Planar families in the 5/3 MMR computed for planetary mass ratio ρ = 1.32 of the K2-21 system. The initial conditions (yellow cross) correspond to the eccentricities provided by Dressing et al. (2017). The cyan curve corresponds to the evolution of the system, when the errors on the orbital elements were not considered. The black curve is centred at a stable (blue) periodic orbit of the family in the configuration (0, π), and corresponds to longitudes of pericentre ϖ₁ = 135.48° and ϖ₂ = −60.61° (values inside the error bars) and mean anomalies M₁ = M₂ = 0°.