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Is Asteroid 2015 BZ509 from another Stellar System?

It’s conceivable that getting humans to an interstellar object may not involve journeying all the way to another star. We’ve learned that wandering asteroids and comets move between stars, as the case of ‘Oumuamua demonstrated, and early research offers the possibility that such objects exist in large numbers. Now we have (514107) 2015 BZ509, which is conceivably an interloper into our system of another sort. Two researchers believe that this asteroid near the orbit of Jupiter is not just passing through, but a captured object from another stellar system.

A comparison with Triton seems apt. One of the most compelling pieces of evidence that Neptune’s largest moon is actually a Kuiper Belt Object is its retrograde orbit. We see the same thing with 2015 BZ509, and for Fathi Namouni (Observatoire de la Côte d’Azur) and Helena Morais (Universidade Estadual Paulista), that sends a clear message. The researchers have offered their work in a new paper in Monthly Notices of the Royal Astronomical Society.

“How the asteroid came to move in this way while sharing Jupiter’s orbit has until now been a mystery,” explains lead author Namouni. “If 2015 BZ509 were a native of our system, it should have had the same original direction as all of the other planets and asteroids, inherited from the cloud of gas and dust that formed them.”

Image: Images of 2015 BZ509 obtained at the Large Binocular Telescope Observatory (LBTO) that established its retrograde co-orbital nature. The bright stars and the asteroid (circled in yellow) appear black and the sky white in this negative image. Credit: C. Veillet / Large Binocular Telescope Observatory.

Namouni and Morais used computer simulations to track the errant asteroid back in time, arguing that 2015 BZ509 has moved this way since the birth of the Solar System some 4.5 billion years ago, an indication that it could not have formed there originally. The case also relies on the fact that the Sun formed in a tightly packed star cluster where movement of objects ejected by gravitational forces within their own system into orbits around other stars would not have been uncommon. Thus this rogue asteroid may well contain information about planet formation and evolution as well as telling us more about the Sun’s original siblings.

So what exactly do we know about this object? 2015 BZ509 is in a resonant, co-orbital motion with Jupiter and represents the first discovery of a retrograde co-orbital asteroid with Jupiter or any other planet. Its orbital eccentricity of 0.3805 takes it inside and then outside of Jupiter’s orbit at its closest approaches (176 million kilometers). The orbital period is 11.65 years and the inclination is 163 degrees, an evidently stable orbit if a complicated one.

Image: Stellar nursery NGC 604 (NASA/HST), where star systems are closely packed and asteroid exchange is thought to be possible. Asteroid (514107) 2015 BZ 509 may have emigrated from its parent star and settled around the Sun in a similar environment. Credit: NASA / Hubble Heritage Team (AURA/STScI).

I’ve long speculated in these pages about interstellar missions that operate not by making fast jumps to other stellar systems but by moving incrementally over thousands of years, taking advantage of the fact that the Oort Cloud extends tens of thousands of AU, if not more, from the Sun. If similar debris were found around nearby stars, a patient species evolving new technologies and biologies as it progressed could simply ‘walk across’ to the next system.

Now we’re learning that we may be able to study interstellar materials without going nearly as far. We’re nowhere near ready to send humans into Jupiter space, but if this work is confirmed, then targets like (514107) 2015 BZ 509 will inevitably receive future visitors. We have so much to learn about how common objects like these are, but the researchers believe the number could be high.

In the passage from the paper below, the term ‘polar corridor’ refers to objects pushed into polar inclinations all the way out to the Oort Cloud. The ‘clones’ refer to the computer simulations Namuni and Marais conducted, simulating the evolution of one million clones of 2015 BZ 509:

The one million clone simulation provides further evidence that there are currently more extrasolar asteroids in the Solar system. In effect, if more objects were captured along [with] 2015 BZ509 by Jupiter early in the Solar system’s history, the less stable orbits must have left the co-orbital region by way of chaotic diffusion into the polar corridor. This occurs because the N-body problem is time-reversible and unstable clones of 2015 BZ509 that are followed into the future exit the co-orbital region and end up in the polar corridor. The prominent presence of the polar corridor in the simulation over the age of the Solar System mainly in the trans-Neptunian region implies that it is currently populated by extrasolar asteroids.

