Exoplanets, or extrasolar planets, are planets located outside the Solar System. So far, more then 5500 exoplanets have been discovered in more than 4000 different planetary systems. Searching for life on exoplanets is by no means an easy task, as they are located at great distances from Earth. To simplify this problem in astronomy and astrobiology, the concept of the habitable zone is defined. The habitable zone of a star is the range of orbits around a star within which water can exist in a liquid state on the surface of a planet, with sufficient atmospheric pressure. Understanding the habitable zone of different stellar systems allows for the sorting of a large number of exoplanets and the identification of those within it. Currently, the only data we can collect from exoplanets are related to the planetary atmospheres. A thorough understanding of different types of atmospheres provides us with insights into the diversity of planetary systems and can serve as a guide in better understanding of the Earth's atmosphere. In this work the concept of planetary atmosphere and habitable zone is explained, providing insights into possible exoplanets located outside the habitable zone of their stellar system capable of maintaining water on their surface.