An equation for the probability of detecting intelligent life on other planets was devised by Frank Drake in 1961. This equation estimates the probability a signal from another planet can be detected if they are advanced enough. The equation is N=RxFpxncxF1xFixFcxL.
This equation has eight variables. N is the number of civilizations that can communicate with the Earth. It is derived from the product of variables on the right side of the equals sign. The first variable of the product on the right hand side is R. It is the rate at which stars that have the capability of having planets that can have life forms inhabit them. The stars must also have a long enough lifetime for intelligent life to develop.
The next variable, Fp, is the number of stars that actually do have planets. The variable after that is nc. All stars have a zone that has the right temperature for planets to have liquid water. This variable is the number of those planets that could also have life as we know it.
The next variable, F1, is the number of the planets of the variable nc that actually do have all the conditions necessary for life. The variable Fi is the number of planets where intelligent life does actually develop. The variable Fc is the number of planets that have technology advanced enough that could be detected by radar from the Earth. The variable L is the length of time this advanced technology is detectable.
There are pictures of Mars that could be evidence of water on the red planet. Ice has been found on Mars at the north and south poles, inside craters, and in long, narrow strips beneath Mars surface. The ice could only be liquid water when the area is warm enough to melt the ice, but there are only a few days in the summer on Mars when it will be warm enough. But there are very small microbes that can live in ice. Some were discovered in Antarctica recently. The bacteria was found thousands of meters below the ice about 120 meters above the underground Lake Vostok.
The only requirements for life to exist are water, carbon, and energy. There is also a theory that life could be silicon based on planets other than the Earth at high temperatures. The theory was discovered by James Emerson Reynolds in 1893.
There are many movies about life on other planets. One of the most famous is War of The Worlds by H.G. Wells. This book has Martians travel to Earth by asteroid-like rocket ships. But the Martians are hostile; they have ray guns that they shoot from their flying saucers that is almost impossible to stop. There is a good movie starring Gene Barry that closely follows the book. The television show Star Trek's co-star Leonard Nimoy plays a people friendly alien called Spock. He is from the planet Vulcan.
A liquid lake, although not water, was discovered on Titan, a moon of Saturn. It is located in the northern part of the moon. The satellite Cassini caught a beam of reflected sunlight that confirms a liquid lake. Icy water was discovered below the surface of another smaller moon of Saturn named Enceladus. This is strong evidence for life. A satellite that was intentionally crashed on the crater of the moon Cabeus October 9, 2011 found what might be a significant amount of water on the moon.