Some planets such as Mercury and Mars have little or no atmosphere so water would evaporate almost immediately. Other planets such as Venus are so hot that water would also evaporate. The outer planets are very large and have dense atmospheres, but may or may not have water in a liquid state unless the material out of which they were formed already had some water to begin with. Also, the water could get locked up or chemically altered in other compounds. Only on the Earth are the temperature and atmospheric conditions known to be exactly right for liquid water to exist on its surface.