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A Huge Population of Interstellar Comets in the Oort Cloud

TAOS II is the Transneptunian Automated Occultation Survey, designed to spot comets deep in our Solar System. It may also be able to detect comets of the interstellar variety, of which we thus far have only one incontrovertible example, 2I/Borisov. And TAOS II, as well as the Vera C. Rubin Observatory (both are slated for first light within a year or so) could have a lot to work with, if a new study from Amir Siraj and Avi Loeb (Center for Astrophysics | Harvard & Smithsonian) is correct in its findings.

I cite Borisov as thus far unique in being an interstellar comet because the cometary status of ‘Oumuamua is still in play. On my way to looking at his paper on Borisov, I had an email exchange with Avi Loeb, from which this:

Observations with the Spitzer Space Telescope of `Oumuamua placed very tight limits on carbon-based molecules in its vicinity, implying that it was not made of carbon or oxygen. This led to suggestions that perhaps it is made of pure hydrogen or pure nitrogen, but these would be types of objects we had never seen before. Borisov appeared to be just like a regular comet that we had seen many times before. Clearly, `Oumuamua and Borisov are of very different composition and origin (irrespective of whether `Oumuamua is natural or artificial in origin).

Image: Comet 2I/Borisov. Credit: NASA, ESA and D. Jewitt (UCLA).

The paper refers to Borisov as “the first confirmed interstellar comet with a known composition,” but if this comet is alone in our catalog, it’s unlikely to remain that way long. Siraj and Loeb argue that there exist more interstellar objects in the Oort Cloud than objects born in the Solar System. Indeed, Loeb in his email cited “a hundred trillion Borisov-like interstellar comets” in this vast space, which extends from roughly 2,000 AU perhaps as far out as 50,000 AU, with some sources citing an outer edge as far as 200,000 AU. That should ring a few bells — Alpha Centauri is 268,000 AU from the Sun, meaning our Oort Cloud could mingle with any similar cloud in that system.

The prospect of studying interstellar objects without leaving our own system is enhanced by these results, even if the calculations contain significant uncertainties. There should be many Borisovs, a small number of which should enter the inner system. This is a reversal of earlier thinking that interstellar visitors should be rare, all part of a reevaluation of the subject forced by the detection of ‘Oumuamua and 2I/Borisov in recent years, and the coming upgrades in equipment and surveys mentioned above.

We are only now getting into position to be able to see these objects and identify their true nature. The detection of Borisov in 2019 allowed scientists to calculate a number density for such objects per star based on a statistical analysis of the likelihood of a single object like this being within 3 AU of the Sun. Other researchers had applied this kind of calibration to ‘Oumuamua, with the number density implied by both being approximately the same. Similarly, the population of bound Oort Cloud comets can be inferred through observations of long-period comets. Figure 1 in the paper shows the comparison.

Image: This is Figure 1 from the paper. Caption: Comparison of the relative abundance per star of bound Oort cloud objects, as implied by the observed rate of long-period comets (Brasser & Morbidelli 2013), and interstellar objects, as implied by the detection of Borisov (Jewitt et al. 2020), with a differential size distribution for power-law index, q, values of 2.5, 3, and 3.5, displayed for reference. The error bar indicates the 3σ Poisson error bars for the implication of a singular interstellar object detection on the abundance. The shaded band correspond[s] to the plausible range of nucleus radii for Borisov, given the central value for Borisov’s abundance. The error bounds on the abundance of bound Oort cloud objects are not resolvable on this plot. Credit: Siraj and Loeb.

These are calibrations with, as the paper notes, uncertainties of several orders of magnitude, but even adjusting for these, interstellar objects still prevail in the Oort. They also come with limits that can be tested by observation. Interstellar objects experience negligible gravitational focusing because of their speed (on the order of 30 kilometers per second) and the nature of their orbits as related to their distance from the Sun. Bound Oort Cloud objects should have characteristic orbits that can be differentiated from the orbits of objects that have entered the Oort from elsewhere.

Note: ‘Gravitational focusing’ refers not to gravitational lensing but to the likelihood that two particles will collide based on their mutual gravitational attraction. The authors are saying that bound Oort objects are significantly affected by gravitational focusing. We wind up with a wide dispersion in these two populations:

Given that the number density of interstellar objects may be ∼103 larger than that of bound Oort cloud objects far from the Sun, the Oort cloud objects may be still a factor of ∼10 more abundant than interstellar objects in the inner Solar system, due to the unequal influence of gravitational focusing on the two populations. The fact that interstellar objects outnumber Oort cloud objects per star is consistent with the Oort cloud having lost most of its initial mass. However, the degree to which interstellar objects outnumber Oort cloud objects is still very uncertain. Stellar occultation surveys of the Oort cloud will be capable of confirming the results presented here, by differentiating between the two populations through speed relative to the Sun…

