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Molecular handedness (chirality) is the potential of some compounds to form molecules incorporating the same combination of elements, but in two forms that geometrically differ, being reflections of each other, in the manner that a person's hands appear to be mirror-reflections of each other.
In many processes that create such compounds, both hands of molecules are produced in roughly equal numbers. However, for some key molecules, life on Earth incorporates and uses only one of two such possibilities (i.e., homochirality, consistent handedness), either specifically synthesizing the one that it uses or reducing its disintegration, either way resulting in differing abundances. For this reason, molecular handedness is a proposed biosignature. In some cases, the polarization of EMR can reveal an imbalance of chirality at the source, but in others, detecting it requires in situ investigation, which makes it more of a candidate for detecting life in solar system bodies than for extra-solar planets.