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Of course, it is impossible to say anything definitive about this life, assuming that it exists. However, general arguments can be made on the nature of alien life based on our knowledge of physics, chemistry and biology:
Liquid water - Liquid water is probably the key factor in creating life in the universe. When astronomers search for life or evidence of life on other planets, the mantra is "Follow the water!" Liquid water, unlike most liquids, is a "universal solvent." It can dissolve an astonishing amount of chemicals. That being said, it is an ideal mixing bowl for increasingly complex molecules. Water is also a simple molecule that is found around the universe. Other solvents are quite rare.
Carbon - Carbon is a likely component in creating life. Carbon has 4 bonds. Thus, it has the ability to bind to 4 other atoms. This can create molecules of incredible complexity. It is easy to form long carbon chains. These become the basis for hydro-carbon and organic chemistry. Other elements with 4 bonds do not have as rich as a chemistry.
DNA - The self-replicating molecule, DNA, is the fundamental basis of life. Self-replicating molecules, in chemistry, are extremely rare. It took hundreds of millions of years to form the first DNA molecule on Earth. This probably occurred deep in the oceans.
Frank Drake
In 1961, Frank Drake, from Cornell University, was the first to make a rough estimate of the frequency of life in the universe, given these requirements. Drake's equation will give you the frequency of civilizations per galaxy.
There could be between 100 and 10,000 planets in the Milky Way Galaxy alone able to harbor intelligent life. If these intelligent life forms are pretty uniformly scattered across the Milky Way, we should expect to find a neighboring one some few hundred light-years from Earth.
In 1974, Carl Sagan estimated that there could be up to a million such civilizations in the Milky Way Galaxy alone.
SETI is the Search for extraterrestrial intelligence. It dates back to a 1959 paper by Giuseppe Cocconi and Philip Morrison. In the paper they proposed that the best way to eavesdrop on extraterrestrial communication was to listen to microwave radiation at a frequency between 1 and 10 gigahertz. Below 1 gigahertz, signals would be washed out by the radiation emitted by fast moving electrons. Above 10 gigahertz, noise from oxygen and water molecules in our own atmosphere would block out any signals.
They selected 1,420 gigahertz. This was the most promising frequency as this was the emission frequency for ordinary hydrogen gas. This is the most plentiful element in the universe. Frequencies around this range are nicknamed the "watering hole." Sadly, searches for intelligence near the watering hole was unsuccessful.
In 1960, Frank Drake initiated Project Ozma. This project attempted to search for signals using the 25-meter radio telescope in Green Banks, West Virginia. Sadly, no signals were ever found.
In 1971, NASA made the ambitious proposal to fund SETI research. It was known as Project Cyclops and involved 1500 radio telescopes and $ 10 billion.
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