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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 space1,2. It is reasonable to expect that the same happened for planetary systems other than our own. Detection of such interstellar objects would allow us 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 known interstellar object, discovered by the Pan-STARRS1 telescope in October 2017 (ref. 3). The discovery epoch photometry implies a highly elongated body with radii of ~ 200 × 20 m when a comet-like geometric albedo of 0.04 is assumed. The observable interstellar object population is expected to be dominated by comet-like bodies in agreement with our spectra, yet the reported inactivity of 'Oumuamua implies a lack of surface ice. Here, we report spectroscopic characterization of ‘Oumuamua, finding it to be variable with time but similar to organically rich surfaces found in the outer Solar System. We show that this is consistent with predictions of an insulating mantle produced by long-term cosmic ray exposure4. 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.

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Fig. 1: Optical reflectance spectra of 1I/2017 U1 obtained with the WHT + ACAM and VLT + X-shooter.
Fig. 2: WHT + ACAM (red data points) and VLT + X-shooter (grey data points) spectra of 1I/2017 U1 compared with reflectances of main-belt asteroids.
Fig. 3: Comparison of our X-shooter 1I/2017 U1 spectrum from Fig. 2 with ranges of reflectance spectra of outer Solar System bodies.
Fig. 4: Thermal modelling of 1I/2017 U1 during its flyby of the Sun, demonstrating the survivability of sub-surface ice.


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We thank the observatory staff at the Isaac Newton Group of Telescopes and the European Southern Observatory for responding quickly to our observing requests. Particular thanks go to R. Ashley, C. Fariña and I. Skillen (Isaac Newton Group) and G. Beccari, B. Haeussler and F. Labrana (European Southern Observatory). A.F., M.T.B. and W.C.F. acknowledge support from Science and Technology Facilities Council grant ST/P0003094/1 and M.T.B. acknowledges support from Science and Technology Facilities Council grant ST/L000709/1. C.S. acknowledges support from the Science and Technology Facilities Council in the form of an Ernest Rutherford Fellowship. B.R. is supported by a Royal Astronomical Society Research Fellowship. The WHT is operated on the island of La Palma by the Isaac Newton Group of Telescopes in the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias. The ACAM spectroscopy was obtained as part of programme SW2017b11. This paper is also based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under European Southern Observatory programme 2100.C-5009.

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A.F. led the application and organization of the WHT observations, analysis of these data and writing of the paper. C.S. led the application for VLT observations, organized the observing plan and assisted with analysis and writing. B.R. performed the thermal modelling of 1I/2017 U1. B.Y. was co-investigator on the telescope proposals, assisted in writing the VLT proposal and reduced the X-shooter data. M.T.B. and W.C.F. assisted in interpretation of the spectra in terms of known TNO properties and helped with writing the paper. M.H. reduced the WHT data. T.S. reduced the VLT data and provided the comparison spectrum of Echeclus. R.J. was co-investigator on the telescope proposals and contributed to the analysis and interpretation, especially with respect to observational selection effects. P.L. assisted in interpretation of the variable spectra and helped with writing the paper.

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Correspondence to Alan Fitzsimmons.

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Fitzsimmons, A., Snodgrass, C., Rozitis, B. et al. Spectroscopy and thermal modelling of the first interstellar object 1I/2017 U1 ‘Oumuamua. Nat Astron 2, 133–137 (2018).

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