In practice most people who claim this title mainly speculate on the biology of alien life forms, usually with an eye toward fiction or philosophical implications. There is no clear example or specimen of extraterrestrial life, however there is an ongoing dispute regarding a meteor sample found in Antarctica labeled ALH84001 that may be a factual basis for this as an actual science. If continuing exploration of our universe eventually finds conclusive proof of extraterrestrial life, this may likely become something of a much more established scientific discipline.
In fiction the word refers to a scientist who studies alien life forms, and is sometimes confused with a xenologer, who studies the alien itself. For instance, in the novel Speaker for the Dead, written by Orson Scott Card, the Speaker for the Dead, Ender Wiggin, goes to the planet Lusitania to speak the death of a xenobiologist named Pipo, and then later his xenologer son Libo.