Glass positive of the planet Pluto; Lowell Observatory 42-inch Reflector. Focal length 220 ins. March 2d 4h 56m. C.O. Lampland 12"x8". Prepared from silver print (from R.A.S.No. 416) lent by Sir James Jeans.

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.