via National Geographic.
Astronomer Mike Brown of CalTech discoverer of several Kuiper Belt Objects (KBOs) in the distant reaches of our solar system, has led a new analysis of the dwarf planet Eris (formerly called Xena and 2003 UB313) that places its mass firmly ahead (up to a third larger) of our old friend Pluto, demoting it not only from planet status but also as the King of the KBOs. The key was getting the mass of Eris' moon, Dysnomia.
"The key to finding Eris' mass was its tiny moon, Dysnomia. Brown and Schaller used Keck and Hubble to capture images of the moon's position over time. The data suggest that Dysnomia is Eris' only moon and orbits every 15 days. The researchers also calculated the total mass of the pair—about 1.27 times that of Pluto—and revealed that the objects are made of around 70 percent rock, making them as dense as Pluto and Triton, a moon of Neptune."
Of course, it has been Brown's work on the region that has ignited the controversy over the definition of the word planet and the status of our formerly ninth planet in the first place. However, there are still those who favor the initial IAU (International Astronomical Union) definition that would allow Pluto to maintain its planetary status, and add both Eris and Ceres (formerly an asteroid) all now both "dwarf planets". However, after much discussion, that definition was shelved in favor of another more restrictive one that resulted in the creation of the new "dwarf planet" category that the three solar objects now dubiously enjoy.