New Interstellar Comet Tracked To Its Origin Region: “It’s Much Older Than The Solar System”

Interstellar visitor Comet 3I/ATLAS was discovered just a week ago and ever since, astronomers across the world have been hard at work following it, trying to gather as much data on it as possible. The initial data on this object already suggested marked differences between it and the previous two known interstellar objects that crossed the Solar System. Thanks to a new model, it looks like it comes from a different region of the galaxy altogether. 

Let’s rewind the clock by a week. Matthew Hopkins defended his PhD thesis on modeling interstellar objects in the Milky Way on Monday. On Tuesday, he told his supervisor, Professor Chris Lintott, he’d take some time off before working on the edits. Instead, Comet 3I/ATLAS decided to make itself known right at that moment. So Hopkins went back to work using his doctoral research to gain unprecedented insight into this new interstellar interloper. 

"It's very exciting!" Hopkins, who is lead author of the new paper, told IFLScience. "I've been anticipating the chance to compare my predictions to new data for four years, and 3I/ATLAS is already giving us new insights into this fascinating galaxy-spanning population." 

Hopkins and Lintott are two of the six authors of a new paper that uses the new model to trace the origin of Comet 3I/ATLAS, and it appears to be coming from a completely different region of the galaxy than our previous interstellar visitors. IFLScience had a chance to read the yet-to-be-peer-reviewed paper ahead of publication.

[Comet 3I/ATLAS] is probably from an old star in the thick disc, and we think that it's likely that this thing's been out there for longer than the age of the Solar System.

Prof Chris Lintott

“What we're able to do with Matthew and our collaborators’ work is predict the population of interstellar objects that should be out there,” Professor Lintott told IFLScience. “We've now seen three of these, but we think there are a billion billion billion – 10²⁷  – of these in the galaxy. And so using that model, we're able to say what's interesting and what's unusual about this particular object.”

One striking difference between this object and the other two interstellar visitors is its speed. Comet 3I/ATLAS is moving almost twice the speed of the others, with estimates around 57 kilometers (36 miles) per second. ’Oumuamua, discovered in 2017, moved at about 26.33 kilometers (16.36 miles) per second, while  Comet 2I/Borisov, discovered in 2021, was moving a tad faster at 32.2 kilometers ( 20 miles) per second.

Two new preprint papers published on the ArXiv server today suggest that this comet is remarkably different from expectations. Observations from the European Southern Observatory's (ESO) Very Large Telescope (VLT) suggest a much redder object, more similar in color to the Centaur population of asteroids than most Solar System comets. A different paper focused instead on similarities with many of the Solar System objects, including some comets, and stresses that the differences are most marked against ‘Oumuamua and Comet Borisov.

I think the thing that's most exciting for me is that if it is from the thick disk, we're seeing an object from a part of the galaxy we've never seen one before. 

Prof Chris Lintott

“This thing's coming in much faster than the other two, but it is actually within the range of velocities that we would predict in objects. So we don't think that's notable, but it's moving fast up and down relative to the plane of the galaxy in a vertical velocity, so it gives us a clue about where it's from,” Professor Lintott explained. “Our model predicts that it's from a star in the thick disc of the galaxy.”

The spiral arms of galaxies like the Milky Way are located in the thin disk. That’s also where the Sun is. Spiral galaxies also have a structure called the thick disk, a region above and below the plane of the Milky Way, where older stars tend to reside.

"[Comet 3I/ATLAS] is probably from an old star in the thick disc, and we think that it's likely that this thing's been out there for longer than the age of the Solar System," Professor Lintott told IFLScience. "There's a two-thirds chance that it's older than 7 billion, and that would explain the colour. So these things get processed by cosmic rays and turn red. That seems to fit in, though I think the colours in the VLT paper are slightly odd, so we should check that," he added.

Good models need to have testable hypotheses, and the team has a pretty straightforward one for Comet 3I/ATLAS: water. 

“Our model says that older stars tend to produce water-rich interstellar objects. So if we're right, as this thing comes further towards the Sun, we should get a lot of cometary activity. The task coming up is to do a better job of chemistry,” Professor Lintott says.

“I think the thing that's most exciting for me is that if it is from the thick disk, we're seeing an object from a part of the galaxy we've never seen one before. The other two share a different origin.” 

With the idea that there are a billion billion billion interstellar objects across the galaxy, it is not surprising that scientists have estimated that there are 10,000 interstellar objects within the orbit of Neptune on any given day. Most of these are too dark to be spotted by our telescopes, but they are likely there. 

The Vera C. Rubin Observatory has recently demonstrated that it can discover over 2,000 new asteroids in a matter of hours across a few nights. As more cutting-edge telescopes come online, it will be easier to discover more of these objects. 

“The reason I got excited about studying stellar objects is that there's this theory that they get incorporated into the material that forms stars and planets, so they may kickstart planet formation. So it's possible that our Earth started with something like this arriving, which is just kind of a cool and exciting idea!” Lintott told IFLScience.

“The second thing is that we're having an enormous amount of fun. This is what I thought astronomers did when I was a kid. We found a thing, we're pointing telescopes at it and then we're arguing about it and it's just the best!”

The paper describing this work has been submitted to The Astrophysical Journal Letters and will be posted to the ArXiv tomorrow.