Thus we can look to planned surveys of the sort mentioned above to test the abundances of the two classes of objects, and can expect more visitors of the Borisov kind, even if such comets are far more common in the Oort Cloud than in the inner system. Siraj points out that such an abundance of interstellar objects indicates that planetary formation leaves a great deal more debris than previously thought:

“Our findings show that interstellar objects can place interesting constraints on planetary system formation processes, since their implied abundance requires a significant mass of material to be ejected in the form of planetesimals. Together with observational studies of protoplanetary disks and computational approaches to planet formation, the study of interstellar objects could help us unlock the secrets of how our planetary system — and others — formed.”

The paper is Siraj & Loeb, “Interstellar objects outnumber Solar system objects in the Oort cloud,” Monthly Notices of the Royal Astronomical Society Vol. 507, Issue 1 (October, 2021) L-16-L18 (abstract).

{ 14 comments… add one }
  • Alex Tolley August 25, 2021, 13:24

    fairly recently I read an article that suggested that the Oort cloud did not exist, it was purely inferred by the aphelions of long period comets. IIRC the argument was that these comets were possibly KBOs or other bodies even closer to the sun that had been perturbed by the giant planets and thrown out into deep space, but with velocities below solar escape. We were just seeing the return of these comets.

    Just as most comets failed to reach escape velocity, a fraction must have escaped their home system and become interstellar like I2/Borisov.

    This is where I want to have direct observational data on cometary clouds to show that do exist. Sparse as their mass might be, these clouds should exist around other stars, and I would hope that some means of detecting them can be devised so that we can see them directly, just as we now see dust and planetesimals around young stars as planets begin to form.

    I hope that TAOSII and the Vera C. Rubin telescope can detect objects in the Oort and show that there is indeed such a cloud out there. If there isn’t then novels like Alistair Reynold’s “Pushing Ice” will be made obsolete.

    • SM August 26, 2021, 5:20

      Isn’t your first paragraph basically describing the Oort Cloud? I thought most Oort objects were exactly that, and the whole idea was that if we see a few there must be a vast cloud spending most of their orbits at 10000 AU+.

      • Alex Tolley August 26, 2021, 9:55

        if we see a few there must be a vast cloud spending most of their orbits at 10000 AU+.

        The article I read suggested that that inference is wrong, i.e. that the comets we see do not come from a vast cloud of objects but rather their long periods are due to gravitational near ejection from the solar system of icy bodies from beyond the snow line. IOW, the Oort cloud does not exist and our model of comet incursions into the inner system is wrong.

        • SM August 26, 2021, 13:31

          I follow, I just thought that was always the idea behind the Oort Cloud, the population of bodies nearly ejected during formation, spending most of their time far out. Then very small perturbations near aphelion could raise the perihelion significantly, meaning for many they would forever remain far beyond the Kuiper Belt.

  • wdk August 25, 2021, 15:19

    Having to examine the definitions of the Oort Cloud and Kuiper Belt frequently for discussions off-line, it is useful to picture the former with the sun at its center and an arrow pointing to our neighboring binary system. The Oort Cloud does not dwindle into nothingness in the diagram, though the textbook versions do not proffer a corresponding
    illustrative Oort Cloud for our neighbors. Whether the fraction of “interstellar” objects is as large as Siraj and Loeb suggest or not, considering Oort Cloud dimensions and flybys of close stars, it is hard to believe that there would not be interactions. In fact several months back we discussed the a mini-binary system flyby that should have
    penetrated the Oort, shaking some loose, if it didn’t have contributions of its own.
    Since the sun’s cloud of cometary material appears to be so large on the local interstellar scale, instead of wondering whether it has captured entries from other stars, would it be appropriate to consider it as part of the interstellar mediu? Doubtless, interstellar space is empty in a profound way. But yet we have these large accumulations of kilometer scale objects. Did they necessarily have to form with the sun or perhaps could they have been drawn into the whirlpool of the sun’s formation.
    A pre-solar formation hypothesis pursued for Oort cloud objects might
    suggest how life building blocks span inter stellar gulfs. Or perhaps ‘Oumuamua’s strange features could be explained looking back into stellar formation processes farther. Or else searching for different details thought less significant prior.

  • Geoffrey Hillend August 25, 2021, 20:22

    The JWST has a mid infra red instrument and near infra red spectrometer, so it can see ort cloud objects.

    • Andrei August 26, 2021, 9:29

      Perhaps, these objects are very faint both from the distance and also very small. The best way to detect Oort cloud objects might be at another star, where we could see this very thin cloud from the side – adding to the signal. Whereas those near us are spread out all around the sky, moving extremely slow.
      The effect of being in the middle can be compared to standing near a smoking person, you do not see the smoke well. But if he is on the bus stop on the other side of the road, the smoke look thick.

