet can’t leave our solar system. And the scientists found that if the comet was moving so slowly and was no more than 6 light-years away from Kruger 60, it wasn’t just passing through. They thought it most likely came from a star system. Comet Borisov used to go around those stars in the same way that comets in our system go around ours.

Ye Quanzhi, an astronomer and comet expert at the University of Maryland who was not part of this project, told Live Science that the evidence linking Comet 2I/Borisov to Kruger 60 is very strong based on what we know so far.

“If you have a comet from another star system and want to find out where it came from, you need to check two things,” he said. “First of all, has this comet been close to a planetary system? Because if it’s coming from there, its path must pass through where that system is.”

Even though the 5.7 light-year gap between the new comet and Kruger seems bigger than a “small gap” (it is more than 357,000 times the distance between Earth and the sun), it is close enough to be considered “small” for these kinds of calculations, he says.

“Second,” Ye continued, “comets are usually thrown out of a planetary system when their gravity interacts with that system’s major planets.”

In our solar system, that might look like Jupiter catching a falling comet, sending it on a short, partial orbit, and then tossing it into space between the stars.

“This speed of ejection can only go so fast,” Ye said. “It can’t be infinite because planets have a certain mass,” and how hard a planet can throw a comet into the void depends on how much mass it has. He also said, “Jupiter is pretty big, but you can’t have a planet 100 times bigger than Jupiter because then it would be a star.”

 

Ye says that this mass threshold limits how fast comets can move through the space between stars. And, if their estimates of the comet’s path are right, the authors of this study showed that Comet 2I/Borisov passed close enough to Kruger 60 in terms of speed and distance to suggest that it came from there.

Ye said, “Studying interstellar comets is exciting because it gives us a rare chance to study other solar systems with the same tools we use to study our own.”

Astronomers can look at Comet 2I/Borisov through telescopes, which may give them information about the surface of the comet. They can find out if it acts like comets in our solar system do (so far, it has) or if it does anything strange like ‘Oumuamua. That’s a whole area of study that’s normally not possible with distant solar systems, where small objects only appear — if at all — as faint, discolored shadows on their suns.

Because of this research, anything we find out about Comet Borisov could teach us something about Kruger 60, a nearby star system where no exoplanets have been found yet. ‘ Oumuamua, on the other hand, seems to have come from the general direction of the bright star Vega, but astronomers at NASA’s Jet Propulsion Laboratory think it came from a new star system, though they don’t know which one. If these results are true, Comet Borisov will be the first object from another star system that has been tracked back to its home system.

But the people who did the research were careful to say that these results are not yet proof. Astronomers are still gathering information about Comet 2I/path Borisov’s path through space. More information may show that the original trajectory was wrong and that the comet came from somewhere else.

The paper that tries to figure out where the comet came from hasn’t been reviewed by other scientists yet, but it is available on the arXiv preprint server.