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A new astronomical discovery means that we will either have to lose Pluto as a planet, or admit that our solar system has far more planets than the textbooks say

By Philip Ball   March 2006

Downsizing Pluto When Pluto was discovered in 1930, it was entirely reasonable to call it a planet. It seemed, after all, to correspond to the so-called “Planet X” that was wrongly thought to be perturbing the orbits of Uranus and Neptune. Pluto’s own orbit was odd—more elliptical than that of other planets, and set at a rakish tilt to the plane of the solar system—and the planet was very far away and rather small, barely two thirds the diameter of our moon. Nevertheless, it became accepted as the ninth planet, even if its discovery four years before Holst’s death came…

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