Constraints derived from stellar and planetary evolution models on the mass of heavy elements present in planets Kepler-9b (top) and Kepler-9c (bottom) as a function of their age. The areas correspond to the ensemble of models that fit the constraints within 1σ (green), 2σ (blue), and 3σ (purple). The 2–4 Ga age range is highlighted because it is strongly favoured by gyrochronology. The error bar on each panel represents the total mass of the corresponding planet and its 1σ uncertainty.