2003 UB313 - Size, Mass and Density

Size, Mass and Density

In 2005, the diameter of Eris was measured to be 2,397 km, give or take 100 km, using images from the Hubble Space Telescope (HST). The size of an object is determined from its absolute magnitude (H) and the albedo (the amount of light it reflects). At a distance of 97 AU, an object with a diameter of 3,000 km would have an angular size of 40 milliarcseconds, which is directly measurable with the Hubble Space Telescope. Although resolving such small objects is at the very limit of the telescope's capabilities, sophisticated image processing techniques such as deconvolution can be used to measure such angular sizes fairly accurately.

This makes Eris around the same size as Pluto, which is about 2,330 km across. It also indicates an albedo of 0.96, higher than that of any other large body in the Solar System except Enceladus. It is speculated that the high albedo is due to the surface ices being replenished because of temperature fluctuations as Eris's eccentric orbit takes it closer and farther from the Sun.

In 2007, a series of observations of the largest trans-Neptunian objects with the Spitzer Space Telescope gave an estimate of Eris's diameter of 2,600+400
−200 km. The Spitzer and Hubble estimates overlap in the range of 2,400–2,500 km, 4–8% larger than Pluto. However, astronomers now suspect that Eris's spin axis is pointing toward the sun, at the moment—a possibility that would keep the sunlit hemisphere warmer than average and skew any infrared measurements toward higher values. So the outcome from the 2010 Chile occultation is actually more in line with the Hubble result from 2005.

In November 2010, Eris was the subject of one of the most distant stellar occultations yet achieved from Earth. Preliminary data from this event cast doubt on previous size estimates. The teams announced their final results from the occultation in October 2011, with an estimated diameter of 2,326+12
−12 km. However, when using data from this event for comparison to Pluto, there is a range of figures available for Pluto's radius/diameter that can be selected. This is due in part to Pluto's atmosphere which interferes with making measurements of its solid surface (as opposed to gaseous haze). The mass of Eris can be calculated with much greater precision. Based on the currently accepted value for Dysnomia's period—15.774 days— Eris is 27 percent more massive than Pluto. If the 2011 occultation results are used, then Eris has a density of 2.52±0.05 g cm−3; substantially denser than Pluto, and thus must be composed largely of rocky materials.