Final answer:
Planet Z is most similar to Jupiter, as they both have large sizes, thick atmospheres, and low densities.
Step-by-step explanation:
Planet Z, with its large size, thick atmosphere, and low density, is most similar to Jupiter, which is a planet in our own solar system. Jupiter is the largest planet in our solar system and also has a thick atmosphere composed mainly of hydrogen and helium. Its low density is due to its gaseous nature, with no solid surface. Besides their similar characteristics, both Planet Z and Jupiter are known as gas giant planets.