sentence1: however , it was believed that the interpretation of the signals and the characterization of the detected multiple-planet systems would be difficult due to the complexity of the magnification pattern in the central region combined with the large number of lensing parameters required to model multiple-planet systems .
sentence2: they noted , however , that characterizing the detected multiple-planet systems by analyzing the central perturbations would be difficult due to the complexity of the magnification pattern combined with the large number of lensing parameters required to model multiple-planet systems .