"Extremophiles" is a bit of a parochial term - this is the name for organisms that live happily in environments that we consider extreme. Too cold, too hot, too acid, too something to support life, in our terms. Studying the lifeforms that disagree with us on what is a good place to live has started a new field of astrobiology and a new appreciation of the possibility of life existing in the wider universe.
Comets are an astronomical event/phenomenon that have exerted quite a hold on the imagination of people in the past & it's only relatively recently that we have any understanding of what they are or why they happen. The In Our Time programme that discussed them primarily focused on the astronomy but did touch on the omens and portents side of them as well. The experts on the programme were Monica Grady (Open University), Paul Murdin (University of Cambridge) and Don Pollacco (University of Warwick).