 Models of internal heating via radioactive decay suggest that Eris could have an internal ocean of liquid water at the mantle–core boundary.In July 2015, after nearly ten years of Eris being considered the ninth-largest object known to directly orbit the sun, close-up imagery from the "New Horizons" mission more accurately determined Pluto's volume to be slightly larger than Eris's, rather than slightly smaller as previously thought.Eris is now the tenth-largest object known to directly orbit the sun by volume, but remains the ninth-largest by mass.