Re: Have aliens visited earth?
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id say its probable for other intelligent life to exist somewhere. getting here is highly unlikely.
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Actually, the conditions for life to exist are extremely rare. Life as we know it is extremely fragile and it appears, based on probabilistic arguments, that the conditions for life to exist are not that common in the universe. Microbes quite likely exist in abundance but complex life not. This argument was first set forth in the book Rare Earth: Why Complex Life is Uncommon in the Universe by Peter Ward, Donald Brownlee.
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As the previous poster said, the number are just too big not for it to have happened elsewhere. 100 billion (10^11) stars in the Milky Way alone, probably 100 biliion galaxys. There are four worlds where life may originiate in our system alone (Earth, Mars, Europa, Titan). Say 25% of all systems have planets, so you've got 100 billion planets in the Milky Way where it has been/will be possible, 10^22 in the universe.
One could be vast orders of magitude off on probablity estimates and still have almost uncountable intelligent civilizations in the universe, let alone planets where there is mere life.
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