Secular evolution over 10⁸ yr of the β Pic b+c two-planet system corresponding to solution #1 from Table 1. Upper row, from left to right: eccentricities, mutual inclination, and (over the first 50 000 yr) the characteristic resonant argument σ_7:1 (see text). Our definition of σ_7:1 implies a 60° degeneracy. In our calculation, we always keep the values between 0 and 60°. Hence the variations from 0 to 60° appearing here actually mean circulation of σ_7:1. Lower row: short-term semi-major axis variations of the two planets over the first 50 000 yr.