The study of the planet Jupiter.
All of the planets (except Earth and Uranus) are named after Roman gods; the study of each planet is named from the corresponding Greek gods (with some odd exceptions that we will not go into here).
The Greek god that corresponds to Jupiter is Zeus.
So, Zeusology? Nope. Maybe Zeusology was just too weird a word. I cannot find a trustworthy source that explains the relationship between the names Zeno and Zeus. They are related, and Zeno may actually mean 'of Zeus', as some sites claim. Whatever the relationship, zenology is the 'official' word for the study of Jupiter.
Zenology, along with most other words for the study of various planets, is not longer in popular use, and you are unlikely to see it outside of science fiction and lists of odd words.