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14 January, 2018

Spectroscopy and Thermal Modelling of the First Interstellar Object 1I/2017 U1 ‘Oumuamua

During the formation and evolution of the Solar System, significant numbers of cometary and asteroidal bodies were ejected into interstellar space. It is reasonable to expect that the same happened for planetary systems other than our own. Detection of such interstellar objects would allow astronomers to probe the planetesimal formation processes around other stars, possibly together with the effects of long-term exposure to the interstellar medium.

1I/2017 U1 ‘Oumuamua is the first unambiguously detected interstellar body to visit our Solar System. Planetary scientists have long expected that the interstellar object population is dominated by ice-rich small bodies like comets; however, ‘Oumuamua didn't exhibit any comet-like activity when it passed close to the Sun at the beginning of September 2017.

Astronomers using ACAM on the William Herschel Telescope (WHT) report in a paper published in Nature Astronomy, the spectroscopic characterisation of ‘Oumuamua, finding it to be variable with time but similar to organically rich surfaces found in the outer Solar System. This is consistent with predictions of an insulating mantle produced by long-term cosmic ray exposure. This coating could have protected an ice-rich interior from being vaporised during its recent passage close to the Sun. An internal icy composition cannot therefore be ruled out by the lack of activity, even though ‘Oumuamua passed within 0.25 AU of the Sun.

Interstellar object ‘Oumuamua (circled) as seen by the WHT. Background stars and galaxies appear as streaks due to the telescope following ‘Oumuamua as it moved across the sky. Credit: A. Fitzsimmons, QUB/Isaac Newton Group, La Palma. Large format: PNG. Movie: MOV.

Professor Alan Fitzsimmons (Queen's University Belfast, UK) commented: "We have discovered that the surface of ‘Oumuamua is similar to small solar system bodies that are covered in carbon-rich ices, whose structure is modified by exposure to cosmic rays. We have also found that a half-metre thick coating of organic-rich material could have protected a water-ice-rich comet-like interior from vaporising when the object was heated by the Sun, even though it was heated to over 300 degrees centigrade."

Dr Michele Bannister and her team observed ‘Oumuamua while it was still within reach of the largest telescopes in the world and their findings are being published in the Astrophysical Journal Letters. They found the object was the same colour as some of the icy minor planets they had been studying in the outskirts of our solar system. This implies that different planetary systems in our galaxy contain minor planets like our own.

Dr Michele Bannister said: "We've discovered that this is a planetesimal with a well-baked crust that looks a lot like the tiniest worlds in the outer regions of our solar system, has a greyish/red surface and is highly elongated, probably about the size and shape of the Gherkin skyscraper in London. It's fascinating that the first interstellar object discovered looks so much like a tiny world from our own home system. This suggests that the way our planets and asteroids formed has a lot of kinship to the systems around other stars."

These observations allow a better characterisation of the structure and the history of this unique object, while also shedding light on the population of interstellar objects floating around our Solar System.

More information:

Alan Fitzsimmons, Colin Snodgrass, Ben Rozitis, Bin Yang, Méabh Hyland, Tom Seccull, Michele T. Bannister, Wesley C. Fraser, Robert Jedicke & Pedro Lacerda, 2017, "Spectroscopy and thermal modelling of the first interstellar object 1I/2017 U1 ‘Oumuamua", Nature Astronomy, doi:10.1038/s41550-017-0361-4 [ Nature | ADS ].

Michele T. Bannister, Megan E. Schwamb, Wesley C. Fraser, Michael Marsset, Alan Fitzsimmons, Susan D. Benecchi, Pedro Lacerda, Rosemary E. Pike, J. J. Kavelaars, Adam B. Smith, 2017, "Col-OSSOS: Colors of the Interstellar Planetesimal 1I/'Oumuamua", ApJ Letters, 851, 38 [ ADS ].

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Last modified: 14 January 2018