If Eris, the most remote dwarf planet, were the same distance from the Sun as Pluto is, the two worlds might look almost identical. Instead, Eris has one of the brightest surfaces in the solar system, so it reflects almost all of the sunlight that strikes it back into space. Eris follows a highly stretched-out orbit around the Sun. It was closest to the Sun more than two centuries ago. Sunlight warmed the little world enough to vaporize some of the nitrogen and methane ice on its surface, creating a thin, cold atmosphere. As Eris again retreated from the Sun, though, the atmosphere refroze, depositing fresh, bright ice on the surface. As Pluto moves farther from the Sun, its atmosphere should freeze as well, making Pluto’s surface resemble that of present-day Eris. Eris’ brightness helped prod astronomers into creating the “dwarf planet” category in the first place. When it was discovered 10 years ago this week, astronomers assumed its surface was about as reflective as Pluto’s is. Sin
