Right now, two astronomers are claiming that there may be alien life present on the Philae Comet. Chandra Wickramasinghe and his colleague Max Wallis claim that many characteristics of the comet indicate alien life is possible, like its rich crust. The two are saying that the comet could be home to microbes, which may be similar to "extremophiles" who live in the most undesirable, uninhabitable places of the Earth.
However, other scientists are now coming forward with alternative explanations for Wickramasinghe's claims; one comet scientist, Professor Alan Fitzsimmons, said that he believes their study to flawed.
Scientists are rightly fascinated by the search for life elsewhere, and the possibility of the delivery of life to Earth from elsewhere remains an intriguing line of enquiry. However all measurements from Rosetta or by Earth-based telescopes on comets can be explained by much simpler chemical and physical processes than involving extra-terrestrial life. These scientists seem to have forgotten Carl Sagan’s maxim that ‘extraordinary claims require extraordinary evidence’. They don’t have it.
Dr. Wallis gave a lecture on the subject of the potential life on the comet and concluded it saying, "...If there is any active biology in the comet, we’d hope to detect it."
The Guardian has a longer, more in depth article on the matter, but in short, most comet scientists believe that the very phenomena Wallis and Wickramasinghe are suggesting indicate life, is the very phenomena that is much easier explained, and far more likely, to be non-biological mechanisms.