# Early formation of moons around large trans-Neptunian objects via giant impacts

## Abstract

Recent studies1,2 have revealed that all large (over 1,000 km in diameter) trans-Neptunian objects (TNOs) form satellite systems. Although the largest Plutonian satellite, Charon, is thought to be an intact fragment of an impactor directly formed via a giant impact3, whether giant impacts can explain the variations in secondary-to-primary mass ratios (the ratio between the body and its main satellite) and spin/orbital periods among all large TNOs remains to be determined. Here we find that hydrodynamic simulations of giant impacts can reproduce the secondary-to-primary mass ratio of the satellite systems of large TNOs when the impact velocity is approximately the same as the escape velocity. We also reveal that the satellite systems’ current distribution of spin/orbital periods and small eccentricity can be most easily explained when their spins and orbits tidally evolve, initially as fluid-like bodies and finally as rigid bodies. The preferred duration of fluid-like behaviour is approximately 104−106 yr, and it depends on the secondary-to-primary mass ratio and the initial orbital elements. These results suggest that all satellites of large TNOs were formed via giant impacts before the outward migration of Neptune4 and that they were fully or partially molten during the giant impact era.

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## Data availability

The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.

## Code availability

The codes are available from the corresponding author upon reasonable request.

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## Acknowledgements

We are grateful to H. J. Melosh for useful comments and suggestions. S.A. acknowledges T. Nakamoto for his fruitful comments. This work is supported by the Astrobiology Center Program of National Institutes of Natural Sciences (JY290107). S.A. is supported by the Grant-in-Aid for JSPS Research Fellow (JP17J06861). R.H. acknowledges the financial support of JSPS Grant-in-Aid for JSPS Fellows (JP17J01269) and for Early-Career Scientists (JP18K13600). H.G. is supported by a MEXT KAKENHI Grant (JP17H06457).

## Author information

Authors

### Contributions

S.A. and H.G. developed the idea for the study and H.G. developed the hydrodynamic code for the giant impact simulations. S.A. and R.H. performed the hydrodynamic simulations and S.A. performed the semi-analytical tidal evolution calculations with support from R.H. All of the authors contributed to the interpretation of the results.

### Corresponding author

Correspondence to Sota Arakawa.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Peer review information: Nature Astronomy thanks Jay Melosh and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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## Supplementary information

### Supplementary Information

Supplementary text, Supplementary Figs. 1–6, Supplementary Table 1 and Supplementary references.

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Reprints and Permissions

Arakawa, S., Hyodo, R. & Genda, H. Early formation of moons around large trans-Neptunian objects via giant impacts. Nat Astron 3, 802–807 (2019). https://doi.org/10.1038/s41550-019-0797-9

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