Range of surface density values Σ₀ that allow the individual Kepler-223 planet pairs to be captured into the correct resonance as a function of the semimajor axis of the outer planet of each pair. The error that is related to the mass error of the planets is indicated in red (Mills et al. 2016). The dotted black lines clarify that within the errors, no value for Σ₀ can be found that allows each separate pair to be captured into the correct resonance if the pairs are initially wide of the 2:1 MMR. The dotted blue vertical lines indicate that this does not hold if the planets are instead placed just wide of the correct MMR, which eliminates the lower limit for Σ₀.