Make no mistake, most of the digital clones the researchers put into motion fell victim to gravitational forces that would have prevented the kind of orbital resonance we see with this object. But a few did remain in stable configurations all the way back to the earliest days of the Solar System. How strong a case this makes for an interstellar origin is still in doubt. Scott Tramaine (Institute for Advanced Study), for example, questions whether gravitational nudges coupled with a collision could account for 2015 BZ509, with no interstellar involvement at all.

For more on Tremaine’s doubts and a look at the possible involvement of the still undiscovered Planet Nine, see Lee Billings’ essay in Scientific American. Billings also speculates about robotic mission possibilities. It’s clear that 2015 BZ509 is going to remain newsworthy — and controversial — as we try to build our census of interstellar objects near the Sun.

The paper is Namouni & H. Morais, “An interstellar origin for Jupiter’s retrograde co-orbital asteroid,” Monthly Notices of the Royal Astronomical Society: Letters (2018). Abstract.


{ 44 comments… add one }
  • Cira Goettig May 22, 2018, 11:25

    Surely you meant “the Oort cloud extends tens of thousands of AU, if not more” , not tens of thousands of light years?

    • Paul Gilster May 22, 2018, 11:36

      Yes indeed, a sloppy typo, and thank you for finding it early. I’ve just made the correction.

  • djlactin May 22, 2018, 12:17

    Could the resonance survive the migrations of planets that are purported to have occurred as the solar system evolved?

  • Alex Tolley May 22, 2018, 13:00

    What evidence would clinch its origin outside our system? Isotopic composition? Something else?

    • Tony Mach May 23, 2018, 12:33

      I guess isotopic composition is one good method (if we can get hold of some samples…).

      Another would be to determine the age – if it formed before our Solar System, well then…

  • ljk May 22, 2018, 15:00

    If we are going to be sending space probes specifically to examine unusual objects circling the outer planets, then we must include Methone on that list:


    • Charley May 22, 2018, 17:02

      May 2012, the Cassini spacecraft obtained its first close-up photographs of Methone, revealing an egg-shaped moonlet with a remarkably smooth surface, with no visible craters.[9] The moons Pallene and Aegaeon are thought to be similarly smooth.[10] Methone has two different sharply defined albedo regions, one distinctly (~13%) darker centered on Methone’s leading point.[4] It brighter area has an albedo of ~0.70.[4] UV and IR spectra gave no indication of a color difference between the two regions, suggesting that a physical rather than compositional difference may be responsible.[4] Increased exposure to electrons from Saturn’s magnetosphere has been proposed to be responsible for thermal anomalies on the leading hemispheres of Mimas and Tethys,[11] and a similar irradiation anisotropy might be behind Methone’s albedo pattern.[4]

      Methone’s mean radius is 1.45±0.03 km.[4]

      Assuming that Methone is in hydrostatic equilibrium, i.e. that its elongated shape simply reflects the balance between the tidal force exerted by Saturn and Methone’s gravity, its density can be estimated: 0.31+0.05
      −0.03 g/cm3, among the lowest density values obtained or inferred for a Solar System body. This indicates that Methone is composed of icy fluff, material that might be mobile enough to explain the lack of craters

    • J. Jason Wentworth May 23, 2018, 8:44

      A publicly-popular mission might be called (informally) “Methone and Friends”; it could be a small Saturn orbiter–possibly powered by a small RTG/rechargeable batteries system–that would make close approaches to Methone and the other small, anomalously-rounded moons in its neighborhood. It could even land on them, due to their utterly negligible gravitational fields (the “ski pole-type” landing gear of the DSI and/or Planetary Resources asteroid mineral assaying/surveying landers would be more than adequate). Also, it could ease into the rings (or orbit just above them) and examine individual ring particles–the pictures taken from just above that “plain” of ice chunks, with Saturn and some moons in the background, would be breathtaking!

      • ljk May 23, 2018, 11:47

        In the 1990s I envisioned a probe in the shape and size of a soccer ball that could dropped into the rings of Saturn, where it would literally bounce off the debris, sampling and imaging them along the way.

