A mirror at the edge of the solar system
On January 5, 2005, a team at the Palomar Observatory in California discovered an object in the far reaches of the solar system that was more massive than Pluto. This object, named Eris, is one of the brightest bodies known. It reflects about 96% of the light that strikes it, an albedo comparable to fresh snow and surpassed only by Saturn's moon Enceladus. This extreme reflectivity points to a surface covered in young, pristine ices.
Spectroscopic analysis reveals a surface dominated by methane ice, mixed with frozen nitrogen. This composition is similar to Pluto's, but Eris appears almost uniformly white, unlike Pluto's reddish, varied terrain. The reddish color on Pluto is attributed to tholins, complex organic molecules created when solar radiation and cosmic rays break down methane. The lack of these dark deposits on Eris suggests its surface is being actively and constantly renewed. This presents a scientific puzzle, as Eris is currently about 96 astronomical units from the Sun—three times farther than Pluto. At such a great distance, the Sun's weak light provides little energy to drive surface activity.
The dwarf planet has a diameter of approximately 2,326 kilometers, making it slightly smaller than Pluto by volume, but its mass is 27% greater. This indicates a much denser, rock-heavy interior. Eris follows a long, highly eccentric 559-year orbit that takes it as close as 38 AU to the Sun and as far out as 97.5 AU.
The mystery of the fresh ice
The reflective surface of Eris challenges planetary scientists. Space weathering should have darkened its methane ice over millions of years, yet it remains bright. Several theories attempt to explain this ongoing resurfacing.
One hypothesis involves atmospheric collapse. Eris's highly eccentric orbit causes dramatic temperature swings, from about 30 K to 56 K (−243°C to −217°C). When Eris is closer to the Sun (a position it will next reach in the year 2257), some of the frozen methane and nitrogen on its surface may sublimate, turning directly into a thin gas atmosphere. As Eris travels toward its farthest point from the Sun (aphelion), this atmosphere would freeze again, falling back to the surface as a bright layer of ice. This process would effectively give the dwarf planet a new coat of paint with each long orbit.
Another possibility involves internal geological activity. Recent data from the James Webb Space Telescope detected specific isotopes in the surface methane that suggest it was not primordial but rather produced by geothermal processes deep inside Eris. This could mean there is enough residual heat from its formation or from radioactive decay in its rocky core to drive cryovolcanism—eruptions of water, methane and ammonia from below the surface. Such eruptions would continuously pave over the weathered ice. This evidence for a warm interior suggests Eris could even harbor a subsurface liquid water ocean.