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Identifying Asteroids from Other Stars

Objects of interstellar origin in our own Solar System continue to draw attention. Comets from other stars like 2I/Borisov give us the chance to delve into the composition of different stellar systems, while the odd ‘Oumuamua still puzzles astronomers. Comet? Asteroid?

Now we have a paper from Fathi Namouni (Observatoire de la Côte d’Azur, France) and Maria Helena Morais (Universidade Estadual Paulista, Brazil) targeting what the duo believe to be a population of asteroids captured from other stars in the distant past. Published in Monthly Notices of the Royal Astronomical Society, the paper relies on a high-resolution statistical search for stable orbits, ‘unwinding’ these orbits back in time to explain the location of certain Centaurs, asteroids moving perpendicular to the orbital plane of the planets and other asteroids.

Centaurs, most of which do not occupy such extreme positions, are a population of asteroids moving between the outer planets in what have until now been considered unstable orbits. The ones Namouni and Morais focus on are not recent interstellar ‘interlopers’ like ‘Oumuamua and Borisov, but objects that may have been drawn into the Sun’s gravitational pull at a time when the Solar System was still in formation, some 4.5 billion years ago. In those days, the Sun would have been part of a star cluster, with many young stars in close proximity. The authors believe they can identify 19 asteroids that once orbited other stars and now orbit the Sun.

Image: A stellar nursery in the Lobster Nebula (NGC6357), where star systems exchange asteroids as our Solar System is thought to have done 4.5 billion years ago. Credit: ESO / VVV Survey / D. Minniti. Acknowledgement: Ignacio Toledo (CC BY 4.0).

The authors delve into the dynamical evolution of Centaurs through statistical searches for stable orbits, to find out whether any could have survived from the days of the Solar System’s formation. In 2018, Namouni and Morais produced a paper using these methods that led to the identification of a retrograde co-orbital asteroid of Jupiter called (514107) Ka‘epaoka‘awela as an object of likely interstellar origin. The new study extends their simulations to the past orbits of 17 high-inclination Centaurs and two trans-Neptunian objects (2008 KV42 and (471325) 2011 KT19) with polar orbits. From the paper:

The statistical distributions show that their orbits were nearly polar 4.5 Gyr in the past, and were located in the scattered disc and inner Oort cloud regions. Early polar inclinations cannot be accounted for by current Solar System formation theory as the early planetesimal system must have been nearly flat in order to explain the low-inclination asteroid and Kuiper belts. Furthermore, the early scattered disc and inner Oort cloud regions are believed to have been devoid of Solar system material as the planetesimal disc could not have extended far beyond Neptune’s current orbit in order to halt the planet’s outward migration. The nearly polar orbits of high-inclination Centaurs 4.5 Gyr in the past therefore indicate their probable early capture from the interstellar medium.

The conclusion that these high-inclination Centaurs had polar inclinations at the time of the Solar System’s formation can be tested by further observations to firm up their orbits in comparison to the simulated results. It’s a natural leap from that possibility to the Sun’s birth cluster of stars, which would have supplied plentiful source material in the form of asteroids and comets available for capture. Thus the idea that all Centaurs are on unstable orbits is contradicted by 4.5 billion year orbits for high-inclination Centaurs, as well as some lower-inclination Centaurs like Chiron, which the authors also factored into their computations:

Either Chiron is an outlier that belonged to the planetesimal disc and whose cometary activity by some unknown mechanism increased its inclination far above the planetesimal disc’s mid-plane, or it could be itself of interstellar origin. Asteroid capture in the Sun’s birth cluster does not necessarily favour objects whose orbits have or evolve to polar or high-inclination retrograde motion (Hands et al. 2019). An astronomical illustration of the principle may be found in the distribution of the irregular satellites of the giant planets. Applying the high-resolution statistical stable orbit search to low-inclination Centaurs is likely to shed light on the possible common capture events that occurred in the early Solar system.

The paper is Namouni et al., “An Interstellar Origin for High-Inclination Centaurs,” Monthly Notices of the Royal Astronomical Society Volume 494, Issue 2 (May 2020), pp. 2191–2199. Abstract / preprint. And on the interesting question of Jupiter’s retrograde co-orbital asteroid and its possible interstellar origins, the paper is Namouni and Morais, “An interstellar origin for Jupiter’s retrograde co-orbital asteroid,” Monthly Notices of the Royal Astronomical Society: Letters 21 May 2018 (abstract).

{ 11 comments… add one }
  • Michael Fidler April 27, 2020, 11:52

    Well the press says that alien asteroids are actually living in our solar system!!!

    Astronomers find ‘alien’ asteroids living in our solar system.

    Alien residents.
    Morais also identified an interstellar “immigrant” living in our solar system in 2018.

    Hope they have their green card or they might get ICED… :-0

    • Patient Observer April 27, 2020, 22:04

      There is a tendency in popular press astronomy articles to anthropomorphize events – “angry star harasses neighboring planets”, “teenage stars act up in stellar nursery”, etc. It is often worse than that as the facts are often presented inaccurately. Sometimes, astronomers themselves contribute to this weirdness in an effort to offer explanations comprehensible or relatable to the layman.

