There is no 'theory' or life on other planets. At the present time it is only a speculation since we have no evidence or data to suggest it does exist. Personally, I BELIEVE that there is life on many planets in our galaxy, but that most of this is bacterial life. Bacteria existed on Earth from 3.8 billion years ago and were the dominant life form until only 600 million years ago, so it is very hardy and easy to form on worlds with conditions like that of the Earth. To get 'intelligence' seems a big step and a very 'iffy' one at that. We are very lucky to be here.