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).
I suspect these interstellar objects are from Oort clouds around other stars as they can’t form in deep space.
https://www.almaobservatory.org/en/press-releases/largest-oort-cloud-comet-ever-observed-unveils-its-secrets-with-almas-powerful-gaze/
What I hope is that at some point we can build a ISO object chaser, lander and return sample like a comet mission. We get a good sized sample. What happens if we radiometric date that sample a bunch of time and it is 20 billion years old. Our hypothesized age of the universe will be proven wrong. If they really are ISO objects have far have them come. If they are from nearby, then they should be 4.5 billion years old or roughly the age of our solar system. If not, then they might be older. This is a question that can only be answered with radiometric dating.
Planets around older stars would have older rocks.
This would rely on very long 1/2 life radionucleides with decent abundances in the comet material.
Commonly used long 1/2 life isotopes with 1/2 life in years for terrestrial dating.
IDK what their abundances in comets would be. Enough?
potassium-40 1.251E9 -> argon-40, calcium-40
uranium-238 4.468E9 -> lead-206 (eventually)
thorium-232 14.05E9 -> lead-208 ( ditto )
What other long 1/2 isotopes would be good for comets?
ICPMS instruments available to the commercial market routinely measure at sub-nanogram level. Both uranium and thorium are easily determined in almost anything. I have seen isotope results for uranium in tree bark for example. So I am not sure determining the isotope ratios in cometary material would be a challenge for state of the art instruments.
Since the Great Galactic Ghoul’s other name is Murphy, as soon as it is launched we wouldn’t see another Interstellar Object for another decade or two.
I would like to see a chart showing the location of spacecraft and asteroids to ATLAS path.
Is it “behind” New Horizons such that the probe could get ahead of it?
Are any Mars craft fuel fat enough to gradually raise its orbit to get near the object?
Some years ago, there was a CD post on the probability that panspermia could spread life across the stars. The calculation was that this was exceedingly low, and therefore, presumably, not a competitor mechanism to abiogenesis.
This was before the discovery of interstellar objects entering our system, of which we now have 3, and likely to be of higher frequency as our telescopes prove more capable.
This situation makes me ponder again the possibility of panspermia, mediated by interstellar comets/objects, and the mechanisms by which they may spread life.
We know terrestrial bacteria can be found even in the stratosphere. Some will inevitably be “blown off” the Earth by high-altitude winds, the solar wind, etc. Therefore, there should be a very sparse distribution of bacterial cells/spores in space, spreading outwards into interstellar space with ever-decreasing density.
We know that some bacteria can survive in space, as demonstrated by bacteria living on the outside of the ISS.
An interstellar comet entering a system may by chance sweep up bacterial spores, with increasing probability the deeper into the system they go, where an inhabited planet exists. This helps build up an inventory of bacteria.
If the comment penetrates inside the HZ, it will shed some of those bacteria in its dust tail. Some of those bacteria will enter the atmosphere of a habitable world.
The surface of the comet will cool and settle, burying and protecting its bacterial payload as it exits the system.
With interstellar comets passing through different systems, sometimes collecting bacteria well outside the orbit where a tail will form, and shedding bacteria when the tail forms, sometimes inside the orbit of a habitable world, which may pass through its tail.
The primary habitable world may not even need to be the first recipient of bacterial spores, as other bodies may become repositories of the bacteria shed by the comet, which in turn eventually enter the atmosphere of the habitable world. (This reminds me of the spread of infectious bacteria in hospitals, with intermediate surfaces temporarily holding bacteria until they are passed on, eventually reaching a sick patient in a distant ward.)
Is the frequency of interstellar comets passing through systems, collecting and shedding bacteria (other life), a mechanism for panspermia that might exceed the probability of abiogenesis?
Unlike the survival of bacterial spores unprotected in the interstellar medium, a comment may prove a more benign habitat, which may even provide a opportunity for bacteria to multiply during the phase when the comet is warm enough to shed a tail, and allowing the bacteria to burrow deeper into the comet further protecting them as the comet returns to interstellar space.
Might it be possible for a probe to such a comet to locate any life, and even return a sample, to determine whether it is from Earth (harvested during its entry to our system) or alien (from another system)?
A sample of a bacterium with both proteins and DNA would likely be sufficient to determine origin. If the genes produce the proteins with the terrestrial genetic code, it may well be a terrestrial bacterium and fit in our phylogenetic tree of life. If the needed genetic code is different, or the bases and amino acids are not completely identical to terrestrial life, then we may have evidence of a separate abiogenesis, and proof that ET life exists. Perhaps the bacterium has the same bases, amino acids, and genetic code, but the gene sequences appear to be rather different from our terrestrial bacteria. Perhaps this is evidence of a shared abiogenesis, but with divergent evolutionary pathways due to a different planetary environment.
Harvesting such bacterial passengers from interstellar objects might be the best hope that we have of collecting ET life directly, as these comets are the delivery mechanism that manages the long times between systems at STL speeds, but reach us with a frequency that vastly improves the search space for ET life samples that cannot be obtained any other way.
To my mind, a probe to an asteroid or comet near Earth would be the best way to determine if bacteria are swept up from bacteria that leave Earth. Sterilizing the probe would be very important to prevent contamination. The Hayabusa 2 probe to Ryugu, the OSIRIS-REx probe to asteroid Bennu, and the Stardust probe to comet Wild 2 seem to be the way to go in this regard.
It was probability this post, Alex:
https://www.centauri-dreams.org/2021/11/16/probing-the-likelihood-of-panspermia/
Abraham Loeb does not believe this object is as large as some are saying. He thinks it must be <0.6 km in diameter, and it just looks bigger because of its coma.
Otherwise, it means the galactic mass in small objects is untenably large.
The other alternative is that it is here because of technological intervention.
https://arxiv.org/abs/2507.05881