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    March 2021: ASU scientists determine origin of strange interstellar object

    'Oumuamua, which flew past Earth in 2017, is likely a fragment of nitrogen ice from the surface of an exo-Pluto

    Research papers 1I/'Oumuamua as an N2 ice fragment of an exo‐Pluto surface I: Size and Compositional Constraints, Alan P. Jackson and Steven J. Desch

    and 1I/'Oumuamua as an N2 ice fragment of an exo‐Pluto surface II: Generation of N2 ice fragments and the origin of 'Oumuamua, Steven J, Desch and Alan P. Jackson,

    published in Journal of Geophysical Research: Planets, March 2021

    Press releases by Arizona State University and the American Geophysical Union

    This work was widely covered in the press, including in print at The Associated Press, The Guardian, CNN, and The BBC; on radio at CBS News Radio LA, and the BBC World Service; and on television at Al-Jazeera English.


    The first confirmed interstellar object, 1I/'Oumuamua was discovered on 19th October 2017 by the Pan-STARRS 1 telescope at Haleakala Observatory in Hawai'i. It had long been expected that comets of interstellar origin should exist, since we know that comets are thrown out of our own Solar system, but finding the first object of interstellar origin was nonetheless very exciting, especially so as it did not match our expectations.

    We had expected interstellar objects to be comets, since the majority of bodies ejected from the Solar system today are comets, and indeed the second interstellar object discovered, 2I/Borisov, matches this expectation. In contrast, observations of 'Oumuamua did not detect a cometary tail, giving the impression that it was an object that lacked volatile ices, more like an asteroid rather than a comet, leading many people including myself, to investigate scenarios in which a larger number of asteroid-like objects might be ejected than we believe happens in our Solar system. Additional observations later in 2018 by Micheli et al. however revealed a small deviation in the orbit of 'Oumuamua from a purely ballistic trajectory. Deviations like this are often seen for comets as a result of the gases sublimating off the surface giving them a small push away from the Sun, but since we had not detected a tail of sublimated gases from 'Oumuamua so this was puzzling.

    In addition, 'Oumuamua has an unusual shape. Initial analysis of the rotational light curve of the object suggested a cigar-shaped object around 8 times as long as it was wide. This was later refined, suggesting it was more likely a disk-shaped object about 6 times as wide as it was thick, but nonetheless this is still significantly more extreme than any known Solar system object.

    Our new work provides an explanation for all of the unusual features of 'Oumuamua as naturally arising if it is composed of nitrogen ice, a substance that is commonly found in the outer solar system on the surfaces of bodies like Pluto and Triton. A nitrogen-ice body would undergo significant erosion as it passed through the Solar system, losing around 95% of its mass. The nitrogen gas released as the ice sublimated would accurately reproduce the observed non-gravitational acceleration. Moreover, the erosion of material from the outer surface of the body leads to an increase in the axis ratio over time, much as a bar of soap is slowly worn down to a thin sliver through much use. In addition, the reflectivity at which the observed properties of 'Oumuamua are reproduced matches that observed for the surface of Pluto.


    Image credit: Painting by William K. Hartmann, based on commission from Michael Belton

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    March 2018: 'Oumuamua likely came from a binary system

    Interstellar asteroid that recently flew past Earth likely came from a system with two stars

    Note: No longer current

    Research paper Ejection of rocky and icy material from binary star systems: Implications for the origin and composition of 1I/'Oumuamua, authors Alan P. Jackson, Daniel Tamayo, Noah Hammond, Mohamad Ali-Dib, Hanno Rein

    published in Monthly Notices of the Royal Astronomical Society: Letters, March 2018

    Press releases by the University of Toronto, and the Royal Astronomical Society

    This was covered quite widely in the press, including The Associated Press, CBC News, The Guardian, and The Daily Mail


    Original content:

    Discovered on 19th October 2017 by the Pan-STARRS 1 telescope at Haleakala Observatory in Hawai'i, 'Oumuamua is an intriguing object. With an eccentricity of 1.2 and travelling at around 26 km/s relative to the Solar system it is the first confirmed body of interstellar origin to pass through the Solar system. In addition it is highly elongated, as can be seen in the artist's impression, and has displayed no detectable coma or tail-like activity, despite having passed within 0.25 AU of the Sun (within the orbit of Mercury), making it a volatile-poor asteroid rather than an ice-rich comet. It was this latter point that particularly interested me and my colleagues at the University of Toronto.

    The overwhelming majority of objects that are ejected from our own Solar system out into interstellar space are icy comets that originate from the outer regions like the Kuiper belt, not the inner parts of the Solar system where rocky asteroids are found. This is because the dominant contributors to ejecting material from the Solar system are Neptune and Jupiter, which are themselves in those outer regions. To be more efficient at ejecting material from the warm, inner regions we need a system with a different architecture to ours, one that has a large body in the inner regions. A Jupiter like planet in the inner parts of the system would work, but planet surveys have told us that Jupiter-like planets close to their parent stars are not very common. Binary stars on the other hand are common, with something like half of all stars being members of a binary or multiple star system. Moreover the companion star is there right from the very beginning of the life of the system, so material can start being ejected as soon as planetesimals form, rather than having to wait until a large planet can be formed. As such we found that material ejected from binary stars should dominate the population of interstellar bodies, especially for rocky ones like 'Oumuamua, which then means that 'Oumuamua most likely was born in a binary star system.


    Update:

    Further work later in 2018 by Micheli et al. showed that matching the trajectory of 'Oumuamua through the solar system required a comet-like non-gravitational acceleration, suggesting that 'Oumuamua might actually be a comet rather than a volatile-poor asteroid. See our more recent press release above for a more up-to-date hypothesis for this interesting object.


    Image credit: ESO/M. Kornmesser, artist's impression of 1I/'Oumuamua

Image Credit: ESO/M. Kornmesser, artists impression of 1I/'Oumuamua