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A Population of Interstellar Asteroids?

It was hard enough to find ‘Oumuamua, the first object on an interstellar trajectory discovered within our own Solar System. The emergence of new resources like the Large Synoptic Survey Telescope (LSST) should help us develop a preliminary catalog of such interlopers, thought to be not uncommon if we can identify them. But tracking down objects that wandered from one star to another and found their way into residence in our system is another matter entirely.

In April we looked at a study of an unusual set of Centaurs, asteroids whose orbit perpendicular to the orbital plane of the planets and other asteroids raises questions about their origin. A letter to Monthly Notices of the Royal Astronomical Society had appeared, written by Fathi Namouni (Observatoire de la Côte d’Azur, France) and Maria Helena Moreira Morais (Universidade Estadual Paulista, Brazil). The scientists identified what seems to be a population of asteroids that were probably drawn into the Sun’s gravitational pull at the time of the Solar System’s formation, when the Sun’s birth cluster stars would still be nearby. See Identifying Asteroids from Other Stars for a preliminary look at the idea.

The study has now been expanded and appears as a full paper in the same journal, calling renewed attention to the dynamical evolution of this particular branch of the Centaur family. Let’s quickly note that not all Centaurs fit into parameters implying an interstellar origin. Similar to asteroids in size but likely icy in composition, most Centaurs revolve in relatively unexceptional but ultimately unstable orbits in the outer Solar System between Jupiter and Neptune.

Namouni and Morais are interested in the small number of high-inclination Centaurs — and a few at lower inclination — whose computed orbital history points to an origin far from home. The subsequent orbits of these objects may be more stable than we have realized. Says Morais:

“The Solar System formed 4.5 billion years ago in a stellar nursery, with its systems of planets and asteroids. The stars were close enough to each other to foster strong gravitational interactions that led to an exchange of material among the systems. Some objects now in the Solar System must therefore have formed around other stars. Until recently, however, we couldn’t distinguish between captured interstellar objects and objects that formed around the Sun. The first identification was made by us in 2018.”

Image: NGC 299 is an open star cluster of a kind similar to the one that produced the Sun, located within the Small Magellanic Cloud. Open clusters are weakly bound collections of stars, all formed from the same massive molecular cloud of gas and dust. Because of this, all the stars have the same age and composition, but vary in mass because they formed at different positions within the cloud. In the close conditions of such a birth cluster, comets and asteroids may have moved readily from star to star. Image credit: ESA/Hubble & NASA.

The object Morais talks about as the duo’s first detection is a retrograde co-orbital asteroid of Jupiter called (514107) Ka‘epaoka‘awela, whose name translates from Hawaiian into the delightful “mischievous opposite-moving companion of Jupiter.” Using computer simulations to unwind Centaur orbits back 4.5 billion years to the formation of the Solar System, the researchers find 19 Centaurs of likely interstellar origin. Again we’re presented with the chance to study objects from another star that may be reachable much closer to home.

The authors contest earlier theories about the origin of Centaurs as being from the primordial planetesimal disk. The paper goes on:

When 4.5 Gyr-stable orbits were found in Paper I for Ka‘epaoka‘awela with a majority of retrograde orbits in Jupiter’s co-orbital region, it was concluded that Ka‘epaoka‘awela is likely of interstellar origin as no internal Solar system dynamical process could produce asteroid orbits with an inclination of 162° at Jupiter’s location at that early epoch. Ka‘epaoka‘awela could be a representative of a class of asteroids captured from the interstellar medium by the Sun and Jupiter owing to the strength of Jupiter’s co-orbital resonance at large inclination that is responsible for shielding the asteroid from disruptive perturbations from the other planets…

And note the whole issue of orbital stability, for an important point in the authors’ work is their contention that there are long-term, stable orbits for interstellar objects that are not as extreme as most of the Centaurs studied here and thought to be interstellar. Indeed, Chiron may be a case of such:

The existence of 4.5 Gyr-stable orbits for high-inclination Centaurs, the two polar TNOs and Chiron widens the significance of that earlier finding in that stability over the age of the Solar system is possible even for Centaurs that are not protected by strong resonances. Furthermore, high inclination is not a prerequisite for Centaur stability over the age of the Solar system as Chiron’s example demonstrates.

Chiron, in fact, gets more and more interesting. On an eccentric orbit that crosses the orbit of Saturn and extends close to that of Uranus, the object seems to have the composition of a comet. The authors see Chiron either as an outlier that was once part of the planetesimal disk or an interstellar object, and urge further study of low-inclination Centaurs as we learn more about the kind of capture events that occurred in the early days of the Solar System. Some Centaurs may be more orbitally stable, and more interesting, than we have previously thought.

The paper is Namouni & Morais, “An interstellar origin for high-inclination Centaurs,” Monthly Notices of the Royal Astronomical Society Volume 494, Issue 2 (May 2020), pp. 2191–2199 (abstract/full text).


Comments on this entry are closed.

