Why is Liquid Water Necessary for Habitability?
Liquid water is necessary for habitability, as it provides a medium for the interaction of chemicals. While exotic life can exist at hotter temperatures or higher pressures, such as the microbes found 0.5 miles under the West Antarctic ice sheet or the extremophiles near hydrothermal vents, those discoveries were made because humans could probe those extreme environments directly. They would not be detected from space.
Humans depend upon remote observations entirely when it comes to locating habitable conditions or life beyond our solar system. Surface liquid water may form a livable condition that can possibly promote growth. These lives can then interact with the atmosphere above, creating remotely discoverable bio-signatures that the Earth-based telescopes can find. These bio-signatures may be Earth-like gas compositions (ozone, oxygen, methane, water vapor, and carbon dioxide) or the structure of the ancient Earth (mostly carbon dioxide and methane, and no oxygen) 2.7 billion years ago.
Earth is one such planet where this has happened already. Therefore, the goal of astronomers is to detect the planets that are close to Earth’s size as well as orbiting at distances from the star suitable for the existence of water in its liquid form on its surface.