      • Michael Fidler April 28, 2020, 9:23

        Tell me about it, I started reading Sky & Telescope back in the 60’s and it was all was nice and informative. The MTV generation of teenagers are sitting around bored out of their mind so looking for the aliens gets their attention. Back then it was once a month, now new enigmas once an hour, to read thru it you just jump to last paragraph to find any information. Spoon feeding them is great but sure is monotonous.

  • Robin Datta April 27, 2020, 14:20

    If such (?exo-) asteroids “just pass through” the solar system often enough, they might be used like “beasts of burden” to carry instrument systems or even humans in suspended animation or as generation ships.

    One of many questions to arise is whether (and how) to steer it.

    • James Jason Wentworth April 29, 2020, 5:10

      It’s a time-honored idea. Dr. Shepherd’s proposed ion-drive “slow-boat” multi-generation starship (his paper appeared in JBIS), and Larry Niven’s proposed (non-fictional) Ark II “seedship” starship, both had modified asteroid hulls, to protect the interior from radiation and dust impacts. Also:

      Niven’s Ark II proposal called for placing water tanks at the selected asteroid’s center, rotating it, and slowly melting it using a plastic film solar mirror (essentially a station-keeping solar sail), so that the water–while evaporating–would slowly “inflate” it. Afterward, docking ports, antennas, and nuclear engines would be installed, and the interior would be outfitted to accommodate the human, animal, plant, and microbe “passengers,” who/which would be brought through to birth at appropriate times before “starfall.”

  • charlie April 27, 2020, 14:51

    How can we have any confidence in these particular calculations that backtrack Orbits into billions of years ago when there is been a considerable quantity of chaotic behavior in the various planets as well as other material bodies in our own solar system and then we make pronouncements that we have the so called Centaur asteroids having been projected to be from such and such a given star system? We seem to be stretching the issue quite a bit and this makes me wonder as to how confident we can be in these calculational results.

  • Michael Fidler April 28, 2020, 9:51

    One point that could be creating a whole new generation of alien interlopers, stars coming close to solar system:

    How a Passing Star 70,000 Years Ago Changed the Solar System.

    Nearby Star Is On A Collision Course With Our Solar System.

    We now have a new comet that is brightening fast, magnitude 5.9 as of today with a beautiful tail, Comet C/2020 F8 SWAN.

    In the morning sky but further south will be coming north in the next week.

  • Alex Tolley April 28, 2020, 11:48

    I tend to agree with charlie. The best I would hope for is that these Centaurs can be targeted as of interest for sampling missions to determine if they are really from another system or not. Hopefully, isotopic analysis would establish this. One really wants multiple, orthogonal analyses to establish a truth, rather than just one analysis that is suggestive.

    I recall a few years ago an analysis of gene duplication rates indicated that the first gene must have appeared around teh age of the universe, far longer than the age of the Earth. This supposedly meant that life must have originated elsewhere. This was a very bold assertion, that ignored likely errors in the analysis, and was unsupported by other observations.

    This orbital analysis strikes me as a rather similar issue. I suppose the good news is that if Centaurs are extrasolar, they are going to be a lot easier to reach than chasing after comets on hyperbolic orbits out of our system.

  • James Jason Wentworth April 29, 2020, 5:49

    A question about this sentence in the fourth paragraph (“In 2018, Namouni and Morais produced a paper using these methods that led to the identification of a retrograde co-orbital asteroid of Jupiter called (514107) Ka‘epaoka‘awela as an object of likely interstellar origin.”):

    Retrograde co-orbital…does this mean that Ka‘epaoka‘awela (Chiron, Chariklo, Nessus, Pholus, et al are Haoles :-) ) moves in Jupiter’s orbit–or very nearly so, like Saturn’s periodically “orbit-swapping” moons Janus and Epimetheus–but *backwards*? If so, how could it have avoided colliding with Jupiter or one of its moons for so long? Also:

    Since velocities of only about 1 km/s are sufficient for objects in a natal star cluster (what will later become an open cluster, after they’ve drifted farther apart) to move between its stars in the cluster’s “early days,” then if the dynamics permit it (they seem to, so their orbital analyses say), some or even all of those 19 Centaurs could conceivably be from one of our Sun’s siblings, and:

    If so, *that* would be worth developing a space-based launching laser and lightsail probes for, to facilitate quicker trips than chemical rocket propulsion could provide–and with braking at the targets (and indefinite multi-target rendezvous capability, since there would be no onboard fuel)–to analyse those Centaurs’ chemical compositions, isotope (including radioisotope) ratios, densities, photometric and polarization properties, and other parameters.

  • James Jason Wentworth April 29, 2020, 5:57

    Some years ago, there were articles saying that Halley’s Comet and Comet McNaught were likely from other stars, because their isotope ratios were different from those of other comets. It was either confirmed, or was disproved by later observations, but I don’t recall seeing any articles to either effect.

  • andy June 9, 2020, 3:18

    A new paper on this subject on the arXiv: Morbidelli et al., “No evidence for interstellar planetesimals trapped in the Solar System


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