2i/borisov

An interstellar object – a comet from a distant star system – is passing through the space between Jupiter and Mars, and according to a recent studyComet 3I/Atlas may be 3 billion years older than our Solar System.

Comet 3I/Atlas appears to hail from somewhere toward the center of the Milky Way (which makes sense, because most of the galaxy is “toward the middle” from Earth). And according to astronomer Matthew Hopkins and his colleagues, the comet entered our Solar System at a steep angle, which suggests that it came from a region of the galaxy called the “thick disk.”

Most of the stars, gas, and dust that make up our galaxy orbit around the center in the same plane, a spiral-armed disk about 400 light years deep. But about 10% of the Milky Way’s stars (by mass) orbit in the 1000 light years “above” and “below” the thin disk, like the stellar bread on a galactic sandwich. The thick disk is home to older stars with simpler chemical makeup than our young Sun, and there’s very little interstellar gas or dust drifting between them. In other words, interstellar comet 3I/Atlas didn’t just come from an alien star system – it came from a cosmic neighborhood very different, and much older than, our own.

“We think there’s a two-thirds chance this comet is older than the Solar System, and that it’s been drifting through interstellar space ever since,” said astronomer Chris Lintott, a coauthor of the study, in a recent press release.

But though Comet 3I/Atlas may be the oldest interstellar object we’ve ever seen, it’s not the first – it may even be one of thousands.

Interstellar Object ‘Oumuamua: A Messenger From Afar, Arriving First

Interstellar object 1I/’Oumuamua was 21 million miles from Earth and already on its way out of the system when astronomers first spotted it in October 2017. That meant we got just a fleeting glimpse of the long, thin, red-hued chunk of rock as it tumbled into the cosmic distance – just enough to stir up wild speculation about alien space probes, in fact.

After ‘Oumuamua swung past the Sun, it accelerated slightly. Astronomers watching the asteroid’s progress calculated that the pull of the Sun’s gravity couldn’t have accounted for that burst of speed.

‘Oumuamua moved more like a comet than an asteroid; as comets get closer to the Sun, their icy nucleus starts to evaporate, releasing plumes of gas into space – which in turn give the comet a push that can speed it up or change its course. But ‘Oumuamua moved like a comet, it didn’t look like one. All that erupting gas and dust usually forms a cloud, or coma, around the comet’s nucleus, along with a tail pointing away from the Sun.

Harvard University astrophysicist Avi Loeb proposed that ‘Oumuamua was actually a thin solar sail (a sail designed to catch solar radiation instead of wind), which had caught the solar wind and used it to accelerate. Other astronomers pointed out that the idea made no sense, because ‘Oumuamua was tumbling as it passed through the Solar System, and a tumbling solar sail wouldn’t have been very effective at all.

It turned out that, according to a 2023 study‘Oumuamua was really a comet all along – just a weird one. As it flew through interstellar space, cosmic rays had broken apart about a third of the water molecules trapped inside ‘Oumuamua, creating a lot of loose hydrogen molecules. When ‘Oumuamua approached the Sun and started venting gas, the hydrogen was too light to drag any dust along with it as it erupted, so the comet’s coma and tail were invisible but could explain the bizarre acceleration.

Interstellar Object Borisov: A Rogue Comet From A Dim Red Star

Another piece of a distant star system, a rogue comet probably born around a red dwarf star, swept through our Solar System in late 2019, streaming a tail of gas and dust 100,000 miles long. (The comet itself, 2I/Borisov, was only about a mile wide when astronomers first spotted it.)

“We reasoned that Borisov is likely a representative of the star system it comes from,” Auburn University astronomer Dennis Bodewits said in a 2020 press release from NASA. In other words, 2I/Borisov’s chemical makeup could offer some clues about the alien star it once orbited. The comet contained a surprising amount of carbon monoxide ice (some comets in our Solar System contain carbon monoxide ice, too, but not nearly as much of it), according to data from the Hubble Space Telescope and the National Radio Astronomy Observatory’s Atacama Large Millimeter/submillimeter Array (ALMA), Because carbon monoxide needs much colder temperatures to freeze than water does, 2I/Borisov must have formed somewhere very cold: less than −337.04° Fahrenheit.

