Simplify the following text for a Grade 5 native English-speaking person.
Original: In our solar system , core size can range from about 20 % ( the Moon ) to 75 % of a planet 's radius ( Mercury ) .
Simplified: A core 's size can vary from each planet or other object . The moon 's core is 20 % of its radius , butMercury 's core is 75 % of its radius .