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.