        • J. Jason Wentworth May 24, 2018, 0:13

          Using an electronic architecture like that of researcher Mark Tilden’s insect-like robots (containing analog circuits that demonstrated self-organization—and even learning and memory—even though these devices contain fewer than ten transistors and have no computers), such “ball probes” could be quite small, so that a Saturn orbiter could carry multiple probes. They could explore in “swarm fashion,” examining the rings and their “spokes”; the differences between the different-colored rings would be very interesting to see–and sample–at such close range.

        • Patient Observer May 24, 2018, 20:39

          The view from within Saturn’s rings has always been fascinating for me. With a purported thickness of less than 100 feet (incredible), with enough density to block a significant amount of light and particle sizes from inches to feet, it has a decidedly human scale. I wonder about the relative motion of the particles – would it be inches per second or more chaotic with particles bouncing around albeit slowly? Would the particles be glass-smooth or collision shards with sharp edges? The science may not be much but the visuals of a probe that orbited within the rings would be stunning.

          • J. Jason Wentworth May 25, 2018, 0:46

            The science could be quite substantial (the latest estimate is that the rings may be just 10 meters–33 feet–thick!). The August 1970 issue of “National Geographic” (see: http://www.amazon.com/National-Geographic-Magazine-August-Vol-138/dp/B000QMZF0M ) contains an article, “Voyage to the Planets” (it covers the Pioneer 10/11 and Grand Tour missions), that includes paintings of the “thick-ring” (closely-spaced ice chunks) and “thin-ring” (ice cylinders worn from bumping and rolling against each other) models of Saturn’s rings, and:

            It is possible that at different locations (“bands”) within the rings, both models–or perhaps a combination of them–are correct, somewhere (I suspect that the particle sizes vary at different locations [distances from Saturn] within the ring system). Looking along the thickness of the rings, they may look rather like the band of the Milky Way (in some places, the particles might even be tiny enough to resemble stars, although up to car-size ice boulders are also expected). The particles’ relative motions–which are probably influenced by electrostatic and/or magnetic forces [like the elevated “spokes”], as well as by the moons’ interacting gravitational tugs–would be well worth studying scientifically, as well as for what we would see…

            • ljk May 25, 2018, 9:12

              Saturn’s ring particles average about three feet across with some going up to the size of a house. After that you start getting into moonlet and moon territory.

              • J. Jason Wentworth May 25, 2018, 21:31

                I hadn’t seen “house-sized” mentioned for some years (but my reading about studies of the ring system aren’t all-inclusive, either). In situ ring system studies will hopefully be objectives of at least some future outer planet orbiter missions. Comparisons between them would be useful (Jupiter’s gossamer ring’s particles appear to be very fine, about the size of cigarette smoke particles, while Uranus’s and Neptune’s rings, while narrower, seem more Saturn-like in terms of particle sizes), and:

                You’ve just brought up a “quietly brooding in the dark” potential nomenclature problem: What constitutes a moonlet, and what differentiates a moonlet from a ring particle? An ice chunk the size of Phobos or Deimos in a ring system would definitely be called a moonlet, BUT:

                It seems that within a certain size range, one can say of a moonlet, “I know one when I see one” (rather like the Supreme Court’s definition of obscene pornography… :-) ). A ~1 mile-wide object in a ring system is also a moonlet, but what if it’s orbiting a much smaller primary body (like the ~4,600′ wide Dactyl, orbiting the ~19.5 mile asteroid Ida)? In that situation, Dactyl would appear to be–at that “scale”–a full-fledged moon rather than a moonlet. (Astronomy presents logophiles with simultaneously challenging and delightful problems, such as the adjectives for objects’ names. For some planets, like Venus, a simple derivation–such as “Martian” for Mars–will not do; Cytherean denotes anything of or from Venus. Some moons’ names, like Pan, Ananke, Nereid, and so forth, are even more fun…)

