This was a series of books (space opera) about a man who becomes a spaceship captain, because he is needed to conduct business on other planets (androids cannot be captains).
An evil, always plotting, android tries to use our hero. The android is cast away and in revenge uses Earth's total destruction as a threat.
There are some alien artefacts shaped like metal discs.
Earth doesn't know about life in space. Main character is only one taken to space. He took some nanobots... so he is a bit stronger....