Well, actually it is a long debate that there could be many earth-like planets where other alien life forms can exist. Actually Drake Equation was trying to formulate this mind blasting question for around 50 years. Now, a leading Cambridge Institution astronomer Alan Boss states that the number of earth-like planets could be at least 100 hundred billion trillion which results in an unavoidable fact for extraterrestrial life according to him. He also added that if there is an habitable planet, it would certainly appear some kind of life form there by time, in a few billion years. He believes that we will find such planets with the proven fact of simple life forms. How about the intelligent life besides bacteria? Well, according to him, it would be a big coincidence to find such creatures at the same time of ours because we will survive further 100,000 years further, very unlikely !