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Carbon monoxide gas produced by a giant impact in the inner region of a young system


Models of terrestrial planet formation predict that the final stages of planetary assembly—lasting tens of millions of years beyond the dispersal of young protoplanetary disks—are dominated by planetary collisions. It is through these giant impacts that planets like the young Earth grow to their final mass and achieve long-term stable orbital configurations1. A key prediction is that these impacts produce debris. So far, the most compelling observational evidence for post-impact debris comes from the planetary system around the nearby 23-million-year-old A-type star HD 172555. This system shows large amounts of fine dust with an unusually steep size distribution and atypical dust composition, previously attributed to either a hypervelocity impact2,3 or a massive asteroid belt4. Here we report the spectrally resolved detection of a carbon monoxide gas ring co-orbiting with dusty debris around HD 172555 between about six and nine astronomical units—a region analogous to the outer terrestrial planet region of our Solar System. Taken together, the dust and carbon monoxide detections favour a giant impact between large, volatile-rich bodies. This suggests that planetary-scale collisions, analogous to the Moon-forming impact, can release large amounts of gas as well as debris, and that this gas is observable, providing a window into the composition of young planets.

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Fig. 1: Cleaned emission maps of the HD 172555 system.
Fig. 2: Mass–temperature degeneracy of the CO data.

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Data availability

The ALMA programme number for the presented data is 2012.1.00437.S and data can be found in the online ALMA archive. The cleaned .fits files are available upon request from the corresponding author.

Code availability

RADMC-3D is available at and emcee is available at Custom code, including the ring model and non-LTE code, is available at


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We are grateful to J. Biersteker for discussions on the liberation of atmospheres in the aftermath of giant impacts. This paper makes use of ALMA data ADS/JAO.ALMA\#2012.1.00437.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada), NSC and ASIAA (Taiwan) and KASI (Republic of Korea), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. K.I.Ö. acknowledges support from the Simons Foundation (SCOL \#321183). G.M.K. is supported by the Royal Society as a Royal Society University Research Fellow.

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Authors and Affiliations



T.S. led the optically thin modelling and discussion. L.M. led the radiative transfer modelling. T.S. and L.M. were involved in data reduction, processing and writing of the manuscript. All authors contributed to discussions of the results and commented on the manuscript.

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Correspondence to Tajana Schneiderman.

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The authors declare no competing interests.

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Peer review information Nature thanks Inseok Song and the other, anonymous, reviewer(s) for their contribution to the peer review of this work. Peer reviewer reports are available.

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Extended data figures and tables

Extended Data Fig. 1 Posterior probability distributions for the model parameters obtained from the emcee fitting process.

All parameters are well constrained, with best-fit values listed in Extended Data Table 1. This model was fitted to spectral data retaining original channel widths, assumed a Gaussian prior on the inclination, and assumed a stellar mass of 1.76 M.

Extended Data Table 1 Best fit values (50 ± 34 percentile) to the optically thin model of gas emission. Left column indicates values derived from the MCMC run where a Gaussian prior was applied to the inclination. Right column indicates values derived from the MCMC run where flat priors were applied to all model parameters

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Schneiderman, T., Matrà, L., Jackson, A.P. et al. Carbon monoxide gas produced by a giant impact in the inner region of a young system. Nature 598, 425–428 (2021).

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