QUESTION:
We need to know who discovered each planet and when.

ANSWER from "Grolier Multimedia Encyclopedia-Rel 6", researched by Ginger Fiel
Mercury, Venus, Mars, Jupiter and Saturn were known to the ancients.

English astronomer William Hershel accidentally discovered Uranus in 1781.

John Couch Adams & Urbain Jean Joseph Leverrier in 1846 predicted the
existence of Neptune which was found later that year.

In 1930 Clyde Tombaugh discovered Pluto.