    • J. Jason Wentworth May 23, 2018, 9:10

      These oddball asteroids in/near Jupiter’s orbit (plus Hidalgo also has an unusual orbit, and then there’s Chiron and Pholus) could be explored using a series of Pioneer 6 – 9 type spin-stabilized solar orbiting probes in eccentric heliocentric orbits. In addition to fields & particles instruments, they could have “push broom” spin-scan cameras, IR and UV photometers, and other “solid body examining” spacecraft instruments. For operating far from the Sun at the outer reaches of their orbits, they could have small RTGs as well as body-mounted solar cells. Just as Pioneer 7 made a (distant) “target of opportunity” flyby of Halley’s Comet in 1986 (so did ISEE-3/ICE, after its 1985 flyby of Comet Giacobini-Zinner), these Sun-orbiting probes could explore such targets of opportunity; indeed, their orbits could be selected to ensure close, relatively frequent flybys of objects on the targets list.

  • andy May 22, 2018, 15:11

    I’m with the sceptics on this one. A 1:-1 resonance doesn’t seem like something that would naturally emerge from an interstellar capture, which would more likely result in a non-resonant orbit.

    Incidentally, there are other known examples of retrograde resonant asteroids: according to Morais & Namouni (2013) the asteroids 2006 BZ8 and 2008 SO218 are in retrograde resonances with Jupiter (2/-5 and 1/-2 respectively), and 2009 QY6 is in a 2/-3 resonance with Saturn. Are these addressed in the paper?

    • Paul Gilster May 22, 2018, 17:44

      I’m with you on this claim, andy. And no, I don’t find the three asteroids you mention in the paper. Count me among the skeptics, at least based on what we have so far.

      • Harry R Ray May 23, 2018, 9:37

        Correct me if I am wrong, but I believe the reason these other three asteroids were not mentioned is that their orbits have been determined to be UNSTABLE, and that they will be EJECTED from them in roughly ten thousand years. The question I have is whether the authors took into account the total amount of collisions this object should have had STATISTICALLY over 4.5 billion years. 176 million kilometers is quite far from Jupiter, so that even its massive gravitational influence could not counteract the effects many impacts, even if they were from small objects. Even microscopic deviations from such a complicated orbit would surely over that period of time, created SUBSTANTIAL orbital instability, meaning that NO INITIAL ORBIT POSSIBLE could remain stable for 4.5 billion years! Unless, of course, it has the ability to make microscopic adjustments to its orbit to COUNTER the effects of these collisions, which, of course, would be an entirely DIFFERENT topic of conversation.

  • DCM May 22, 2018, 16:02

    We should, as soon as we can, set up a kind of semi-autonomous robot station near Pluto that will launch probes to anything that drifts by, detect signals that might be overwhelmed by the sun’s radiation at Earth and Mars distance, and even recover samples to analyze there and ship here.

    • ljk May 23, 2018, 13:49

      Just wondering why Pluto as opposed to some other place out in the KBO? Or how about free-floating?

      And then there is the nasty question of who builds and pays for this idea? I like it, don’t get me wrong, I just know how reality has a way of getting in the way of these concepts.

      • DCM May 23, 2018, 15:46

        Pluto is something we know about and know where it is at any given time.
        There usually has to be an economic or military drive in such things. Maybe people and governments can be convinced we’re actually being bombarded by interstellar asteroids — someone can claim we’re entering some kind of debris field. Or some day we might have even wealthier and more extravagant rich people than now who’ll do it as a gift to the world. At that time there’ll probably be people on Mars as well so people of the worlds.

    • J. Jason Wentworth May 24, 2018, 0:35

      Now that nanoprobes are feasible (Dr. Mason Peck’s “spacecraft-on-a-chip” Sprites), such a probe launcher is also feasible. The probes could use thin-film alphavoltaic or betavoltaic (atomic fission battery) power in the gloom of the outer solar system, and could use laser communication (Dr. Peck’s “standard” Sprites–which have flown in Earth orbit–have on-chip solar cells and use radio communication). The launcher could be a “push-pull,” lunatron/hermitron type electromagnetic launcher (like O’Neill’s mass driver), which could be powered by a Topaz-, SNAP-10A-, or Kilopower-type space-rated fission reactor.

  • Robin Datta May 22, 2018, 21:00

    The question of bits & pieces of interstellar celestial bodies, beyond the “whether” for a particular item extends to which, where, whence, when – and how. Efforts to identify and categorise that part of the flow of matter through the universe expands the view. Could a metiorite or two from another part of the galaxy, among the rocks in the wilderness be as instructive as a mission to an asteroid?

