Article: The girl who named Pluto.(Venetia Burney)

After the eighth planet, Neptune, was discovered in 1846, the hunt for a ninth planet began. In science, X means the unknown. Since astronomers weren't even certain that a ninth planet existed, they called it Planet X.

In 1929, Lowell Observatory in Arizona hired Clyde Tombaugh to search for Planet X. On February 18, 1930, Tombaugh was studying some of his sky photos. He compared two photographs of the same part of the sky taken days apart. On the photos, he spotted a dim object that had moved slightly during that time.

"That's it!" Clyde Tombaugh told himself. He knew he had found a planet for the simple reason that stars are too far away to move ...

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