Earth is Quarantined!

It has taken mankind over 50 years to build two huge generational ships that are to carry our best and brightest out to the stars. They set out with the best wishes of those that remain behind and are soon largely forgotten. It is only when both ships are slowed and then stopped, their engines non-functional but life-support intact that we all find out that aliens DO exist. And their intentions are anyone's guess!
Why have they stopped the ships? HOW have they stopped them? Who are they and what do they want?
A message is finally sent telling all mankind that their entire solar system has been quarantined for the safety of the rest of the galaxy. We must prove that we are fit to live among the other races of the galaxy. What test do we need to meet? What standards apply?