Constraints on planet mass and density ratios in the orbital expansion phase with the radius of the planets according to Table 1. The green region corresponds to the area satisfying a_cde > 1.56, where orbital expansion to the observed period ratio is possible, while the planets cannot reach the observed period ratio if the planet mass ratio falls in the gray region. The colored dots represent different mass models in Table 2. The conversion to density ratios (upper and right axes) has assumed the radii of the planets according to Table 1.