Dwarf planet Eris revealed in new light

The dwarf planet Eris is one of the most reflective objects in our solar system, astronomers say in a study uncovering a trove of information about the mysterious object beyond Pluto.

Eris was identified in 2005, but its size and other characteristics have been difficult to measure because it is located nearly three times farther from the sun than Pluto and about 96 times farther from the sun than the Earth. However, new data gathered as it passed in front of a faint star in 2010 has allowed astronomers to get to know Eris a little better.

Based on the new measurements, astronomers led by Bruno Sicardy at Pierre and Marie Curie University and the Paris Observatory estimated in the journal Nature Wednesday that Eris is almost perfectly spherical and 2,326 kilometres in diameter, plus or minus six kilometresalmost exactly the same size as Pluto and about one-fifth the size of the moon.

Eris had been estimated to be 27 per cent heavier than Pluto based on its interactions with its moon, Dysnomia.

That, combined with its newly measured diameter, suggests Eris is mostly made of rock surrounded by a 100-kilometre thick layer of ice that makes up roughly 15 per cent of its mass.

The new observations of Eris also showed that it reflects 96 per cent of the light that falls on it. That makes it one of the most reflective objects in the solar system, after Enceladus, one of Saturn's moons, the European Southern Observatory said in a news release.

A possible explanation for the brightness is that it is caused by a thin, reflective icy layer of nitrogen or methane that may make up Eris's atmosphere when it is at its closest point to the sun, about 5.7 billion kilometersaway or 38 times the distance between the Earth and the sun. Right now, when it is close to its farthest point from the sun, as it is now and is extremely cold, the atmosphere may have "collapsed" or condensed into a reflective frost on the planet's surface, the paper suggests.

Based on the new measurements, combined with data from the Spitzer space telescope and the IRAM scientific satellite, the researchers estimate that the surface temperature on the side of Eris facing the sun is no more than238 C.

The researchers were able to observe Eris and collect the new data about it by predicting when it would pass in front of a faint background star.

They had forecasted the rare event, called occultation,for Nov. 6, 2010. They arranged to record the event from 26 sites, but because of weather and other factors, they were only able to see it from two sites in Chile: San Pedro de Atacama, where they observed it with two telescopes, and the European Southern Observatory's La Silla Observatory, home of the Belgian TRAPPIST telescope.

By recording the timing of the drop in light caused by Eris passing in front of the star at the sites, the researchers were able to calculate its size. That, in turn, could be used in combination with previous measurements to calculate many of the planet's other characteristics, such as its density, reflectivity and temperature.