Why is peptide bond planar?

Peptide bonds are planar due to their partial double bond characteristics existing between the nitrogen and carbon atoms of the -CONH bond. In peptide bonds, resonance occurs when all nonbonding electrons and empty orbitals are on the same plane, like a double bond. Thus, this resonance causes more stability of the peptide bond, and that is why they typically exist in a trans, planar configuration. Because peptide bonds are planar, there is very little rotation or twisting involved around the amide bond which links the alpha amino acid nitrogen of one amino acid molecule to the carbonyl carbon of the adjacent amino acid. It is important to note that the partial double bond character is from the length of the bond; it is approximately 0.13 Angstrom shorter than the C-N single bond, but not as short as a regular double bond.