How to make Objects so as they do not leave an area around another object

Hello, I need to make it so that some objects do not leave a circular area of which the center is another object. So these objects can have random movement until they reach the boundaries of the circumference around the center object. I have been looking at insideUnitSphere but I do not think that this is the correct function for the job! Any help will be appreciated :) Thanks in advance!