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The carbon monoxide-rich interstellar comet 2I/Borisov


Interstellar comets offer direct samples of volatiles from distant protoplanetary disks. 2I/Borisov is the first notably active interstellar comet discovered in our Solar System1. Comets are condensed samples of the gas, ice and dust that were in a star’s protoplanetary disk during the formation of its planets, and inform our understanding on how chemical compositions and abundances vary with distance from the central star. Their orbital migration distributes volatiles2, organic material and prebiotic chemicals around their host system3. In our Solar System, hundreds of comets have been observed remotely, and a few have been studied up close by space missions4. However, knowledge of extrasolar comets has been limited to what could be gleaned from distant, unresolved observations of cometary regions around other stars, with only one detection of carbon monoxide5. Here we report that the coma of 2I/Borisov contains substantially more CO than H2O gas, with abundances of at least 173%, more than three times higher than previously measured for any comet in the inner (<2.5 au) Solar System4. Our ultraviolet Hubble Space Telescope observations of 2I/Borisov provide the first glimpse into the ice content and chemical composition of the protoplanetary disk of another star that is substantially different from our own.

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Fig. 1: Co-added HST/COS spectra of 2I/Borisov acquired between 19 and 22 December 2019.
Fig. 2: Volatile production rates as a function of time relative to perihelion.
Fig. 3: The composition of volatiles in the coma of 2I/Borisov compared with other comets in our Solar System.

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Data availability

All data are publicly available on the Mikulski Archive for Space Telescopes ( under HST proposal 16049, PI D.B.


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This study is based on observations with the NASA/ESA Hubble Space Telescope obtained from the Mikulski Archive for Space Telescopes at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract number NAS5- 26555. Support for programme number GO-16049 was provided through a grant from the STScI under NASA contract number NAS5-26555. We thank A. Vick, D. Sahnow, E. Nance and C. Mannfolk at STScI for their help with scheduling the challenging HST observations. We further acknowledge the effort of many observers who supplied astrometry of 2I/Borisov to the International Astronomical Union’s Minor Planet Center to support the planning of the HST sequences, in particular E. Jehin, Q.-Z. Ye, M. Micheli, D. Tholen, T. Lister, K. Meech and S. Sheppard. Part of this research was conducted at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with NASA. This research has made use of data and/or services provided by the International Astronomical Union’s Minor Planet Center.

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Authors and Affiliations



D.B. and J.W.N. performed the data reduction and analysis and wrote the paper. P.D.F. conducted the CO modelling and interpretation. J.-Y.L. led the acquisition and initial verification of the data. D.F. led the orbital characterization of 2I/Borisov required for HST pointing. K.E.M. contributed to the comparison of the elemental composition. Z.-X.X. coordinated the Swift observations and provided a comparison with H2O production rates. M.T.B., J.W.P. and W.M.H. contributed to experiment design, calibration and data interpretation. All authors discussed the results and commented on and revised the manuscript.

Corresponding author

Correspondence to D. Bodewits.

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The authors declare no competing interests.

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Peer review information Nature Astronomy thanks Edwin Bergin, Martin Rubin and Bin Yang for their contribution to the peer review of this work.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Extended data

Extended Data Fig. 1 Observing logs of HST observations and water production rates from simultaneous observations with Swift/UVOT (ref. 9).

T-Tp indicates the time past perihelion; Rh indicates the heliocentric distance and dRh the comet’s radial heliocentric velocity; D indicates the distance to Earth; Ncol is the derived average column density in the field of view; Q(CO) and Q(H2O) indicate the CO and water production rates corresponding to these column densities; and the radius is the radius of the field of view at the distance of the comet.

Extended Data Fig. 2 Co-added HST/COS spectra of 2I/Borisov.

Top left, Dec. 11–14, 2019 UTC; Top right, Dec. 19–22; Bottom left, Dec. 30, 2019; Bottom right, Jan. 13, 2020. Black lines indicate the smoothed HST/COS data, and red lines the best-fit CO fluorescence model.

Extended Data Fig. 3 Upper limits on sulphur in 2I/Borisov.

Synthetic atomic sulphur blended triplet emissions at 1813 Å plotted for three different column densities of sulphur. The filled region of the spectrum represents one standard deviation.

Extended Data Fig. 4 Production rates used to determine the elemental composition of volatiles in the coma of 2I/Borisov.

Water production rates were extrapolated to the dates on which the other production rates were measured using an empirical fit.

Extended Data Fig. 5 Elemental composition of volatiles in the comae of different comets compared with 2I/Borisov.

See text for references.

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Bodewits, D., Noonan, J.W., Feldman, P.D. et al. The carbon monoxide-rich interstellar comet 2I/Borisov. Nat Astron 4, 867–871 (2020).

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