Let’s run through what we know about 3I/ATLAS, now accepted as the third interstellar object to be identified moving through the Solar System. It seems obvious not only that our increasingly powerful telescopes will continue to find these interlopers, but that they are out there in vast numbers. A calculation in 2018 by John Do, Michael Tucker and John Tonry (citation below) offers a number high enough to make these the most common macroscopic objects in the galaxy. But that may well depend on how they originate, a question of lively interest and one that continues to produce papers.

Let me draw on a just released preprint from Matthew Hopkins (University of Oxford) and colleagues that runs through the formation options. Pointing out that interstellar object (ISO) studies represent an entirely new field, they note that theoretical thinking about such things trended toward comets as the main source, an idea immediately confronted by ‘Oumuamua, which appeared inert even as it drew closer to the inner system and even appeared to accelerate as it departed. The controversy over its origin made 2I/Borisov a relatively tame object, it being clearly a comet. 3I/ATLAS looks a lot more like 2I/Borisov than ‘Oumuamua, though it’s larger than either.

Protoplanetary disks are a possible source of interstellar debris, but so for that matter are the Oort-like clouds that likely surround most main sequence stars, and that would largely be released when their hosts complete their evolution. ‘Oumuamua has been analyzed as a fragment of a small, outer-system world around another star, or even as a ‘hydrogen iceberg,’ and I see there is one paper suggesting that ISOs may be a part of galactic renewal, contributing their materials into protoplanetary disks and nascent planets.

The Hopkins paper underlines the ubiquity of such objects:

A standard picture has emerged, in which planetesimals formed within a protoplanetary disk are scattered by interactions with migrating planets or via stellar flybys, early in the history of a system (Fitzsimmons et al. 2023). The number density inferred from observations of the first two ISOs, in addition to studies of scattering in our own Solar System, suggest that such events are common, with ≳ 90% of planetesimals joining the ISO population (Jewitt & Seligman 2023). Such objects spread around the Milky Way’s disk in braided streams (Forbes et al. 2024), a small fraction of which intersect our Solar System. The observed ISO population is thus truly galactic, rather than being associated with local stars and stellar populations.

Image: ESO’s Very Large Telescope (VLT) has obtained new images of 3I/ATLAS, an interstellar object discovered in recent weeks. Identified as a comet, 3I/ATLAS is only the third visitor from outside the Solar System ever found, after 1I/ʻOumuamua and 2I/Borisov. Its highly eccentric hyperbolic orbit, unlike that of objects in the Solar System, gave away its interstellar origin. In this image, several VLT observations have been overlaid, showing the comet as a series of dots that move towards the right of the image over the course of about 13 minutes on the night of 3 July 2025. The data were obtained with the FORS2 instrument, and are available in the ESO archive. Credit: European Southern Observatory.

I’m struck anew by how much our view of our Solar System’s place in the cosmos has changed. The size and density of the Kuiper Belt only swam into focus when the first KBO was discovered in 1992, although the belt had been hypothesized by Kenneth Edgeworth in the 1930s and Gerald Kuiper in 1951. The vast Oort Cloud of comets that envelops our entire system was posited by Jan Hendrik Oort in 1950. Now we’re looking at populations of objects at minute sub-planetary scale existing between the stars in unfathomable numbers.

Hopkins and team point out that the Rubin Observatory Legacy Survey of Space and Time (LSST) will dramatically increase the number of confirmed ISOs. So then, what do we have on 3I/ATLAS? The early work on the object identifies it as a comet with a compact coma, a cloud of gas and dust surrounding the nucleus. It’s also bigger than its two predecessors, perhaps as large as 10 kilometers, as opposed to ‘Oumuamua and Borisov’s roughly 0.1 kilometers, although a more precise number will emerge as we learn more about its composition and albedo. It enters the Solar System at a higher speed than the latter ISOs, but one well within the distribution model used in this paper.

Interestingly, the object shows high vertical motion out of the plane of the galaxy, ruling out the idea that it comes from the same star as ‘Oumuamua or Borisov. That velocity points to an origin in the Milky Way’s thick disk – stars above and below the disk within which the Solar System resides. It is the first object to be identified as such. Says Hopkins:

“All non-interstellar comets such as Halley’s comet formed with our solar system, so are up to 4.5 billion years old. But interstellar visitors have the potential to be far older, and of those known about so far our statistical method suggests that 3I/ATLAS is very likely to be the oldest comet we have ever seen.”

The team’s model (based on Gaia data, disk chemistry and galactic dynamics) was developed during Hopkins’ doctoral research. It emerges as the first real-time application of predictive modelling to an interstellar comet. It likewise predicts that 3I/ATLAS will have a high water content. We’ll be able to check on that as observations continue. Co-author Michele Bannister, of the University of Canterbury in New Zealand, points out that 3I/ATLAS is already showing activity as it warms during its approach to the Sun. The gases the comet produces as it moves toward perihelion at 1.36 AU in October will tell us more.

The paper is Hopkins et al., “From a Different Star: 3I/ATLAS in the context of the ̄Otautahi–Oxford interstellar object population model,” submitted to Astrophysical Journal Letters and available as a preprint. The paper on the density of the interstellar object population is Do, Tucker & Tonry, “Interstellar Interlopers: Number Density and Origin of ‘Oumuamua-like Objects,” Astrophysical Journal Letters Vol. 855 (6 March 2018), L10. Full text. Also be aware of a new paper by Avi Loeb at Harvard that I haven’t yet had time to review. It’s “Comment on “Discovery and Preliminary Characterization of a Third Interstellar Object: 3I/ATLAS” (preprint).