Final answer:
There are 120 different planet types.
Step-by-step explanation:
Astronomers classified the planets into six sizes with five atmosphere types and four average temperature ranges. To determine the number of different planet types, we need to multiply the number of sizes, atmosphere types, and average temperature ranges. With six sizes, five atmosphere types, and four average temperature ranges, the total number of different planet types would be 6 x 5 x 4 = 120.