1915 - Percival Lowell publishes "Memoir of a Trans-Neptunian Planet." In it, he predicts that there is a planet beyond Neptune. He calls it Planet X. He thinks the planet is seven times heavier than Earth. Pluto isn't that heavy, so he couldn't figure out its location even though the planet appeared in some sky photos he took.
1930 - American astronomer Clyde Tombaugh discovers Pluto by comparing photos of the sky made at different times. This is the first (and the last, so far) planet discovered in the 20th century.
