A Black Widow's Best Friend?

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Science  23 Sep 2011:
Vol. 333, Issue 6050, pp. 1712-1713
DOI: 10.1126/science.1212489

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The question “what is a planet?” revisited in 2006 by the International Astronomical Union (IAU) led to the controversial downgrading of Pluto to dwarf planet status (1). In practice, astronomers will happily assign “planet” to any object of planetary mass, or size. Indeed, for the vast majority of the 1000 or so planets known outside our solar system, or exoplanets, the object's mass or diameter (rarely both) is its only measured property (2). On page 1717 of this issue, Bailes et al. (3) report the discovery of a Jupiter-mass companion orbiting the millisecond pulsar PSR J1719-1438, forcing us to further rethink the very meaning of what constitutes a planet.