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Hypothetical types of biochemistry
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<p>Several forms of <a href="/facts/Biochemistry/fa2pWEBp">biochemistry</a> are agreed to be scientifically viable but are not proven to exist at this time. The kinds of <a href="/facts/Life/DNw7NUId">living organisms currently known on Earth</a> all use <a href="/facts/Carbon/ukrgQexo">carbon</a> compounds for basic structural and <a href="/facts/Metabolism/WxDrm8k5">metabolic</a> functions, <a href="/facts/Water/0lkXnJPK">water</a> as a <a href="/facts/Solvent/LoMPMz8S">solvent</a>, and <a href="/facts/DNA/nB89Iauz">DNA</a> or <a href="/facts/RNA/HxPeNfNj">RNA</a> to define and control their form. If <a href="/facts/Life/DNw7NUId">life</a> exists on other <a href="/facts/Planet/rbPjE6Bx">planets</a> or <a href="/facts/Natural_satellite/AE4KQKbq">moons</a> it may be chemically similar, though it is also possible that there are organisms with quite different chemistries – for instance, involving other classes of carbon compounds, compounds of another element, or another solvent in place of water. </p><p>The possibility of life-forms being based on "alternative" biochemistries is the topic of an ongoing scientific discussion, informed by what is known about extraterrestrial environments and about the chemical behaviour of various elements and compounds. It is of interest in <a href="/facts/Synthetic_biology/rHSNe8j3">synthetic biology</a> and is also a <a href="/facts/Science_fiction_theme/2SqxkwEc">common subject in science fiction</a>. </p><p>The element <a href="/facts/Silicon/cgWx0hdr">silicon</a> has been much discussed as a hypothetical alternative to carbon. Silicon is in the same group as carbon on the <a href="/facts/Periodic_table/umdGIfUb">periodic table</a> and, like carbon, it is <a href="/facts/Tetravalent/ck7cHByj">tetravalent</a>. Hypothetical alternatives to water include <a href="/facts/Ammonia/MtLtJFtz">ammonia</a>, which, like water, is a <a href="/facts/Chemical_polarity/KlIJ0zjB">polar</a> molecule, and cosmically abundant; and non-polar <a href="/facts/Hydrocarbon/Dcg95Ju5">hydrocarbon</a> solvents such as <a href="/facts/Methane/kBHneYnh">methane</a> and <a href="/facts/Ethane/BGCzc6yh">ethane</a>, which are known to exist in liquid form on the surface of <a href="/facts/Titan_(moon)/lg0l9PCH">Titan</a>. </p>