This image shows the eight planets and the four dwarf planets named so far: Pluto, Makemake,
 Eris and Ceres. The orbits are not drawn to scale. (Alexander Novati, Jet Propulsion Laboratory
/September 23, 2008
)


Eris, the dwarf planet whose 2005 discovery led to Pluto losing its status as a planet, has passed in front of a star, providing astronomers with the clearest view of it since it was identified.

It is about the same size as Pluto and is one of the brightest objects in the solar system, according to the new analysis, released Wednesday by the journal Nature.

Scientists' picture of Eris had remained fuzzy because its distance from Earth is so vast: It is about three times farther out from the sun than Pluto. Some estimates pegged Eris as about 25% larger than Pluto, but it was too far away to tell for sure.

"It's very difficult, because it's so small in the sky," said lead author Bruno Sicardy, a planetary scientist at Pierre and Marie Curie University and Observatory in Paris.

With such small, far-off objects, astronomers wait for what's known as a stellar occultation, in which the object will cross over a star, essentially casting a shadow over the Earth. The amount of starlight blocked by the object allows scientists to calculate the object's size.

Witnessing this stellar occultation last year required being in the right place at exactly the right moment during the brief time window that Eris was scheduled to block the star.

To spot the star-crossing, Sicardy's team asked telescope operators at 26 different sites around the world to make observations. Just three telescopes at two of those sites, both in Chile, managed to catch the event.

From the data, the researchers were able to calculate that the dwarf planet's diameter is about 1,445 miles — on a par with Pluto, which is somewhere between 1,429 and 1,491 miles across.

The fact that Eris is smaller than previously estimated means that the amount of light scientists had detected coming from it originated from a smaller-than-anticipated surface area — and therefore its surface is brighter than anyone had thought.

In fact, the new calculations make Eris one of the brightest objects in the solar system, even though its surface should have been darkened from bombardment by cosmic rays and micrometeorites.

The authors think its shininess is due to a millimeter-thick layer of methane-and-nitrogen frost coating the dwarf planet's surface. This frost, they say, was probably once an atmosphere 10,000 times thinner than Pluto's that froze onto the surface in the frigid temperatures as Eris traveled away from the sun on its 557-year orbit.

Though they have Eris' size narrowed down, scientists still don't know whether it's smaller or larger than Pluto — because Pluto's size is not known precisely.

But even if Eris ends up being the smaller, "Pluto is never going to go back to being a planet," said Amanda Gulbis, a planetary scientist at the South African Astronomical Observatory who was not involved in the study. "The definition has been set."

Regardless of the question of size, Eris is about 27% heavier than Pluto. This means it must contain relatively more rock and less ice, said Caltech astronomer Mike Brown, whose team discovered Eris in 2005.

This could be because Eris was once a much larger planet whose less dense outer layers were blown off by an impact — much as is thought to have happened with the small but uncommonly heavy Mercury, the sun's closest planet.

"We really think [Eris and Pluto] should have been made at the same time out of the same materials — so really, it's bizarre that they're so different," Brown said.

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