    • DCM May 24, 2018, 11:51

      I wonder if we could necessarily distinguish an interstellar wanderer from local rocks except by speed and trajectory. Unless it had been modified by some process like life wouldn’t it be chemically much the same as any lump of rock? We need to analyze bits of every piece of space flotsam we find and chart the proportions of different substances. They’d probably average out much the same with a few anomalies to whet interest.

  • Michael Fidler May 22, 2018, 21:35

    How can they tell if it has been there for 4.5 billion years or 4.5 years? I’d be interested in a 3d plot of its orbit and what or if it comes close to any other objects.

    • Michael Fidler May 22, 2018, 22:02

      What about light curve, rotation rate, spectrum and the possibility of satellite’s. If we sent probes to another planetary system, would this be a good orbit to survey the whole system and would Jupiters massive gravity field and this type of orbit be useful for changing the orbit of the probes? I’m thinking how hard would it be able to use Jupiter’s Lagrange points to travel the Lagrange highways of least energy

  • J. Jason Wentworth May 23, 2018, 8:28

    Paul Gilster wrote:

    “I’ve long speculated in these pages about interstellar missions that operate not by making fast jumps to other stellar systems but by moving incrementally over thousands of years, taking advantage of the fact that the Oort Cloud extends tens of thousands of AU, if not more, from the Sun. If similar debris were found around nearby stars, a patient species evolving new technologies and biologies as it progressed could simply ‘walk across’ to the next system.”

    To popularize this idea, I suggest a new video game that is a cross between “Asteroids” and “Frogger” (Kerbal Space Program already has a game module called “Centauri Dreams,” whose objective should be obvious… :-) ).

  • Harold Shaw May 23, 2018, 8:41

    Remnants from the first generations of star systems may be present in our solar system. I am sure others have considered this, but I just realized it is possible and am awestruck.

    • ljk May 23, 2018, 13:51

      And remnants of early Earth may also be on the Moon blasted out from impacts on our planet long ago. This may include fossils.

      • J. Jason Wentworth May 24, 2018, 0:48

        …And, just maybe–although it’s a long shot–“meteorite-transplanted microbial life.” Arthur C. Clarke and Patrick Moore both pointed out that hardy terrestrial microbes, and even some macroscopic (although small) plants, could live on the Moon, particularly in favorable micro-climates. They noted that if life ever got started on the Moon, perhaps in some long-vanished, *real* lunar sea–it may have adapted to the changing conditions and still exist there in some form. I wouldn’t bet a large sum that they’re right, but nature has surprised us many times before.

        • Michael May 24, 2018, 10:18

          There was never a Luna sea of water as it was way too hot, however some water would have been incorporated into the molten mass. This water could have made its way out towards the surface over time and got trapped in the impermeable layers like dykes and collected in pockets.



          • Alex Tolley May 24, 2018, 13:37

            It is conceivable that at some depth the temperature is a fairly constant value above freezing for in situ water allowing lithophilic microbes to exist. These would probably have come from Earth in the past. While I doubt there is any life in the mantle, it might well be worth a look at some point in the future when we have a lunar presence that can put some effort into very deep drilling.

          • J. Jason Wentworth May 25, 2018, 0:25

            Thank you for posting those article links! I was being partly figurative with regard to seas, for the reason you mentioned; such lunar “seas,” if any, would have been more like underground lakes, although probably more like discontinuous bodies of water diffused through fissures in subsurface rock, like what Alex mentioned.

  • ljk May 23, 2018, 8:57

    Is Humanity Ignoring Our First Chance For A Mission To An Oort Cloud Object?

    Ethan Siegel

    May 22, 2018 @ 10:00 AM

    In 2003, scientists discovered an object beyond Neptune that was unlike any other: Sedna. While there were larger dwarf planets beyond Neptune, and comets that would travel farther from the Sun, Sedna was unique for how far it always remained from the Sun. It always remained more than twice as distant from the Sun as Neptune was, and would achieve a maximum distance nearly 1,000 times as far as the Earth-Sun distance. And despite all that, it’s extremely large: perhaps 1,000 kilometers in diameter.

