Percival Lowell

This confirmed what had been increasingly suspected: Pluto's gravitational influence on Uranus and Neptune is negligible certainly not nearly enough to account for the discrepancies in their orbits. Craters on the Moon and on Mars have been named after him. From 1914 to 1916 a 9-inch (23 cm) telescope on loan from Sproul Observatory was used to search for Planet X.

The choice of the name Pluto and its symbol were partly influenced by his initials PL.