Is there just one Earth in the universe? Or are there many? Why hasn't SETI worked? It takes many special parts to make an Earth: for example a solar system with iron and nickel; large planets like Jupiter and Saturn; A correct-sized moon to stabilize the planet. The list is actually enormous. But there are many stars with solar systems. At an average 100 billion stars in 100 billion galaxies, surely more than one planet got it right. Well, maybe. Current discoveries in our galaxy:-http://www.nature.com/news/2011/110202/pdf/470024a.pdf