Final answer:
Pluto was discovered through a systematic search by Clyde Tombaugh at the Lowell Observatory in 1930. It took a long time to find Pluto because its discovery required careful observation and comparison of photographs over multiple nights to detect its movement among the stars.
Step-by-step explanation:
Pluto was discovered through a systematic search in the early twentieth century. Astronomers noticed that Uranus had slight departures from its predicted orbit, which indicated the presence of an undiscovered planet. Percival Lowell and other astronomers became interested in finding this ninth planet, and in 1930, Clyde Tombaugh discovered Pluto at the Lowell Observatory.
The search for Pluto took a long time because it required careful observation and comparison of photographs over several nights to detect its movement among the stars.