NASA and other scientists believe that the key of determining whether or not a planet has life or potential life lies in one factor: water. “Water is a polar molecule and a solvent, two properties that are important for certain chemical reactions critical to life,” says NASA chief scientist Ellen Stofan. This theory leads scientists to believe that planets or moons such as Europa, which has water and ice, could have been home to living beings. For more on this topic, please click here:: In Search for Alien Life, Follow the Water