When Kepler saw the smaller planets orbiting Jupiter, they reminded him of the bodyguards surrounding a king on a journey, so he named them satellites.
Literal
Jupiter [possessive-の] surroundings [object-を] is-circling more small planets [object-を] seeing Kepler [topic-は], during-outing [possessive-の] king [possessive-の] surroundings [object-を] surround bodyguards [object-を] remembered [because-ので], it [object-を] satellite [quotative-と] named.
A long, multi-clause sentence rich in relative clauses and embedded modifiers. Notice three stacked attributive frames: 木星の周辺を回っている modifies 惑星, 外出中の王 modifies the bodyguards, and 王の回りを取り囲む also modifies the bodyguards. The chain ends with a causal ので setting up the naming. 衛星 (eisei, 'satellite') is itself the term Kepler is reported to have coined, made of 衛 ('guard') + 星 ('star').