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An Origin for a Far Traveling Asteroid

I used to think the Kuiper Belt object Quaoar was hard to pronounce (“Kwawar”), and even muffed it despite having plenty of time to practice before the recent Tennessee Valley Interstellar Workshop. Pontus Brandt (JHU/APL) had mentioned Quaoar in his talk in Huntsville as a target that lined up in useful ways with a proposed interstellar precursor mission he was presenting, one designed to examine dust distribution from within the system by looking back at our heliosphere at distances up to 1000 AU, seeing it as we see other stars’ dust environments.

So I summarized Brandt’s ideas in my wrap-up talk and couldn’t get Quaoar pronounced properly without multiple tries. But even Quaoar pales into the realm of everyday lingo when compared to 1I/’Oumuamua. Please tell me how to do this. The word is a Hawaiian term for ‘scout,’ and the Ulukau: Hawaiian Electronic Library’s online dictionary tells me it’s pronounced this way: ō’u-mu’-a-mu’-ă. I could work with that and maybe get it right in a talk, with extra practice. At least until I look at it — all those vowels defeat me.

The object originally tagged A/2017 U1, then, is now tagged as interstellar in the combined 1I/’Oumuamua, the 1 indicating it is the first such object to be observed, the I indicating interstellar. It is fitting that 1I/’Oumuamua was the name chosen by the Pan-STARRS team in Hawaii that first brought this object to our attention.

Image: The trajectory of 1I/ ‘Oumuamua, which made its closest approach to Earth on October 14, coming within 24,000,000 km, or about 60 times the distance to the moon. Credit: NASA/JPL-Caltech.

As far as the sheer number of objects making long interstellar journeys, consider what New Horizons PI Alan Stern recently told Astronomy Magazine (see The First Known Interstellar Interloper):

According to Stern, Jupiter, Saturn, Uranus, and Neptune combined probably ejected 1013 to 1014 objects larger than 1 km early in our solar system’s history, when it was still cluttered with debris left over from the planet-formation process. Multiply that by the 1011 stars in the Milky Way, and one comes up with numbers like 1024 to 1025 objects larger than a kilometer. Smaller objects like 1I/‘Oumuamua must be orders of magnitude more plentiful.

We’ve just seen, in the work of members of the Initiative for Interstellar Studies, how reaching such an object might be attempted. Now the question becomes, where did it come from? On that score, we have quick work indeed from Eric Gaidos (University of Hawaii at Manoa), working with Jonathan Williams at the same institution and Adam Kraus (University of Texas at Austin). The trio believes it has traced 1I/‘Oumuamua’s origins and presents its case in a paper submitted to Research Notes of the American Astronomical Society.

Gaidos and team traced the path of 1I/’Oumuamua backwards along its route and took into account the numerous variables along the way in so extended a journey. The likelihood, the researchers believe, is that the object originated in a nearby young stellar cluster. They point to the Carina and Columba Associations, the word ‘association’ referring to stellar associations, which are loosely bound star clusters whose stars share a common origin. They still move together through space but are at this point gravitationally unbound. The estimated distances to the Carina and Columba Associations range from 50 to 85 parsecs (163 to 277 light years); the age of stars within these groups is on the order of 45 million years. An object ejected at 1-2 kilometers per second soon after star formation would thus have had time to reach the Sun.

From the paper:

We suggest that A/2017 U1 formed in a protoplanetary disk in the Carina/Columba associations and was ejected by a planet ≈40 Myr ago. The absence of ice indicates an origin inside the “ice line” of the disk plus an ejection velocity of 1-2 km sec−1 (assuming the cluster was already unbound), constrain the mass mP and semi-major axis aP of the planet.

What kind of planet could have ejected this object? The paper examines the issue both for solar mass stars as well as M-dwarfs:

Permitted values (grey zone) center around a 20-30M⊕ planet forming by a few Myr within a few AU, reminiscent of the core accretion scenario for giant planet formation. In contrast, a “super-Earth” at ∼1 AU could eject ice-free planetesimals from a lower-mass M dwarf.

Image: This is half of the paper’s Figure 1, showing only the projection from a protoplanetary disk around a solar mass star. The paper’s second chart, not shown here, gives equivalent information for an M-dwarf. The caption continues: “Below the red line planets accrete rather than scatter planetesimals. Above the green line planets eject planetesimals at > 1 km sec−1. Below the purple line planetesimals are captured into clouds by the cluster tide. Below the black line planetesimals require > 10 Myr to escape. To the right of the blue lines planetesimals contain ices.” Credit: Gaidos, Williams & Kraus.

As the paper notes, future interstellar interlopers may well have radiants similar to 1I/’Oumuamua. Practicing our skills on this celestial wanderer may thus tune us up for another.

The paper is Gaidos, Williams and Kraus, “Origin of Interstellar Object A/2017 U1 in a Nearby Young Stellar Association?” submitted to Research Notes of the American Astronomical Society (abstract). Our interstellar wanderer seems to be spawning a growth industry in these early days following its detection. See also Zwart et al., “The origin of interstellar asteroidal objects like 1I/2017 U1,” submitted to Monthly Notices of the Royal Astronomical Society (preprint). I haven’t read this one yet and thus won’t comment.

