What is life? How does life begin and evolve? And where do you find it? Astrobiology seeks to understand life in the context of the wider cosmos, and it is combining aspects of biology, geology, astronomy, physics as well as philosophy. Astrobiologists study the origins, evolution, and future life in the universe.
Is the question, whether life exists elsewhere the universe a verifiable hypothesis, and thus a valid line of scientific inquiry? Some scientists claim that astrobiology is a pseudoscience. Pseudoscience is false science and the belief in extraordinary claims without scientific proofs. Scientific ideas depends on experimental and observational confirmation.
One thing that is clear is that astrobiology is more theoretical than scientific. There may be a great deal of mathematical computation which support for example, the string theory, but at the moment such evidence for astrobiology is not there. The only way of finding out whether or not there are life on other planets, is to keep looking. Receiving signals from intelligent life like ourselves may be unlikely, and there are so far no direct evidence of extraterrestrial life. The cosmos could be filled with life. Or life could be a rare combination of accidents. Theoretically there should be many other living planets, according to some calculations in the order of one billion. The Drake equation expresses the probability of intelligent life as the product of factors such as the fraction of planets that might be habitable and the fraction of planets on which life might arise. Considering that there is only one known planet with life forms some critics consider the equation as un-reliable.
The most likely candidates for life in the solar system are the planet Mars, the Jovian moon Europa and Saturn's moon Titan. A growing body of evidence to suggest that Mars has previously had a considerable amount of water on its surface, water is considered to be an essential precursor to the development of life, although it has not been conclusively proved. Titan has been described as having early Earth-like conditions (another criticism of astrobiology has been that it creates tautologies. Earth-like life will evolve in circumstances which are Earth-like enough).
Astrobiologist doubt that life exists in extreme conditions, if and when we do find life on other planets it is likely that it will be somewhere that is not too hot or too cold. However, astrobiologists have teamed up with marine biologists and geologists to search for extremophiles organisms that live in extreme physical and geochemically conditions. They have found bacteria that are able to live without oxygen, or light. Their findings help them to create hypotheses on whether life could exist on certain planets or moons, such as Europa.
It is also possible that if and when we find life it might look completely different from the life based on carbon chemistries. If such life does exist, what does chemistry permit? Life on Earth is based on DNA, a specific genetic code. Could there be non-DNA life? Once we move out in the solar system, chemistry changes. There could be different information systems, different solvents, different membranes. And as we go from hotter to colder, we really should expect radically different chemistries. Exciting, isn't it?