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. It is only when both ships slow, then stop, their engines non-functional, but life-support intact that we all find out that aliens exist. 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?
The aliens send a message to Earth, telling all mankind they have quarantined our solar system for the safety of the rest of the galaxy. They challenge us to prove we are fit to live among the other races of the galaxy. What test do we need to meet? What standards apply?