Galileo's astronomical ideas got him into trouble with church leaders and he was kept prisoner in his house near Florence until he died. His 'crime' was to suggest that the
Earth was not the centre of the universe. We now know that he was right, but the Catholic Church didn't pardon him until 1992, nearly 400 years later. Another of his 'crimes' was to suggest that the Sun was not perfect, but had spots!
Galileo watched the sunspots carefully and saw that they appeared, disappeared and changed their shapes so he knew they could not possibly be planets just passing in front of the Sun.
You can see one of Galileo's first drawings of sunspots in this newspaper article.