What did Galileo first think the protrusions of Saturn were?

Galileo first thought Saturn’s protrusions were two companion stars. In 1610, his telescope showed Saturn as a central planet flanked by two smaller, apparently separate bodies. He described the arrangement as a three-bodied system, because his instrument could not resolve Saturn’s rings clearly enough to reveal their true shape.

The observation became even more puzzling when Saturn’s side bodies seemed to disappear in 1612, as Earth crossed the plane of the rings. Galileo wondered whether Saturn had somehow swallowed its companions. The protrusions later reappeared, deepening the mystery.

The modern explanation is that Galileo was seeing Saturn’s ring system edge-on or at changing angles, but his telescope lacked the resolving power needed to identify it. Christiaan Huygens correctly interpreted the feature as a disk or ring surrounding Saturn in 1655. Galileo therefore discovered the rings observationally, but did not understand what they were. “Two companion stars” is the accepted description of his initial interpretation; “two planets” is a less accurate paraphrase of the same visual impression.