Long Description of Sighting Report

I was out in the garden checking out the icy grass and looked up at the sky to see if it cloudy... Suddenly, they was a green light in the sky travel at a very high speed and disappeared in few more seconds, and I am very much sure it was not one of the usual things we might notice in the sky. And i was telling this incident to a colleague at work. to a surprise, three other collegues have noticed/heard about this incident. I was left in shock having seen all the Alien movies, etc...