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# A kilometre-sized Kuiper belt object discovered by stellar occultation using amateur telescopes

## Abstract

Kuiper belt objects (KBOs) are thought to be remnants of the early Solar System, and their size distribution provides an opportunity to explore the formation and evolution of the outer Solar System1,2,3,4,5. In particular, the size distribution of kilometre-sized (radius = 1–10 km) KBOs represents a signature of initial planetesimal sizes when planets form5. These kilometre-sized KBOs are extremely faint, and it is impossible to detect them directly. Instead, the monitoring of stellar occultation events is one possible way to discover these small KBOs6,7,8,9. However, until now, there has been no observational evidence for occultation events of KBOs with radii of 1–10 km. Here, we report the first detection of a single occultation event candidate by a KBO with a radius of ~1.3 km, which was simultaneously provided by two low-cost small telescopes coupled with commercial complementary metal–oxide–semiconductor cameras. From this detection, we conclude that the surface number density of KBOs with radii exceeding ~1.2 km is ~6 × 105 deg−2. This surface number density favours a theoretical size distribution model with an excess signature at a radius of 1–2 km (ref. 5). If this is a true KBO detection, this implies that planetesimals before their runaway growth phase grew into kilometre-sized objects in the primordial outer Solar System and remain as a major population in the present-day Kuiper belt.

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## Data availability

The imaging data frames of the detected occultation are available from https://www.dropbox.com/s/1iesn4z2pwv110t/imgfile.zip.

## References

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## Acknowledgements

We thank the personnel of the Miyakojima City Museum and Miyako open-air school (Miyako Seishonen no Ie) for providing site infrastructure and access to facilities. We also thank the people of Miyako Island for supporting our observations. We thank Y. Sarugaku for constructive advice. This research has been partly supported by JSPS grants (JP26247074, 15J10278, 15J10864, 26800112, 16K17796, 18K13584 and 18K13606).

## Author information

Authors

### Contributions

K.A. is a principal investigator of the monitoring campaign described here. K.A., K.T., F.U., T.O., T.K., T.W., K.N. and J.W. developed the observation systems. K.A., K.T., F.U., Y.S. and K.I. carried out the monitoring observations. K.A. developed the data reduction pipeline and occultation detection programme, with substantial contributions from K.T. and J.W. The figures and movies, including the supplementary figures and video, were generated by the authors.

### Corresponding author

Correspondence to K. Arimatsu.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

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## Supplementary information

### Supplementary Information

Supplementary Figures 1–6, Supplementary Video 1 caption.

### Supplementary Video 1

Animation of the occultation event candidate obtained with the two OASES observation systems.

## Rights and permissions

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Arimatsu, K., Tsumura, K., Usui, F. et al. A kilometre-sized Kuiper belt object discovered by stellar occultation using amateur telescopes. Nat Astron 3, 301–306 (2019). https://doi.org/10.1038/s41550-018-0685-8

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• DOI: https://doi.org/10.1038/s41550-018-0685-8