The outer solar system is still largely a mystery to astronomers, and no celestial body illustrates this more clearly than the long-lost “Planet X,” Eris. It was Eris’ discovery in 2005 that prompted astronomers to famously downgrade Pluto from a planet to dwarf planet, as they realized that Eris would not be the last larger-than-Pluto object they’d find in the outer solar system—and so unless they planned on making kids memorize the order of a lot of planets, they needed to refine the taxonomy. Eris became either the tenth planet or the first dwarf planet, or both, depending on how you look at it. In any case, its discovery fundamentally changed the way we look at our solar system.
Considering the fact that we’ve never really gotten a good look at Eris, it’s remarkable that astronomers know as much about its composition as they do. It’s perhaps even more remarkable that Eris appears to have a much better chance of sustaining life than any other planet or dwarf planet that we’ve found in our solar system (with the obvious exception of Earth); the surface contains methane, and there’s an excellent chance that, much like the moons of Enceladus and Europa, Eris contains an underground ocean heated by geological activity.
We probably won’t know for sure for some time, because Eris is pretty far away and we haven’t gotten a very good look at it yet. As soon as 2018, the James Webb Space Telescope (JWST) may tell us about the dwarf planet’s surface features, confirm or eliminate the possibility of an underground ocean, pick up on biosignatures, and so on. If it turns out that an actual visit to Eris is in order, that may be harder to arrange; Eris is so far away that it takes 560 years to orbit the Sun, and an unmanned craft launched today with the specific purpose of examining Eris would still take more than a decade to reach it. What we learn over the next few years may tell us whether it’s worth the trip.