Complex sentence: Although knowledge of the nature of planetary systems, including that of the Solar System, has increased considerably, much is unknown about their origin and evolution and current theories of their formation and evolution are relatively new to the field of planetary science.
Split into simpler sentences: Although knowledge of the nature of planetary systems, including that of the Solar System, has increased considerably, much is unknown about their origin and evolution. Current theories of their formation and evolution are relatively new to the field of planetary science.