Dalek Planetarium was the first piece of media in The Dalek Book, a sort of prelude to the first and most prominent comic story in the book, namely Invasion of the Daleks. It saw a group of Daleks educating themselves about the rest of the solar system in preparation for their invasion of it in that story. As such, it featured the first-ever mention of a Dalek Emperor, with the Golden Emperor himself making his actual debut in Invasion of the Daleks.
A Dalek in a hoverbout calls for the attention of a crowd of Daleks watching from below. As a picture of several celestial bodies from the solar system appears on a humongous screens, the flying Dalek explains that they must learn more about "these planets and moons" (Earth's Moon, Mars, Saturn and its moons, Earth itself, Jupiter, Skaro and Venus).
The Dalek instructor then explains that the positions and relative sizes of the planets on the image do not actually correspond to reality; instead, they depict the various celestial bodies' importance to the Daleks. The Dalek then reassures its audience that "our Emperor" will soon disclose the actual astrophysical positions and specs of the planets in the solar system.