Abstract
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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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 https://github.com/dullemond/radmc3d-2.0 and emcee is available at https://emcee.readthedocs.io/en/stable/. Custom code, including the ring model and non-LTE code, is available at https://github.com/tmschneiderman/hd172555_CO_2021.
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Acknowledgements
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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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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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☉.
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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). https://doi.org/10.1038/s41586-021-03872-x
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DOI: https://doi.org/10.1038/s41586-021-03872-x
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