It wasn’t long ago when astronomers who wanted to search for planets outside our solar system were considered daft. A couple of brave scientists began the search, found what they were looking for, and started a whole new area in astronomy. Hundreds of exoplanets have been identified and those who invest their careers in this search are no longer ridiculed. The search has led to a very respectable and fast-growing new field: astrobiology (sometimes called exobiology). So what do astrobiologists expect to find and how do they build their models of alien life?
As far as we know, the fundamental ingredients for life are hydrogen, oxygen, nitrogen and carbon, the most common elements in the known universe. Earth life is composed principally of these four, plus sulphur and phosphorus and traces of other elements, for a total of some 26 elements. It is amazing to realize that such a small number of basic ingredients can give rise to such diversity as we find. The other essential for life seems to be water. On Earth, life exists in all climates and conditions where there is water, and in the very few areas where there is absolutely no water there is absolutely no life. Water is abundant in the universe, although more in the form of ice than liquid.
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