The first planet discovered using a telescope in the 20th century was Pluto. Clyde W. Tombaugh identified it at Lowell Observatory in Arizona on February 18, 1930, during a systematic search for a predicted distant world called Planet X.
Tombaugh compared pairs of photographic plates with a blink comparator, looking for a faint object that shifted against the background stars. The discovery was announced on March 13, 1930, the 149th anniversary of William Herschel’s discovery of Uranus.
Pluto was taught and widely described as the solar system’s ninth planet for decades. In 2006, the International Astronomical Union classified it as a dwarf planet after defining a full planet as an object that also clears its orbital neighbourhood. That modern classification can cause confusion, but in the historical wording of this trivia question, Pluto remains the accepted answer. Ceres, Uranus, and Neptune were discovered earlier.