It can convince you that something's there, but that doesn't mean it's a planet. You believe it's a planet because of what you were taught in school, and maybe that all the things you were taught were consistent and reinforced each-other so it made sense. To the ancient Romans, it was a god, and we still celebrate that god at christmas (a.k.a. saturnalia). What you can see using a telescope is a circle-shaped thing with ring-shaped things around it. But, to decide that's a planet means trusting that that object is in fact multiple times the size of the entire planet earth and more than a billion km away. What have you personally done to verify that the mass of Saturn is actually accurate? Are you just trusting what you were taught, or have you actually verified those claims?

Homosexuality used to be considered a psychiatric disorder, so I wouldn't go by what people used to get hospitalized for. That's just another example of how common knowledge, or "things everyone believes to be true" can change over time.

As I said, we can't verify everything ourselves. We have to trust other people, and while it's good to question what we think we know, we can't question everything all the time.