Simplify the following text for a Grade 7 native English-speaking person.
Original: In the late 20th and early 21st century , many objects similar to Pluto were discovered in the outer Solar System , notably the scattered disc object Eris in 2005 , which is 27 % more massive than Pluto .
Simplified: Later , in the early 21st century , the scattered disc object Eris and other objects like Pluto were discovered . Eris is 27 % more massive than Pluto .