Planetary Science

What counts as a planet?

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Science  18 Dec 2015:
Vol. 350, Issue 6267, pp. 1488
DOI: 10.1126/science.350.6267.1488-c

In 2006, the International Astronomical Union formally defined the term “planet” in such a way that bodies such as Pluto, Ceres, and Eris do not qualify. The definition was controversial, partly because the criteria are somewhat subjective and cannot be directly applied to planets around other stars (exoplanets). Margot has proposed an alternative objective mathematical definition of a planet. Within our solar system, the calculation clearly separates the eight planets from all other bodies. It can also be calculated from data that are already available for 99% of known exoplanets. It remains to be seen whether the new metric will catch on.

Astron. J. arXiv:1507.06300 (2015).

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