Spotlight on Eris

ASTRONOMY

More than five years ago, scores of astronomers inside a big convention hall in Prague waved yellow cards robbing Pluto of its planetary status. From being a full-fledged planet, Pluto was demoted to ‘dwarf planet’.

Pluto’s colder twin Eris was also grouped in the same bracket, ending years of speculation on whether membership in the planetary club would be extended beyond Pluto. The reclassification sparked an emotional debate with many refusing to accept Neptune as the last planet.

“Rarely in the course of modern research has an astronomical discovery generated such widespread debate and emotional reaction among both scientists and general public – it turns out that Eris was aptly named after the Greek goddess of strife and discord,” observes Amanda Gulbis, a senior scientist at Southern African Large Telescope and South African Astro Observatory in Cape Town.

However, not much was known about Eris except that it is slightly larger than Pluto. But exploiting a rare occultation event, when the dwarf planet Eris crosses in front of a distant star, astronomers have now determined Eris’ size and improved human understanding of the dwarf-planet. Occultations provide the most accurate and often the only way to measure the shape and size of a distant solar system body. The candidate star for the occultation was identified by studying images from the 2.2-meter telescope at ESO’s La Silla Observatory. The observations were carefully planned and carried out by a team of astronomers.

Reporting their findings in the October 27 issue of Nature, scientists not only suggest that Eris’ radius is similar to Pluto’s but also uncover the possible reason for its bright surface. “Observing occultations by tiny bodies beyond Neptune in the solar system requires great precision and very careful planning. This is the best way to measure Eris’ size, short of actually going there,” explains Bruno Sicardy, the lead author from Pierre and Marie Curie University, Paris.

Close to spherical
Observations 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 La Silla Observatory and the other was in San Pedro de Atacama. 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.

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 Pluto from planet to dwarf planet in 2006. Eris is currently three times further from the sun than Pluto.

While earlier observations using other methods suggested that Eris was probably about 25 per cent larger than Pluto with an estimated diameter of 3,000 kilometers, the new study proves that the two objects are essentially the same size. Eris’ newly determined diameter stands at 2,326 kilometers, with an accuracy of 12 kilometers.

This makes its size better known than that of its closer counterpart Pluto, which has a diameter estimated to be between 2,300 and 2,400 kilometers. 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’ satellite Dysnomia was used to estimate the mass of Eris. It was found to be 27 per cent heavier than Pluto.

“The heavy density means Eris is probably a large rocky body covered in a relatively thin mantle of ice,” comments Belgian astronomer Emmanuel Jehin, who contributed to the study.

Reflecting 96 pc of light
The surface of Eris was found to be extremely reflective, reflecting 96 per cent of the light that falls on it. 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 millimeter 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 kilometers.

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 minus 238 Celsius at most, and 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 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 Sicardy.