Eris is a twin sister of Pluto

Starry cover provides important information about the distant dwarf planet

Artistic representation of the dwarf planet Eris. © ESO / L. Calçada
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Astronomers have gained new insights into the dwarf planet Eris in the outer solar system. This succeeded during a Vorüberwanderns of the object in front of a star behind it. Accordingly, Eris is almost a perfect twin sister of Pluto in size and seems to have a highly reflective surface. It could be a thin layer of ice - presumably the frozen and frosted atmosphere of the Eris.

In November 2010, the dwarf planet Eris offered the astronomers a special chance: He walked from the ground in front of a weak background star over. Due to the large distance and small size of the dwarf planet, such events, so-called star coverings, are very rare and difficult to observe. Star coverings, however, are the most accurate and often the only way to determine the diameter and shape of objects from the outer reaches of the solar system.

"The observation of star coverings by small bodies beyond the Neptune orbit requires great accuracy and careful planning. However, to determine the size of Eris, this is the best method - apart from a flight there, "explains first author Bruno Sicardy of the Universite Pierre et Marie Curie in Paris.

The Eris star cover was observed at two locations: at the La Silla Observatory of ESO, where the TRAPPIST telescope was used for observation and by two telescopes in San Pedro de Atacama, also in Chile. All three telescopes registered a sudden drop in the brightness of the star as Eris shielded the far-off star's light.

Eris is about the same size as Pluto

Previous measurements with other methods had suggested that Eris, with an estimated diameter of 3, 000 kilometers, would need to be about 25 percent larger than Pluto. The new, more accurate observations show instead that both are about the same size. The diameter of Eris was determined at 2, 326 kilometers. Since the uncertainty of the measurements is just 12 kilometers, the size of Eris is now far better known than that of its counterpart Pluto, whose diameter is anywhere between 2, 300 and 2, 400 kilometers. The diameter of Pluto is more difficult to determine because Pluto has an atmosphere and thus no sharp edge, which could be directly detected in a covering. display

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With the movement of Dysnomia, the Eris' moon, astronomers were also able to determine the mass of Eris. It turned out that Eris is 27 percent heavier than Pluto. From mass and diameter results in a mean density of about 2.52 grams per cubic centimeter, the researchers report. This density value suggests that Eris are mostly made of rock and should be surrounded by a comparatively thin mantle of ice utters Emmanuel Jehin from the Universit de Li ge in Belgium.

Extremely bright ice surface

The surface of Eris throws off 96 percent of the incoming light. Thus, the surface is even more reflective than freshly fallen earth snow and makes Eris together with the Saturn moon Enceladus one of the best reflecting objects in the solar system. The dwarf planet's spectrum shows that its surface should be covered by a mixture of nitrogen-rich ice and frozen methane less than one millimeter thick.

EisThis layer of ice could have been created when the dwarf planet's nitrogen or methane atmosphere was deposited as frost on its surface when Eris moved farther away from the sun on its elongated path, and therefore Cooling, J adds Jehin. If Eris approaches the sunniest point of her orbit at about 5.7 billion miles from the Sun, the ice could turn to gas again.

It's really amazing how much we can find out about such a small and distant object as Eris, by watching with relatively small telescopes how it passes in front of a star. Five years after the introduction of the dwarf planet's new object class, we are finally getting to know one of the first members of this class, "concludes Sicardy. (Nature, 2011; DOI: 10.1038 / nature10550)

(ESO, 28.10.2011 - NPO)