For only the third time, astronomers have found something passing through our solar system that looks like it came from outside the solar system.
This interstellar object, with a temporary, cumbersome designation of A11pI3Z, is still pretty far from the sun, located between the orbits of the asteroid belt and Jupiter.
The first known interstellar object was Oumuamua, which raced through the solar system in 2017. In 2019, Borisov, a comet of interstellar origin, passed by.
On Tuesday, a telescope in Chile spotted what initially looked like an unknown asteroid on a highly eccentric path that might come close to Earth’s orbit. The telescope is one of five around the world that are part of the Asteroid Terrestrial-impact Last Alert System, or ATLAS, which is a NASA-funded project that watches for space rocks that might be on a collision course with our planet.
The observation was submitted to the International Astronomical Union’s Minor Planet Center, which maintains a catalog of small space bodies in the solar system. Other skywatchers quickly took a look.
“Follow-up observations on July 1 and 2 began to reveal that its orbit might be unusual, possibly interstellar,” said Larry Denneau, a co-principal investigator for ATLAS, which was developed by the University of Hawaii.
Then, an amateur astronomer, Sam Deen, spotted A11pI3Z in photos that ATLAS had taken in late June. The additional sightings allowed more precise calculations of the trajectory.
“Now we’ve got dozens and dozens of observations from loads of different people,” said Matthew Payne, director of the Minor Planet Center. “And so it’s like becoming almost 100 percent certain that it’s interstellar.”
The object is surprisingly bright. Although it cannot be seen by the naked eye, modest-size telescopes can spot it.
“This is the most interesting question in my mind right now,” said Avi Loeb, an astrophysicist at Harvard. “What accounts for its very significant brightness?”
That is especially intriguing if the surface turns out to be dark, like that of a rocky asteroid. In that case, the object would have to be big, about 12 miles wide, to reflect the amount of light observed. That would be larger than the asteroid that slammed into Earth 66 million years ago, which unleashed the mass extinction that killed off the dinosaurs.
Finding an interstellar object that big would be a surprise, Dr. Loeb said. It would possess about 10 million times the mass of Oumuamua, he said.
Because small bodies are much more bountiful than large ones, the existence of a 12-mile-wide interstellar object would imply that astronomers should have also seen millions of Oumuamua-size ones.
A11pI3Z could turn out to be an interstellar comet like Borisov, the second interstellar object observed. For a comet, the brightness comes from sunlight bouncing off a plume of gas and dust, and the nucleus would be considerably smaller.
“If we find that it’s a comet, nothing surprising here,” Dr. Loeb said.
Dr. Loeb offered one other possibility. “The final possibility, and I’m getting more speculative here, is that it makes its own light,” he said. “Probably unlikely, but this is what comes to my mind.”
When Oumuamua was discovered in 2017, Dr. Loeb speculated that it could be an alien artifact because of its unusual shape, like a cigar or a disc, and because it seemed to be pushed by a force other than gravity. He has since proposed a possible alien origin for odd material found on the seafloor of the Pacific.
Whether it is a comet or a rock is a question that should be answered within a few days as larger telescopes point to A11pI3Z.
If it is a comet, astronomers will see a tail.
Astronomers will also have months to study it. Analysis of specific colors emitted could identify elements and molecules on its surface. Dr. Loeb said infrared measurements by the James Webb Space Telescope could measure how much heat is coming off the surface.
“If the object is tumbling, we would see the surface area changing over time,” Dr. Loeb said, “and by that, infer, in three dimensions, the shape of the object. That’d be very exciting.”
In contrast, the smaller Oumuamua faded out of view after only a few weeks, leaving many of its mysteries unsolved.
The Vera C. Rubin Observatory in Chile, which will soon begin scanning the full sky every three to four days, should find more interstellar objects, even farther away. This will provide a more complete view of the interstellar objects passing through our solar system.
Something that no one will need to worry about is A11pI3Z hitting Earth. In October, when it makes its closest pass to the sun, it appears that A11pI3Z will still be outside the orbit of Mars. In addition, Earth will be on the other side of the sun, so there will be more than 200 million miles of buffer between Earth and A11pI3Z.
If A11pI3Z turns out not to be a comet, astronomers could be left with a lot of perplexing puzzles to solve.
“The good thing about this object is, because it’s bright, it will give us so much data that nobody would be able to deny it,” Dr. Loeb said. “If it has anomalies, nobody can shove them under the carpet of conventional thinking.”
He added, “Science is about discovering things you don’t expect. It’s the opportunity to stay a child and learn something new. And I love it.”
Kenneth Chang, a science reporter at The Times, covers NASA and the solar system, and research closer to Earth.
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