Artist drawing of Eris and Dysnomia. Remember, the camera adds ten pounds. Copyright Robert Hurt, IPAC
Eris was discovered a few years back, and observations indicated it might be bigger (that is, have a larger diameter) than Pluto. This is pretty hard to do, because it didn’t look like much more than a dot in telescopes; the diameter had to be inferred by its known distance and its brightness. If it’s made of something dark (like organic chemicals, common on distant objects), it must be big to look as bright as it does; if it’s made of something reflective (like snow or ice) then it’s smaller. Subsequent Hubble observations indicated it was indeed bigger than Pluto, and the former ninth planet took one more body blow.
Now it’s known that not only is Eris bigger, it’s more massive. About 30% more massive, in fact.