  • andy July 21, 2020, 14:14

    It’s worth noting that this analysis has already been met with a rebuttal: see Morbidelli et al. (2020) “No evidence for interstellar planetesimals trapped in the Solar system” (preprint), which addresses the claims in this paper and the earlier 2018 one. They claim that the analysis of Namouni & Morais (2018, 2020) is fatally flawed, in particular by assuming that the alternative to interstellar capture is that these objects must have been in their observed orbits for the lifetime of the Solar System, rather than being transient members of a population being replenished in a steady state. They also note that the N&M models require an implausibly high volume density of interstellar asteroids.

    • Paul Gilster July 21, 2020, 17:18

      Good catch, andy. Thanks.

    • Jean Schneider July 22, 2020, 2:56

      Morais and Namouni are writing a counter-rebuttal.
      We will see.

    • andy July 30, 2020, 15:16

      From today’s arXiv batch, here’s another study of the origins of retrograde co-orbitals, again favouring a local source for these objects over an interstellar one. Greenstreet et al. “Transient Jupiter Co-orbitals from Solar System Sources”

  • Alex Tolley July 21, 2020, 14:58

    Life imitating art. The Phoenix asteroids from the movie Dark Star, that “circumnavigate the universe”. The retrograde orbit of Ka‘epaoka‘awela reminds me of the retrograde orbit of Kali, the Earth intersecting asteroid from Clarke’s The Hammer of God.

    Any astroid or comet that was ejected from another world might just contain evidence of life from the original system, deep frozen during its interstellar journey. If that is possible, it would be worth looking at NEAs for evidence that they have collected Earth life from past close encounters and preserved the evidence. If so, such interstellar visitors might have done the same and be worth some aseptic examination.

    • charlie July 21, 2020, 19:08

      “worth some aseptic examination.”
      where – here on earth?

      • Alex Tolley July 22, 2020, 13:13

        Probably, as we have the facilities. But if this didn’t happen for a long while, then on a lunar base or space station. The work would be done in separate labs just as biohazard labs (e.g. Fort Detrick, USA, Porton Down, UK) are designed to prevent contamination of, and from, samples.

        Because any preserved cells or biomolecules are likely to be extremely sparse, the ideal situation is to be able to test in situ to obviate the need for transporting large amounts of sample material. We don’t really have that capability today for a robotic spacecraft. Maybe in future, and with really good sterilizing of the craft before arrival to ensure no terrestrial contamination.

        The discovery of alien life signs clearly from another star system would be very exciting, and with no need to make a sample return journey to the stars would be a lot less expensive. As with terrestrial life, microbes can tell us a lot about the biology without needing to examine complex life.

    • Ljk July 22, 2020, 14:06

      If you would like to learn more about the Phoenix Asteriods from Dark Star, such as their apparent trait of circling the entire Universe in 12 trillion years, see here:


  • Geoffrey Hillend July 21, 2020, 15:40

    The idea that Chiron came from outside our solar system is not a new one. Chiron has a mythology or archetypal meaning which fits that idea, so it is suspect as a projection. If Chiron really is from outside our solar system, then all we have to do is send a space craft there and get a large of piece of it for radiometric dating. It should be measurablely older or younger than 4.5 billion years. Until we do, I will accept the idea with the notion that it potentially could be a myth.

    • Alex Tolley July 21, 2020, 18:26

      I will accept the idea with the notion that it potentially could be a myth.

      As is its namesake, the centaur of Greek mythology. Appropriate.

      As you say, a dating of Chiron would likely settle the matter. Empiricism is better than reasoning, however computationally aided.

  • Bruce D Mayfield July 21, 2020, 19:42

    How would a piece of a sister solar system, born at the same time, place and from the same chemical composition as ours did, be any different from pieces of Sol’s system? How could it be proven, apart from orbital dynamics, that it really was from another system?

    • kph July 22, 2020, 14:18

      Interesting question. At same time assumed as within large confidence intervals of the object’s date, other system’s age measurements. Presence or absence of elements in ratios found locally wouldn’t determine origin in such a twin, sister system scenario.

  • Henry Cordova July 21, 2020, 20:31

    The possibility of a truly foreign body (like ‘Oumuamua) being captured by the sun are highly unlikely, it is almost dynamically impossible. A third body must be available in just the right time and place to make the accounting add up for momentum and kinetic energy. However, in the early solar nebula, crowded with objects from the protoplanetary disk as well interlopers ejected from the disks of nearby forming stars, the opportunities may be more likely.

    Debris from other stars in our own molecular cloud/open cluster may very well be trapped in our system now, and their chemical compositions and isotopic abundances may very well be similar to our own solar system, Still, they will not be identical. Massive stars will quickly evolve and return their metals to the surrounding cloud medium, so older stars like the sun, evolving later, will be chemically different. It has always been assumed that all the stars making up an open cluster, having evolved from the same cloud at ROUGHLY the same time, will all have the same chemistry, but sunlike stars and dwarfs will perhaps be born later, and incorporate the products of nucleogenesis from bright giants that quickly evolved through the supernova or planetary nebula* phase.

