Atmosphere is created by mixture of gases and water vapours around the planet that makes it habitable. On earth, the oxygen percentage is mainly responsible for the habitable atmosphere. Presence of water and water vapours is another factor that makes earth habitable. These factors prevents the sudden temperature and climate changes which would have been there due to outer space effects and rotation and revolution of earth. Without them, earth would have existed but not as a life supporting planet. However these factors have still not been found on Venus and Mars which makes them inhabitable planets for now.