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Pluto demoted as definition of ‘planet’ changes

The International Astronomical Union (IAU) defined the meaning of the term “planet” last Thursday in a vote that ended a centuries-long period during which the term had no precise definition. However, the IAU did not create a definition for planet for use outside Earth’s solar system.

By the new definition, Pluto, which was considered a planet since its 1930 discovery, now merits only “dwarf planet” status. It shares this classification with two other objects, Ceres and UB313, nicknamed Xena. Experts expect that many more objects, mostly from the Kuiper Belt at the edge of the galaxy, will soon receive official classification as “dwarf planets.”

Under the new rules, a full...

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