Complex sentence: The ice giants represent one of two fundamentally different categories of giant planets present in the Solar System, the other group being the more-familiar gas giants, which are composed of more than 90% hydrogen and helium (by mass).
Split into simpler sentences: The ice giants represent one of two fundamentally different categories of giant planets present in the Solar System. The other group being the more-familiar gas giants, which are composed of more than 90% hydrogen and helium (by mass).