Eris is 1,445 miles in diameter, just 10 or so miles wider than Pluto, which rings in at 1,432. So it is still bigger, but only a tad. This size relative to its mass also means it must be a dense, rocky world, the astronomers report. Also noteworthy is its greater-than-expected brightness, the astronomers report in the journal Nature. It is covered in a thin shell of methane ice, and is one of the brightest objects in the entire solar system, they say. The team found Eris' atmosphere is about 10,000 times more tenuous than Pluto's present atmosphere, and they suggest that it fades away as the planet moves farther away from the sun in its elliptical orbit.