      • Geoffrey Hillend August 26, 2021, 17:44

        The way small objects are best detected are with still fame or single photographs over time and the movement of small bodies is revealed since the stars remain fixed. The JWST can do this.

  • Al Jackson August 25, 2021, 22:05

    I remember when working on the Nemesis ‘problem’ back in the 1980s modelers showed that the Oort cloud was made in the formation of the Solar System. A problem arose , why was the Oort cloud still there? In the Sun’s voyage through the Galaxy there were enough encounters with Giant Molecular Clouds that should have stripped the Oort cloud away.
    A solution was found in a proposal that the Sun captured comets from encounters with other stars.
    Levison, Harold; et al. (10 June 2010). “Capture of the Sun’s Oort Cloud from Stars in Its Birth Cluster”. Science. 329 (5988): 187–190

  • Richard McCormac August 26, 2021, 9:07

    These objects have high velocity because they’ve been ejected from other star systems by gravitational interactions with larger bodies in those systems, right? This is how you can distinguish them from other Oort cloud bodies. However, wouldn’t low eccentricity objects that are just barely being held by the sun’s gravity, at the edge of its Hill Sphere exhibit slow transfer speeds? Orbits that far out have orbital velocity as slow as a few hundred meters per second, and would be on the edge of being lost at any time into the influence of another star. Could it be the case that there’s a constant transfer of low velocity objects as well, that kind of wobble their way through the overlap zone of different star’s gravitational influence over billions of years? Essentially Oort clouds co-mingle.

  • Michael Fidler August 26, 2021, 10:26

    The Giant Molecular Clouds should be full of cometary bodies so any system flying thru them would add to the systems entourage of comets.

    Scholz’s Star may have been what caused the 12,900 years ago impacts that change Clovis man and animal life in North America. There is evidence of other cometary impacts further back in history. We are bound to see more findings as techniques and field research find more evidence and more samples are analyzed.

    What is really interesting about our nearest neighbors Alpha Centauri A and B is that they may have a much larger Oort cloud then our Sun. Twice the mass and a close binary may produce very unusual orbits for comets. The red dwarf Proxima Centauri M6V would be flying thru the thicker parts of their Oort cloud as it orbits some 5,000 to 13,000 AU from the famous binary in its eccentric orbit. This may also throw many of their comets our way.

    But what of Proxima b? Too bad we can not view a transit with the JWST to see if the planet is an ocean world or organic paradise from all those comets it may of encounter in its short 11 day orbit around Proxima A. But maybe JWST can see if there is a large cloud of objects that ALMA spotted further out then 1.5 UA.

    I think Amir Siraj and Avi Loeb have done a great job of investigating this subject, something that is strongly needed, to open up discussion and theory. The Rubin Observatory will be opening in the next two years and it will open the Oort cloud to in depth study. Get ready for a few surprises…

  • andy August 26, 2021, 15:00

    Oh great, Siraj and Loeb again. Given their track record, I’m not going to read too much into these results until researchers with a less “interesting” track record confirm it.

  • torque_xtr August 26, 2021, 17:52

    I wonder how much of interstellar population are fragments of terrestrial planets blasted away by impacts. From a study on lithopanspermy (https://www.liebertpub.com/doi/full/10.1089/ast.2013.1028), considerable fraction of material lifted into heliocentric orbits from planetary surfaces gets ejected from stellar systems. No doubt the fraction of planetary fragments in the total population of interstellar objects passing through Solar system is small, and the challenges are daunting, but this still may be the best chance to study exo-lithospheres directly (in this century at least).

    I thought of primordial low-density Kuiper Belt objects. Some of the smaller bodies may be very fluffy, giant snowflakes resembling aerogel in density. Their structure would preserve impacting bodies almost intact. If Stardust panel collected few interstellar grains, then each Kuiper Belt snowflake could accumulate tons of interstellar material, due to billion times greater exposures and up to million times bigger collecting areas. All it takes is to find a suitable km-wide snowflake, fly there, look for stones with a radar, dig them out, identify interstellar ones with isotope-MS and bring them a mere 50 AU back to Earth…

    PS Searching for interstellar meteorites here on Earth, of course, looks much closer to the realm of accomplishable, despite the lack of obvious ones. AFAIK, only a small fraction of meteorites found here on Earth have been directly studied for anomalous elemental/isotopic composition, and there already was one suspected interstellar meteor in 2014. Interstellar fraction is likely much less than one per thousand… But at least they are already here on Earth and not flying by at interstellar velocities! :-)

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