Astronomers surprised by large space rock less dense than water

Kuiper belt object challenges planet-formation theories.

A planetary scientist has identified the largest-known solid object in the Solar System that could float in a bathtub. The rock-and-ice body, which circles well outside the orbits of the planets, is less dense than water — although a bathtub big enough to hold it would stretch from London to Frankfurt.

The body, dubbed 2002 UX25, lies in the Kuiper belt, a reservoir of dwarf planets, comets and smaller frozen bodies beyond the orbit of Neptune. The object’s low density and size — it is 650 kilometres wide — seem to conflict with a leading model for the formation of large solid bodies in the Kuiper belt and throughout the Solar System. Planetary scientist Michael Brown of the California Institute of Technology in Pasadena reports its density measurement in an upcoming issue of The Astrophysical Journal Letters, with a preprint available on the arXiv online repository.

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