Complex sentence: Planet Nine is a hypothetical large planet in the far outer Solar System, first proposed in 2014, which would explain the unusual orbital configuration of a group of trans-Neptunian objects (TNOs) in the outer part of the Kuiper belt.
Split into simpler sentences: Planet Nine is a hypothetical large planet in the far outer Solar System. The existence of the planet would explain the unusual orbital configuration of a group of trans-Neptunian objects (TNOs) in the outer part of the Kuiper belt.