Several different aliens have visited earth from time to time. most regretfully earth is outside the main trading routes, there are no nearby gravity wells of any value nearby -space travel requires the presence of gravity well to curve space-, and more important earth has nothing valuable to trade.
However, recently earth has become a center of attention by it position near the great void between the neighboring arm of our galaxy. I turns out that just across the void is the heart of a rather nasty alien race (let call them X) who are bend on destroying any other civilization.
The X race has been stopped off at a place distant from earth, but there is fear that they may have or are at the verge of developing a new technology that would allow them to jump through the great void witout the aid of gravity wells.
The trading civilizations have decided that it may be on their interst to either place some listening post near earth, or even better deploy some forces to the area.
I was recently told that some scouts ships have been deployed, and the "earth issue" is being discussed at the highest levels. I was also was told to make no mistake that if race X passes-by earthling's can kiss their asses good by as they would be exterminated with extreme prejudice.
