Simplify the following text for a Grade 9 native English-speaking person.
Original: The invariable plane of a planetary system , also called Laplace 's invariable plane , is the plane passing through its barycenter ( center of mass ) perpendicular to its angular momentum vector .
Simplified: The invariable plane of a planetary system is the plane passing through its barycenter ( center of mass ) . This is perpendicular to its angular momentum vector .