Faraway Eris is Pluto's twin: Dwarf planet sized up accurately as it blocks light of faint star

Oct 26, 2011
This artist's impression shows the distant dwarf planet Eris. New observations have shown that Eris is smaller than previously thought and almost exactly the same size as Pluto. Eris is extremely reflective and its surface is probably covered in frost formed from the frozen remains of its atmosphere. Credit: ESO/L. Calçada

(PhysOrg.com) -- Astronomers have measured the diameter of the dwarf planet Eris by catching it as it passed in front of a faint star. This was seen by telescopes in Chile, including the TRAPPIST telescope at ESO's La Silla Observatory. The observations show that Eris is an almost perfect twin of Pluto in size and appears to be covered in a layer of ice. Results will be published in the Oct. 27 issue of the journal Nature.

In November 2010, the distant Eris passed in front of a faint background star, an event called an occultation. These occurrences are very rare and difficult to observe as the dwarf planet is very distant and small. The next such event involving Eris will not happen until 2013. Occultations provide the most accurate, and often the only, way to measure the shape and size of a distant body.

The candidate star for the occultation was identified by studying pictures from the MPG/ESO 2.2-metre at ESO's La Silla Observatory. The observations were carefully planned and carried out by a team of astronomers from a number of (mainly French, Belgian, Spanish and Brazilian) universities using — among others — the TRAPPIST (TRAnsiting Planets and PlanetesImals Small Telescope) telescope, also at La Silla.

"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.

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Credit: ESO

Observations of the occultation were attempted from 26 locations around the globe on the predicted path of the dwarf planet's shadow — including several telescopes at amateur observatories, but only two sites were able to observe the event directly, both of them located in Chile. One was at ESO's using the TRAPPIST telescope, and the other was located in San Pedro de Atacama and used two telescopes. All three telescopes recorded a sudden drop in brightness as Eris blocked the light of the distant star.

The combined observations from the two Chilean sites indicate that Eris is close to spherical. These measurements should accurately measure its shape and size as long as they are not distorted by the presence of large mountains. Such features are, however, unlikely on such a large icy body.

Eris was identified as a large object in the outer Solar System in 2005. Its discovery was one of the factors that led to the creation of a new class of objects called dwarf planets and the reclassification of from planet to dwarf planet in 2006. Eris is currently three times further from the Sun than Pluto.

This diagram shows the path of a faint star during the occultation of the dwarf planet Eris in November 2010. Two sites in South America saw the faint star briefly disappear as its light was blocked by Eris and another recorded no change in brightness. Studies of where the event was seen, and for how long, have allowed astronomers to measure the size of Eris accurately for the first time. Surprisingly, they find it to be almost exactly the same size as Pluto and that it has a very reflective surface. Credit: ESO/L. Calçada 

While earlier observations using other methods suggested that Eris was probably about 25% larger than Pluto with an estimated of 3000 kilometres, the new study proves that the two objects are essentially the same size. Eris's newly determined diameter stands at 2326 kilometres, with an accuracy of 12 kilometres. This makes its size better known than that of its closer counterpart Pluto, which has a diameter estimated to be between 2300 and 2400 kilometres. Pluto's diameter is harder to measure because the presence of an atmosphere makes its edge impossible to detect directly by occultations. The motion of Eris's satellite Dysnomia was used to estimate the mass of Eris. It was found to be 27% heavier than Pluto. Combined with its diameter, this provided Eris's density, estimated at 2.52 grams per cm^3.

"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 .

The surface of Eris was found to be extremely reflective, reflecting 96% of the light that falls on it (a visible albedo of 0.96). This is even brighter than fresh snow on Earth, making Eris one of the most reflective objects in the Solar System, along with Saturn's icy moon Enceladus. The bright surface of Eris is most likely composed of a nitrogen-rich ice mixed with frozen methane — as indicated by the object's spectrum — coating the dwarf planet's surface in a thin and very reflective icy layer less than one millimetre thick.

"This layer of ice could result from the dwarf planet's nitrogen or methane atmosphere condensing as frost onto its surface as it moves away from the Sun in its elongated orbit and into an increasingly cold environment," Jehin adds. The ice could then turn back to gas as Eris approaches its closest point to the Sun, at a distance of about 5.7 billion kilometres.

The new results also allow the team to make a new measurement for the surface temperature of the dwarf planet. The estimates suggest a temperature for the surface facing the Sun of -238 Celsius at most, and an even lower value for the night side of Eris.

"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 , 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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User comments : 9

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1 / 5 (3) Oct 27, 2011
Go Pluto! Four Moons, maybe rings, now as big (or bigger) as Eris. Make it a planet IAU!
1.6 / 5 (7) Oct 27, 2011
I disagree. By the same logic, Eris should also be a planet. Besides, I think the term "Dwarf planet" suits it well. Since it really is just a tiny planetoid (even the Moon is larger in comparison).
1 / 5 (4) Oct 27, 2011
Twice in this article we are told Eris is "... almost exactly ..." the same size as Pluto. Which is it? Almost? Or exactly? Sorry - it simply can't have both!! Check the dictionary dudes. And we're supposed to have faith in these guys measuring the size of an object that's a bazillion kilometres away? Ha!
5 / 5 (4) Oct 27, 2011
Let's just say we have more faith in the accuracy of their measurements than in your ability to discern between the actual science/measurement and a journalistic piece based on said data.
1 / 5 (2) Nov 14, 2011
...at a distance of 128 light-years from the Sun.

Strange. Per http://en.wikiped...se_581_g 20.5

light-years from Earth. That sounds almost doable.
not rated yet Nov 14, 2011
What do you mean by 'doable'? In what timeframe?
At high speeds (let's say at 0.1c) there's already a substantial problem with the occasional lone hydrogen atom creating serious radiation upon impact. Not to mention what would happen if anything like a grain of sand would hit a craft at that speed.

And we'd need something orders of magnitude larger than the floating washing machines we're currently sending into orbit.

Before we do stuff like this we should try for the planets (and moons/asteroids) in our own solar system. That should keep us occupied for the next thosuand years.

1 / 5 (2) Nov 18, 2011
Yes no occupied spacecraft rather a flock of miniaturized probes.
1 / 5 (2) Nov 20, 2011
Might be safer than the space station though considering all the junk up there.
1 / 5 (2) Nov 20, 2011
I wonder what is the typical speed of an asteroid compared with that of space junk?

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