Hypothetical types of biochemistry are forms of biochemistry speculated to be scientifically viable but not proven to exist at this time.[2] The kinds of living organisms currently known on Earth all use carbon compounds for basic structural and metabolic functions, water as a solvent, and DNA or RNA to define and control their form. If life exists on other planets or moons, it may be chemically similar; it is also possible that there are organisms with quite different chemistries[3]—for instance involving other classes of carbon compounds, compounds of another element, or another solvent in place of water.