Since Mercury lacks an atmosphere, its surface is boiling hot during the day and freezing cold at night. Contrary to popular belief, however, the planet is not tidally locked to the Sun. Due to the harsh environment of the planet, Mercurians have been described as very simple organisms in most science fiction stories. In ufology and exobiology, Mercurians are almost never mentioned, as their existence remains unlikely.
- Mercurians are single-celled organisms which inhabit the craters of Mercury. Featured in the Brazilian magazine Superinteressante in June, 1999.
- The Harmoniums are blue-colored, paper-thin creatures that inhabit the caves of Mercury in Kurt Vonnegut 's The Sirens of Titan (1959).
- H. P. Lovecraft's The Shadow Out of Time makes mention of the "bulbous vegetable entities of Mercury".
- Semi-humanoid Mercurians from DC Comics.