For billions of years the Earth has been green and has had an oxygen atmosphere. Aliens would be able to see the planet and know it was alive, because an oxygen atmosphere is created by life, and so is green. However, that wasn't us. We have had a technological civilization that can be seen from other stars for no more than 200 years. We have been sending radio waves into space for about 100 years. These signals have only traveled 100-200 light years from Earth; thus, only a small number of stars are close enough for civilizations there to know we are here. Most stars in the Milky Way are so far away, almost a 100,000 light years, that neither radio waves nor light telling our presence have not reached them, so it is impossible for them to know we are here.