Monocytes are 10-24 micrometer in diameter as compared to red blood cells which are only 7-8 micrometer in diameter. Monocytes are one of the agranular leukocytes and transform into macrophages when they migrate from blood to tissues. Their function is to kill the pathogens and cellular debris by the process of phagocytosis.
The key factor in determining the habitable zone of a planet is looking for a Earth-like planet that is able to have water on it's surface, and make sure that it can still give or support life.