That could point to the system of planets orbiting a type of small, dim star called a red dwarf. “Red dwarfs are much smaller and dimmer than the Sun, so the planet-forming material around them would be colder than the building blocks of our solar system,” explained NASA in its press release at the time. And the odds are in favor of the red dwarf idea, because these dim, cool-burning stars make up about 75% of the stars in our galaxy.

On the other hand, the carbon monoxide ice could also point to someplace like the outskirts of a system like ours; at 3.7 billion miles away from the Sun, dwarf planet Pluto’s surface temperature ranges from -375° to -400° Fahrenheit, and there is carbon monoxide ice on its surface. So it’s possible that 2I/Borisov is actually a chunk of a dwarf exoplanet – another star’s version of Pluto – which got knocked into space by a meteor impact.

Or maybe 2I/Borisov was always just a mile-wide clump of ice and dust that coalesced in the chilly outer reaches of its star system. Either way, something must have boosted the comet to escape velocity, letting it slip the bonds of its star’s gravity and travel through interstellar space. In our own Solar System, migrating gas giants probably boosted some of the comets of the Oort Cloud into their long, lopsided orbits, and they may also have kicked an entire planet out into interstellar space. The same process could have flung 2I/Borisov out of its own star system.

Interstellar Object Atlas: The Oldest Comet Ever Seen

And now, for the third time in less than a decade, another comet from another distant star system is passing through.

Like 2i/borisov, 3I/Atlas is probably a comet. Telescopes here on Earth can’t see 3I/Atlas in much detail yet, but what they can see suggests that it’s surrounded by a haze of gas, some of which is streaming outward to form a short tail, which will get larger as the comet gets closer to the Sun. That means 3I/Atlas is probably made of more ice than rock. And if Hopkins and his colleagues are right about its origins, a lot of that ice should be water, rather than other ices like carbon monoxide, carbon dioxide, or methane.

“The gases that may be seen in the future as 3I is heated by the Sun will test our model,” said co-author University of Canterbury in New Zealand astronomer Michele Bannister, a co-author of the recent study, in a press release.

Comet 3I/Atlas looks larger than either 1I/‘Oumuamua or 2I/Borisov; Atlas is somewhere between 6.2 and 12.4 miles wide, while Borisov was about a mile wide. Cigar-shaped ‘Oumuamua, the smallest of the three, was less than 3,000 feet long and less than 500 feet wide.

And Atlas is also “moving considerably faster than the other two extrasolar objects that we previously discovered,” according to University of Lancashire astronomer Mark Norris in comments to Agence France Presse. ‘Oumuamua was zooming along at around 86,000 miles per hour when it passed the Sun, and Borizov whizzed past at 98,000 miles per hour. Meanwhile, astronomers have already clocked Atlas at around 137,000 miles per hour, and it will be moving even faster by the time it passes by the Sun in October 2025.

Interstellar Objects Pass Through More Often Than We Thought

So why are astronomers suddenly so many interstellar objects wandering through our Solar System? It’s not because we’re the hot new travel destination for wandering space rocks, but because new telescopes – like Vera Rubin – make it possible to see smaller, dimmer, and more distant objects. The presence of interstellar objects in our Solar System isn’t anything new, but our ability to spot them definitely is.

“Astronomers estimate that an interstellar object similar to ‘Oumuamua passes through the inner solar system about once per year, but they are faint and hard to spot and have been missed until now. It is only recently that survey telescopes, such as Pan-STARRS1 [which spotted ‘Oumuamua]are powerful enough to have a chance to discover them,” explains NASA on its webpage for ‘Oumuamua.

And now that the Vera Rubin Observatory is up and running, astronomers like Norris are optimistic about spotting more interstellar objects passing through the Solar System on their way to (and from) parts unknown: maybe as many as several a year.

Meanwhile, a 2022 study suggested that we may actually have a few million samples of other star systems orbiting our own Sun. If University of Edinburg astronomer and statistician Jorge Peñarrubia is right, our Sun may have trapped a few million passing interstellar objects in the outer reaches of our Solar system. Some of them are stuck permanently, while others may make a couple of laps around the Sun and slingshot off into interstellar space again.

It’s a fascinating reminder that our Solar System is part of a wider galaxy – and not as isolated from it as we tend to think.

As John Noonan of the Lunar and Planetary Laboratory at the University of Arizona, Tucson, said in the same 2020 NASA press release,“With an interstellar comet passing through our own solar system, it’s like we get a sample of a planet orbiting another star showing up in our own backyard.”