How do we use what we know about earth, its structure, atmosphere, geographical activities and processes to prove or disprove our theories of the other planet? This is not evident that there exists a planet as same as earth in interstellar space on which life form can exist. But there are some compelling reasons to believe in the life forms in outer space. There are many discoveries on the basis of which scientists assume on life forms on other planets. Many life forms have developed so that that they can survive in heat, cold, poisonous chemicals or even in vacuum, and are known as Extremophiles like tardigrades. In other words, we know life can survive in conditions we've seen on other planets and moons. We just haven't found it yet.
Earth’s structure is mainly consisting of rocks and oceans; we know that life started from oceans and now scientists know that there exists strong evidence that water once flowed freely on Mars and Titan, the closest known example to possess similar surface geology. On Titan even scientist had found the precursors of life. But if we see of atmosphere both of them have non-persistent atmosphere. Kepler 22b is most habitable planet as it orbits a sun-like star in habitable zone i.e. “goldilocks zone meaning they orbit at a distance that could produce temperatures similar to those on Earth”. The mediocrity principle suggests that there is a chance that lucky events may have allowed an Earth-like planet to form elsewhere that would allow the emergence of complex, multi-cellular life. Hence we cannot prove or disprove our theories about other planets until we get rigid evidences rather than strong theories.
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