Glass positive of Pluto discovery

Finding Pluto

From 1930 until 2006, our Solar System had nine planets. Tiny Pluto was discovered by Clyde Tombaugh, who was searching for a predicted ‘Planet X’ that might explain oddities in the orbits of Neptune and Uranus. Tombaugh’s photographs show the same patch of sky a few nights apart. One ‘star’ seems to have moved, indicating that it is actually a planet. Astronomers later discounted the idea of Planet X – Pluto had just been in the right place at the right time. It was controversially demoted to ‘dwarf planet’ in 2006.