An artist's depiction of Eris and its moon Dysnomia. A new study shows the distant object, residing in the Kuiper Belt, is a near-duplicate of Pluto in terms of size.
Credit: NAS
Moon rise over the 50-cm Caisey Harlingten telescope at San Pedro de Atacama, Chile, where one of the Eris occultation light curves was obtained.
Credit: A. Maury
LONDON: The dwarf planet Eris has been revealed as Pluto's twin, with an atmosphere that periodically collapses to generate one of the brightest bodies in the Solar System.
The new findings show that Eris, previously thought to be the largest body in the Solar System beyond Neptune's orbit, is in fact a similar size to Pluto.
The observations of a rare event, in which the distant Eris passed in front of a star, also enabled the scientists to characterise the structure of the dwarf planet, indicating that the atmosphere of large, icy objects in this region of space may collapse and then reappear throughout orbit with changing proximity to the Sun.
"We now have a much more accurate measurement of Eris' size that gives us a radius very close to that of Pluto," said Bruno Sicardy from the Université Pierre et Marie Curie and Observatoire de Paris in Meudon, France and lead author of the paper published today in Nature. "Eris is an example of what Pluto would look like if it were away from the Sun."
The great debate
Eris was discovered in 2005, and initial predictions that it was significantly larger than Pluto sparked a great debate in the field of planetary astronomy. Initially referred to as the 'tenth planet', Eris was the trigger for the demotion of Pluto to non-planetary status as the International Astronomical Union decided to reclassify both objects into a new category of 'dwarf planets'.
Eris and Pluto both occupy the Kuiper Belt, a distant ring of frigid bodies in the region of space beyond Neptune, although Eris is roughly three times further away than Pluto.
Since its discovery, scientists have confirmed that Eris is 27% larger in mass than Pluto, but that the dwarf planets might share a similar structure of a methane-ice-rich cover. Several attempts have been made to accurately determine the size of Eris, and in 2006 two different techniques indicated the newly discovered object was significantly larger than Pluto, but both included a significant margin of error.
Unprecedented brightness
The latest results predict the size of Eris to a much greater accuracy, and indicate that the two dwarf planets are in fact very similar in size - such that the researchers are unable to confirm which is bigger. The observations also indicate that Eris is unusually reflective. "The surface is almost as bright as a white sheet of paper. Very few bodies are as bright in the solar system," explained Sicardy.
The team of astronomers, from France, Belgium, Spain and Brazil, predicted the occurrence of the stellar occultation in which Eris obscured the light from the star, providing an accurate way to measure the size of the dwarf planet from the faint shadow cast on Earth.
Observations of the occultation were attempted from 26 international locations that fell in the shadow's predicted path, and three telescopes at two sites in Chile successfully recorded the sudden drop in brightness.
By combining these recordings, the astronomers calculated that Eris has a radius of 1163 km, highly comparable to estimates of Pluto's size at between 1150km and 1200km. However, the greater mass of Eris indicates it has a much higher density than Pluto, and therefore consists mainly of rock with a relatively thin mantle layer of ice.
That's not Eris
That looks like artwork of the rapidly rotating space rugby ball that is Haumea, aka 2003 EL61.
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