    It’s the first object we’ve ever found that might have originated from the Oort cloud. And we’ll only get two chances if we want to send a mission there: in 2033 and 2046. Right now, there isn’t even a proposed NASA mission looking at the possibility. If we do nothing, the opportunity will simply pass us by.

    Full article here:


  • jonW May 23, 2018, 10:14

    Could someone tell me: is there any special problem in rendezvousing with a body in an orbit with near-polar inclination? Compared to a body the same distance from the sun in the usual plane, it feels somehow further and more difficult to attain…

    • Tony Mach May 23, 2018, 12:45

      An possibility would be a fly-by (think New Horizons at Pluto), when the object is close to the plane of the Solar System. (which happens twice per orbit, but the orbits are very long…)

      The further the object is from the plane, the more delta-V a probe needs to reach such a object. This means bigger rocket (much bigger?) and (one or more) Saturn/Jupiter fly-bys to change the inclination of the probe.

      • J. Jason Wentworth May 24, 2018, 1:31

        Tony Mach (are you related to the famous physicist with that surname?) is right. Satellite launches made to the east–and from the equator, whenever possible–take advantage of the “free tailwind” provided by the Earth’s rotation; just sitting on its launch pad, the rocket is already moving almost 1,000 miles per hour. Polar and Sun-synchronous (near-polar, slightly-retrograde) orbits provide advantages for observing the Earth, but such launches get no velocity boost from the Earth’s rotation, and:

        Likewise, spacecraft launched to planets outside the Earth’s orbit take advantage of the Earth’s 66,000 mph orbital velocity around the Sun. When they’re heading Sunward, to Venus and/or Mercury, they reach escape velocity–from their initial parking orbits around the Earth–traveling *opposite* to the Earth’s direction orbiting the Sun. Also:

        A spacecraft aiming for a polar orbit around the Sun would get no boost from the Earth’s orbital velocity (and reaching a retrograde solar orbit from the Earth would require even more energy; only a very high-specific impulse propulsion system, such as an ion drive or a solar sail, could do this–the planned-but-cancelled U.S. rendezvous mission to Halley’s Comet, whose orbit is retrograde–would have utilized a solar sail or ion drive). In addition:

        Jupiter can be used to inject spacecraft into solar polar orbits–the Ulysses spacecraft (which was originally part of the two-probe International Solar Polar Mission–ISPM–which budget cuts pared down to one probe) conducted a Jupiter flyby which bent its trajectory southward, out of the ecliptic. Also, Pioneer 11’s Jupiter flyby reached periapsis (the closest approach) above Jupiter’s south pole, which hurled the probe into a trajectory to Saturn which cut back across the solar system, far above the Sun’s northern hemisphere.

        • jonW May 25, 2018, 23:42

          Thanks very much for this detailed reply to my question. It all makes sense.

  • Hamilton1 May 23, 2018, 11:42

    Here is the complete 2017 article by Wiegert et al in which co-orbital motion is established. I don’t see any mention of the light-curve though…


  • J. Jason Wentworth May 24, 2018, 1:45

    Thank you for posting the link to this article! It would indeed be very strange if 2015BZ509 has no detectable light curve. (This was one of the hints, in Arthur C. Clarke’s novel “Rendezvous with Rama,” that it wasn’t a natural object [a very faint light curve–due to an impact splash-type stain on Rama’s uniformly gray body–was finally detected at close range, but its indicated rotation period of only four minutes was decidedly non-natural…].) I don’t think 2015BZ509 is artificial, but at our current level of ignorance about this object, that possibility can’t yet be discounted.

  • Harry R Ray May 24, 2018, 9:33

    Now I am really confused. Wiegert et al used the Large Binocular Telescope to track 2015 BZ509, which is right up there with Keck and VLT. My big question is, what was the length, timewise, of their exposures? Maybe the did not take ANY long exposures, because they were JUST tracking it, and not even TRYING to get a spectra. To get ANY KIND OF SPECTRA(let alone a GOOD one) of a 3Km object orbiting that far away would require substantial exposure time.

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