Pluto and Eris: not twins!

In recent days, many media reports about the dwarf planet Eris and the fact that he, according to astronomical observations, a "twin" of the dwarf planet Pluto is. Why not, after all, it is so also in the official press release of the European Southern Observatory. But we actually found out something else: Pluto and Eris are both the same size. Therefore they must be twins but not identical.
Was announced as the discovery of Eris in 2005, there was great excitement. "Scientists Discover Tenth Planet" , the title was the NASA press release . Because Eris after the then state of knowledge was actually larger than Pluto, the smallest known planet - so it would have been a little weird when the great Eris should be classified as an asteroid while the small Pluto is a planet. Things were different, however, astronomers placed firmly in 2006 that neither Eris nor Pluto are planets but should the new group of "dwarf planets" are.

Figuring out how big a celestial body, is relatively easy when he is near. But Eris is so far away from the earth, that he identified himself only as a telescope in the best spot of light. The only way of Mike Brown ( the discoverer of Eris ) and his colleagues was open at that time, was prone to error. They used the brightness of Eris in order to infer the size: the brighter, the better! Of course, the brightness also depends on other parameters, the more ice exisiert on Eris' surface, the more light it reflects the dwarf planet. A small, icy celestial bodies can therefore shine as bright as a big rocky object. Reflecting on the ability (called "albedo") of Eris, however, no one knew then and there was no immediate opportunity to learn more about it. But even if Eris should be completely covered with ice, then measurements showed that the object is larger or the same size as Pluto ought to be. Later, more accurate measurements of Brown & Co confirmed this with the Hubble telescope They watched Eris and Eris was also almost a point. The difference between a point and a tiny screen was minimally but with lots of clever tricks and intelligent data analysis of Brown and his team were able to determine a useful value for the diameter of Eris: about 2400 km. The uncertainty was but 100 miles. Within the error limits of Eris is also the same size as Pluto, the diameter of 2300-2400 km somewhere must lie.

Fortunately, last year an opportunity to the diameter of Eris was as precise as never before to be able to determine: An occultation. As the moon covered the sun and sometimes it is dark in certain areas of the earth, may also obscure another heavenly body around another star. On 6 November 2010 drew over Eris whale in front of a star in the constellation, and obscured it for a few seconds or minutes (depending on where it was located). This could be measured and the duration of the eclipse can be calculated the size of Eris. The bigger the planet, the longer the star remain dark. Once the data has now been thoroughly evaluated, we now know that Eris has a diameter of 2326 kilometers. The inaccuracy is now only 12 kilometers. Whether Eris is now actually bigger than Pluto or not, one can say with absolute certainty, but still do not - this is the diameter of Pluto is not known accurately enough. Because Pluto has a thin atmosphere, the technology can be with him during the occultation not be fully applied. The atmosphere ensures that the brightness does not drop abruptly but slowly, and the measurement is less accurate. As it looks now, Pluto and Eris are about the same size. So it "twins"?

The Pluto and Eris are roughly equal in size, you already knew before. Now we know it in more detail. But the really important results of the measurements is that now you can also determine other parameters of Eris better. Actually, it was thought that Pluto and Eris should be both built pretty similar. They both come from the same region of the solar system and should therefore have a similar density. And since the mass of Eris pretty accurate from the orbital period of Eris' moon Dysnomia could be calculated ( Kepler's Third Law ) and Eris has been found to be 27 percent heavier than Pluto, it was assumed that Eris also a 27 percent greater volume or would have a 9 percent larger in diameter. Now we know that Pluto and Eris are the same size. So if Eris has 27 percent more mass, then it must also be 27 percent more dense than Pluto! Their internal structure must therefore differ significantly from that of Pluto! Pluto has a density of 2 grams per cubic centimeter and is composed of 70 percent rock and 30 percent ice together .. The density of Eris is 2:52 grams per cubic centimeter. The astronomers conclude that the dwarf planet consists mostly of rock, covered by a relatively thin, millimeter-thick layer of ice. Which consists of frozen methane and nitrogen and is perhaps the former atmosphere of Eris, which is frozen out, as the celestial body has moved to its elliptical orbit of the sun