Final answer:
Clyde Tombaugh, the astronomer who discovered Pluto, first noticed its omission from Dr. Tyson's exhibit.
Step-by-step explanation:
The individual who first noticed that Pluto had been left out of Dr. Tyson's exhibit was the astronomer and observatory staff assistant Clyde Tombaugh. Tombaugh, who discovered Pluto in 1930 at the age of 24, was comparing photographs taken on January 23 and January 29 of that year when he identified the motion of a faint object right for a planet beyond Neptune. This discovery was instrumental in the naming of Pluto, which was inspired by the Roman god of the underworld and also serendipitously included the initials of Percival Lowell, founder of the Lowell Observatory where Tombaugh worked.