Thursday, October 27, 2011

Occultation of Minor Planet 136199 Eris Gives Significant Data

Last November 6, 2010, minor planet 136199 Eris occulted the star USNOA2 0825-00375767 in the constellation Cetus. What significance does this relate to? Before this occultation, trans-Neptunian object Eris was assumed to be smaller than Pluto, its presumably larger contemporary. But now, Eris' diameter has been measured and not guesstimated, but rather an exact measurement. The video below shows the occultation, as astronomers would have seen it on that November day.

USNOA2 0825-00375767, with a magnitude of 17.25 is a very faint star, but nothing is too dim for the Belgian TRAPPIST telescope at ESO’s La Silla Observatory to see. This event has been described by ESO as a "very rare and difficult to observe" event becasue minor planets are very small - and an occultation is even rarer. But these things do occur, for Eris will occult again in 2013, ESO reports. Astronomers first observed the star using the MPG/ESO 2.2-metre telescope at ESO’s La Silla Observatory, and a another team carefully predicted the time and what would occur during the occultation. "Observing occultations by the tiny bodies beyond Neptune in the Solar System requires great precision and very careful planning. This is the best way to measure Eris’s size, short of actually going there," explains Bruno Sicardy, the lead author.

Twenty-six places global observed the occultation and were henceforth able to determine the diameter of Eris, which had not yet been accurately determined.  Eris is a dwarft planet, today known to be the most massive, of the Kuiper Belt - the region of the solar system which contains the "remains," as astronomers call it. Eris is a dwarf planet, as well as other well-known objects such as Pluto, Sedna, and Makemake. The planet itself is magnitude 18.7 and it orbits 97.56 AU, or 9,068,768,557.0108 miles from earth. It has one moon, Dysnomia, as shown in the picture below. This moon was used to help determine the mass of Eris - which is 27% heavier than Pluto, astronomers found.

Artist's conception of Eris (with Dysnomia above)

Previously estimated was the diameter of Eris at around 3000 km (or 25% larger than Pluto), but now, the research has proved that both minor planets are essentially the same size, as Eris' predicted diameter was 2326 km. (Pluto's diameter is approximately 2300 or 2400 km, as New Horizons will find out in 2016. Pluto's obstructive atmosphere makes occultation predictions hard to accurately define).

The below picture shows trajectories of the occultation, "the three oblique solid lines show the star trajectories relative to Eris, as seen from San Pedro, La Silla and CASLEO, with the arrow pointing towards the direction of motion," as multiple authors of A Pluto-like radius and a high albedo for the dwarf planet Eris from an occultation, a paper presenting their discoveries, write. The names below are observatories in Chile where this occultation was viewed.

This density means that Eris is probably a large rocky body covered in a relatively thin mantle of ice,” comments Emmanuel Jehin, who contributed to the study. “It is extraordinary how much we can find out about a small and distant object such as Eris by watching it pass in front of a faint star, using relatively small telescopes. Five years after the creation of the new class of dwarf planets, we are finally really getting to know one of its founding members,” concludes Bruno Sicardy.

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