Simplify the following text for a Grade 7 native English-speaking person.
Original: Pluto and its largest moon , Charon , are sometimes treated as a binary system because the barycentre of their orbits does not lie within either body .
Simplified: Pluto and its largest moon , Charon , are sometimes called a `` binary system '' . This is because the barycentre of their orbits does not lie within them .