Eris, the largest dwarf planet in our solar system, made headlines in 2006 when scientists reported that it was larger and heavier than Pluto. New measurements are suggesting that Eris may actually be more like Pluto’s twin. The newly measured radius of Eris puts it within the error range of the accepted size of Pluto.
The dwarf planet Eris was discovered in 2005. Based on imaging, it was thought to have a radius of anywhere from 1,100 to 1,700 km, making it likely to be larger than Pluto (thought to have a radius between 1,150 and 1,200 km). Based on the orbit of its moon, Dysnoima, Eris's mass has been estimated to be about 27 percent larger than Pluto's.
Scientists were able to get a better handle on the size of Eris by observing it as it passed in front of a background star, a technique called stellar occultation. By observing how long Eris obscures the star (given how fast it is moving), you can determine the width of Eris at that point. By simultaneously measuring the occultation from different locations on Earth, astronomers were able to measure the width of Eris from different angles, and thus able to determine its shape.
Astronomers tried to observe Eris's occultation from 26 different locations, and four were successful. They found Eris’s radius to be 1,163 ± 6km. A very impressive precision, considering Eris’s angular diameter is only about 0.03 arcseconds (the same angle a bb takes up at 1km) and it was moving at a paltry 1.5 arcsecond per hour. Something that small and slow does not occult stars very often.
The occultation indicated that there is no appreciable atmosphere on Eris, meaning it's at least 1/10,000 that of Pluto’s. That, combined with the high surface brightness of Eris, suggests that it has a recently collapsed atmosphere—that is, an atmosphere that has frozen onto the surface. If true, as Eris gets closer to the Sun, some of its surface may sublimate, reducing its surface brightness and restoring its atmosphere. Stay tuned; with an orbital period of 557 years, we should know for sure in a few hundred years.