When a speaker says, "Jackson says he believes that there is intelligent life on some other planet in the universe, but how can you believe him? He's not an astronomer; in fact he is not a scientist at all. The only degree he has is a Masters in Business Administration.", then he is guilty of using which Ad hominem.
To add, Ad hominem short for argumentum ad hominem, is in which an argument is rebutted by attacking the character, motive, or other attribute of the person making the argument.