Simplify the following text for a Grade 9 native English-speaking person.
Original: With a diameter of about 950 k , Ceres is by far the largest and most massive body in the asteroid belt , and contains almost a third ( 32 % ) of the belt 's total mass. .
Simplified: With a diameter of about 950 km , Ceres is by far the largest and most massive object in the asteroid belt , and has about a third of the belt 's total mass. .