The search for life in our solar system reached its highest point to date with the successful landing (1976) of two viking spacecraft on Mars. Abroad were experiments designed to look for the biological processes of metabolism, experiment gave positive results, a gas chromatograph failed to detect the presence of
organic molecules, leading many to believe that the prevailing opinion is that Mars most likely does not support even low forms of life.
Because manned interstellar travel is not now feasible and because no evidence has established beyond doubt that Earth has been visited by interstellar travelers (despite persistent reports of Unidentified Flying Objects)
UFO, the search for extraterrestrial intelligent (SETI) outside the solar system must at present be carried out with radio telescope. Such telescopes could detect radio signal transmitted by intelligent beings on distant planets. The central problem with this approach is deciding which stars to listen to and at what frequencies. In a landmark paper (1959) in Nature, Philip Morrison and Giuseppe Cocconi suggested a frequency of 1,420 mHz, corresponding to a wavelength of 21 cm, as a universally recognizable communication channel, because that frequency is emitted when an electron reverses its spin in an atom of hydrogen, the most abundant element in the universe. The very abundance of hydrogen, however, may make this channel too noisy, and thus other, supposedly fundamental frequencies might be used. Once a frequency is decided on, nearly sun-like stars would be logical early targets.
Human until now can't detect any other
place to life like in the Earth, so they should keep this Earth more careful because we don't have any alternative place to life.
