Why is the peptide bond planar?
The peptide bond is formed during a condensation reaction between two amino acids. Amide (CO-NH2) group and carboxylic acid (-COOH) combined up to release water molecules. This bond is planar because resonance is possible when all nonbonding electrons and empty orbitals are in the same plane. As resonance leads it towards stability, it prefers to stay in trans planner configuration.
Resonance of peptide bond
It gives partial double bond character to peptide bond. This reason is confirmed by analyzing the bond length (C-N) which is 103 nm instead of i.e the molecule has ceased to rotate. It is mostly found in Trans configuration.