Facts about the Dwarf Planet Eris

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When Neptune was discovered in 1846 astronomers thought that the orbit of the planet was perturbed by a mysterious massive body further out in the solar body. Although the massive body was never found, and scientists realised that much of the error in the orbit could be explained through a better definition of the mass of Neptune, the search for planets beyond Neptune led to the discovery of Pluto in 1930, the Kuiper belt in 1992 and the Scattered Disc in 1996

The Kuiper belt contains many celestial bodies that are too small to be classified as planets. To date more than 100 objects have been discovered that are more than 100 kilometres in diameter. Scientists estimate that there are over 10,000 such bodies in the Kuiper belt. These objects made of frozen gas are remnants from the formation of the Solar System.

The Scattered Disc resembles the Kuiper belt in that it is thought to contain similar small, frozen celestial bodies. It differs because it lies further from the sun and contains objects that are in highly elliptic and unstable orbits. Whereas the obit of Neptune stabilises the orbit of bodies in the Kuiper belt, it disturbs the orbit of bodies in the Scattered Disc. Some of the comets seen on Earth are believed to have originated in the Scattered Disc. To date (2011) over 200 objects have been found in the Scattered Disk. The Scattered Disc may contain as many objects as the Kuiper Belt.

In July 2005 NASA made an astonishing announcement “10th Planet discovered: Astronomers have found a new planet in the outer reaches of the solar system”. The press release referred to a discovery by Michael Brown and colleagues at the Mount Palomar Observatory in California. This team was making systematic observations from the Scattered Disc. They found the object when they re-examined observations made in October 2003. They found an object that their automatic scanning software had rejected out of hand.  

Brown chose to call the newly discovered object Eris after the Greek goddess for struggle and strife. He did so to reflect the controversy that the object caused in astronomical circles. NASA thought it was a planet. The Initial observations were planet like: it was in orbit around the sun and had a mass slightly greater than that of Pluto. The International Astronomical Union (IAU) thought otherwise. In August 2006 the IAU formally defined a planet. It was a body that is in orbit around the sun, is sufficiently massive to maintain a spherical shape through hydrostatic equilibrium and sweeps its orbit clear of debris. A body which is otherwise a planet but fails to clear its orbit of debris is a dwarf planet. Eris and Pluto are dwarf planets.             

Brown and his colleagues found that Eris has a highly elliptic orbit. It has an orbital period of 557 years. At the moment it is three times further from the sun than Pluto. It will come closest to the sun in 2256 and at that time will be comfortably within the orbit of Pluto. The strangest point is that most objects in the Solar System orbit in roughly the same plane. Eris is in orbit at 44.187° to that plane. Nobody knows why.

Scientists think that Eris was thrown into a high inclination orbit through an ancient gravitational encounter with Neptune. Computer models suggest that Eris originated in the Kuiper belt. Scientists have not yet systematically searched for high inclination objects from within the Scattered Cloud. When they do, they are likely to discover more dwarf planets similar to Eris.

Mike Brown collaborated with a team the W.M.Keck Observatory in Hawaii. They studied Eris using a technique known as adaptive optics. By firing a laser into the upper atmosphere astronomers can compensate for atmospheric dispersion. This allows them to see objects more clearly than otherwise. In October 2005 the group were able to announce that Eris had a moon which they called Dysnomia.

Although scientists are confident that Eris is more massive than Pluto they are unsure which has the larger diameter. In June2007 using evidence from the orbit of Dysnomia they came to the conclusion that it was 27% heavier than Pluto.  When the Hubble Space Telescope was used to determine the diameter of Eris in 2005 the diameter was found to be 2,397 kilometres. This is less than 5% larger than the diameter of Pluto. To do this the Hubble Telescope was operating close to its operating limit, using a specialist technique known as deconvolution. In 2007 a measurement by the Spitzer Space Telescope implied a larger diameter of 2,600 kilometres. Based on evidence from a stellar occultation which took place in 5 November 2010 Michael Brown has given a preliminary estimate for the diameter of Eris as 2,320 kilometres which is close to the Hubble observation. The results are crucial. If the findings of the Hubble Telescope and the occultation are correct Eris has a density compatible with a ro9cky composition. If the space Telescope experiment is correct Eris has a composition of frozen gas.

In 2005 a team at the Gemini North Telescope in Hawaii conducted a study of the infra red spectrum of Eris. They concluded that the surface was composed of solid methane. Eris also has an albedo of 0.86, higher than that of any other large body in the Solar System except Enceladus, the sixth moon of Saturn. A high albedo im0plies that the surface has a fresh white appearance. A clean surface implies that the surface methane is being refreshed.

Scientists are currently investigating the mechanisms that could explain the clean surface. Eris has a surface temperature of between minus 243 and 217 degrees Celsius depending upon its position in its orbit. For much of the orbit the surface temperate is so low that any evaporating methane would fall back as snow. For just part of the orbit when closest to the sun the surface temperature would be sufficient to allow methane to sublime and refresh the surface. Some scientists think that Eris might have an internal source of heat, such as radioactive decay which is melting the methane which is falling back as snow. There is even speculation that Eris could support liquid water beneath the surface.

Currently, apart from a few man made satellites and a few comets, Eris and its moon are the furthermost known objects in the Solar System. It is a bright object which can be seen in the sky using the best amateur equipment.

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