{ 7 comments… add one }
  • andy November 14, 2017, 13:20

    Regarding the name and spelling, Wikipedia gives the following:

    The common name ʻOumuamua was chosen by the Pan-STARRS team. The name is Hawaiian in origin (ʻou means reach out for, and mua, with the second mua placing emphasis, means first, in advance of), and reflects the nature of the object as a “scout” or “messenger” from the past. The first character is not an apostrophe, but a Hawaiian ʻokina.

    I’m also trying to figure out the pronunciation, going by this page, it starts with a glottal stop, and it seems like the “ou” is somewhat like the word “owe” (but without the “w” sound at the end which you usually have in English) – I have seen some conflicting information about that so it would be good to hear someone who actually speaks the language pronounce it!

  • ljk November 15, 2017, 10:45

    Update on `Oumuamua, Our First Interstellar Object

    By: Kelly Beatty | November 10, 2017

    The arrival of an object from beyond the solar system caught astronomers by surprise — but that doesn’t mean that they were unprepared to study it.


    To quote:

    But asteroids are not catalogued by name alone, and this is the first of an entirely new class of object. So, at the suggestion of MPC associate director Gareth Williams, the IAU adopted the identifier “I”, for interstellar. “All parties involved in the discussion agreed that the proposal was suitable,” he says. “Took less than a day to sort out, announcement followed the day after.” As noted in the announcement on MPEC 2017-V17, “Correct forms for referring to this object are therefore: 1I; 1I/2017 U1; 1I/`Oumuamua; and 1I/2017 U1 (`Oumuamua).”

    Those who work the comet and asteroid trade aren’t enamored with the use of “I” — after all, the IAU has gone out of its way to avoid using “I” and “O” in designations because they look too much like the numerals “1” and “0”. Several have suggested substituting “E” (for extrasolar) instead, but Williams defends the IAU’s decision, noting that “E” could be interpreted as “extraterrestrial,” with the implication of intelligence behind the object, and “G” (for “galactic”) is too limiting.

    A Moving Target

    The first characteristic that marks `Oumuamua as being from outside our solar system is its orbital eccentricity, 1.19. If you ever took geometry, you might recall that the eccentricity of an ellipse can’t exceed 1.0. In fact, dynamicist Bill Gray (Project Pluto) regrets that he didn’t pick up on 1I’s interstellar nature a few days sooner — his orbit-calculation software kept rejecting the eventual solution as being impossible.

  • Michael Fidler November 16, 2017, 10:28

    APO Time Resolved Color Photometry of Highly-Elongated Interstellar Object 1I/’Oumuamua.

    “We report on g, r and i band observations of the Interstellar Object ‘Oumuamua (1I) taken on 2017 October 29 from 04:28 to 08:40 UTC by the Apache Point Observatory (APO) 3.5m telescope’s ARCTIC camera. We find that 1I’s colors are g−r=0.41±0.24 and r−i=0.23±0.25, consistent with the visible spectra of Masiero 2017, Ye et al. 2017 and Fitzsimmons et al. 2017, and most comparable to the population of Solar System C/D asteroids, Trojans, or comets. We find no evidence of any cometary activity at a heliocentric distance of 1.46 au, approximately 1.5 months after 1I’s closest approach distance to the Sun. Significant brightness variability was seen in the r observations, with the object becoming notably brighter towards the end of the run. By combining our APO photometric time series data with the Discovery Channel Telescope (DCT) data of Knight et al. 2017, taken 20 h later on 2017 October 30, we construct an almost complete light curve with a most probable lightcurve period of P≃4 h, and reveal the most useful locations during 1I’s rotation phase for additional data to improve constraints on its lightcurve. Our results imply a double peaked rotation period of 8.1 ± 0.02 h, with a peak-to-peak amplitude of 1.5 – 2.1 mags. Assuming that 1I’s shape can be approximated by an ellipsoid, the amplitude constraint implies that 1I has an axial ratio of 4.1 to 6.9, which is strikingly elongated. Assuming that 1I is rotating below its critical break up limit, our results are compatible with 1I having a density > 1.0 g cm−3, or having modest cohesive strength. Astrometry useful for constraining 1I’s orbit was also obtained and published in Weaver et al. 2017.”


    • Michael Fidler November 20, 2017, 10:19

      On the dynamical history of the recently discovered interstellar object
      A/2017 U1 – where does it come from?

      Another where did it come from, is that four or five? Anyway this time something a little more interesting, seems one of are neighbors may be throwing rocks at us!

      “A/2017 U1 is the first interstellar object recorded inside the Solar System. We try to answer the obtrusive question: where does it
      come from? After a careful search in the close vicinity of the Sun we looked a bit further. Finally, we have checked over 200 thousand
      stars and found just a handful of candidates. If we limit our investigation to the Sun surroundings within about 30 pc, the most probable candidate for the A/2017 U1 parent stellar habitat is a star UCAC4 535-065571 but GJ 876 cannot be completely ruled out. However, the origin of A/2017 U1 from a more distant source is still an open question.”



  • Harry R Ray November 20, 2017, 10:33

    There is an OUTSIDE CHANCE that Oumuamua originated in the Gliese 876 system. “On the dynamical history of recently discovered interstellar object A/2017 U1 – where does it come from?” by Pitor A Dybczynski, Malgorzata Krolikowska.

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