|Has anyone ever actually proven the assumption that life can only exist on planets similar to earth? Even on our own planet there are species that thrive in extremes of cold and heat and where there's little light, oxygen or water and other supposedly vital substances and conditions. The only absolute necessity is energy and even planets with no sunlight could supply geothermal energy. |
Of course this isn't something you can go ahead and write a mathematical proof of, but certain assumptions are made. At extremely low temperatures, chemical reactions of almost all types cannot happen. One thing we do know is that without chemical reactions, there is no life, so you can set a lowest temperature cut off point depending on what kind of reactions you think life needs. The general opinion is that life needs quite complex reactions using water, so the temperature needs to be at least above 0.01C. -220C is such a low temperature I'm not sure any chemical reaction would happen at all.
You get a similar thing with high temperatures. When the temperature gets high, chaos reigns. Life represents some kind of order. Too much heat and order is lost. Any temperature above 100C becomes very difficult for life to survive in already. This is why somewhere like Venus (400C or something) is not considered possible for life to exist.
All in all, it is pretty safe to assume the following are requirements for life:
-Reasonable temperature - Probably 0 up to 100 C or for the really extreme among you maybe -100 up to 200 C (though you would only expect microbes at such low temperatures.
-Low gravity (always good to not be being crushed)
-Low pressure (again, good not to be squashed)
-A source of energy
-Shielding from radiation (life doesn't work if radiation is randomly smashing bonds up everywhere)
-Time (for life to evolve in)
-Carbon or silicon (basically the only atoms that allow complex chemistry)
-Lack of toxic compounds
-Reasonable weather (see Venus)
-Liquid water (slightly questionable requirement, but all life on Earth uses it)
The requirements for life are actually pretty specific. The only reason people expect life to be around in other places is from the sheer number of stars, not because making life is easy or statistically likely.