    It is not inconceivable that these Centaurs condensed from the nebulae of Sol’s siblings in the original birth cluster and were captured, and that their composition may give some insight into the chemistry of the Sun’s birth cloud. But its the objects from totally different birth nebulae, like ‘Ouamuamua, that will have the real surprises for us.

    *For the benefit of non-astronomers, “planetary nebulae” have nothing to do with planets, but are the outgassing of matter from dying stars. They are called such because through the telescope, they resemble little planetary discs.

  • Michael Fidler July 22, 2020, 12:23

    Take a close look at this image. It illustrates the the movement of the Sun and Earth’s movement thru galaxy.

    Now what is interesting is that the Sun north pole is some 30 degrees to the galactic plane and pointing in the direction of the constellation of Draco. We move at a speed of some 1/2 million miles an hour with the sun thru the galaxy! Do not worry, everything else in the galaxy is moving at a similar speed, but as we cross thru sections of the galaxy many of those objects are at different speed compared to the Sun.
    So here is the point, we should be seeing a rain of material in the form of micrometeorites to large comets passing us and the most common object that can be observed on earth are meteoroid trails. Since these objects come from outside the solar system the trails may show up as odd angles without a definite meteor radiant point. They should however show a general trend in this angle direction as the earth passes thru them. We have plenty of data on such objects, going back at least a hundred years. A good AI program will have no problem seeing these extraterrestrial meteoroids entering the earth’s atmosphere and be able to compile a definite database showing their origins.

    This angle could also show in some of the larger objects that may have been captured by the sun!
    We need to start looking for the extraterrestrial rain! ;-}

  • wdk July 22, 2020, 12:37

    This discussion is very interesting. And it is an illustration of how our perspectives of the solar system, planets and stars evolves over the decades.

    Back in the 1970s when Chiron 2060 was first introduced to the world as “Kowal’s Object”, an enigmatic asteroid, comet or planet between Saturn and Uranus, it was mainly considered as one of the more remarkable discoveries of Charles Kowal during a stint at Palomar Observatory. I don’t recall people speaking much of “minor planets” and then its discovery was the initial production run for “Centaurs”. Plus, astronomers considering interstellar matter: you had an uphill battle proving any comets or asteroids had wandered into the solar system. Until proven otherwise, and odds of missing, solar exceptionalism. A “minor planet”? Indeed! In the intervening years we have had exoplanet observations galore and ‘Oumuamua.

    Kowal’s initial diameter estimates were several times larger than those now maintained. That seems to suggest he intuited a darker object than what was later considered an oversized comet. The Kuiper Belt probably has many such members. Same with the Oort Cloud perhaps?

    So the problem remains, Kuiper, Oort or interstellar origin: How could it be captured into a solar system orbit?

    Well, in the case of satellite systems, we have a number of examples of large and small satellites captured by planets. Triton is the most remarkable example. Evidence points to a third body there. Phobos and Deimos must have been minding their own business before they were captured by Mars… Did they disintegrate from a larger body? And then, of course, we have our own Moon. Either it got captured or its origin is based on a third body that conveniently disappeared.

    Since the Centaurs, including Chiron, are highly inclined, yet captured,
    low relative motion on solar system entry would help. And a doughnut region rather than a thin belt would help for a local origin. But capture from a close stellar passage – I don’t think that should be ruled out. And for that possible origin, any type of assay would be highly interesting. If Psyche is worth inspection, then Chiron should be worth a visit too.

  • Ron S. July 22, 2020, 21:05

    “How could it be captured into a solar system orbit?”

    If my understanding is correct it should not be too difficult during the early life of the solar system. Since other stellar systems formed from the same nebula have low relative velocity to each other an object getting a small “kick” from one system would have only a slightly hyperbolic orbit in a sister system. With an extant accretion disk of even low density there should be enough friction for the hyperbolic orbit of a small object to close into an ellipse of high eccentricity. More interactions until the loose disk material thins out could tighten and circularize the orbit.

  • wdk July 23, 2020, 11:48

    Ron S.,
    Sounds like you are saying ( as no doubt others) that the most likely
    transit was in the earliest days and stellar neighbors had low relative velocities and close proximity. I would still have hope for a later passage since the constituents might be more distinctive ( e.g., something like Barnard’s Star which is still approaching – but with bettter acuracy).
    That way there would be more separate history – FWIW.

  • sanayadevendra November 8, 2020, 11:53

    The possibility of a truly foreign body (like ‘Oumuamua) being captured by the sun are highly unlikely, it is almost dynamically impossible.

  • ljk January 27, 2021, 12:58

    This map shows the orbits of more than 18000 asteroids in the solar system. This includes everything we know of that’s over 10km in diameter – about 10000 asteroids – as well as 8000 randomized objects of unknown size.

    Each asteroid is shown at its position on New Years’ Eve 1999, colored by type of asteroid.