Small, Heavy, and Shiny— Planetoid Surprises Astronomers
Distant Eris may be more like Mercury than Pluto
By Mark Russell,  Newser Staff
Posted Oct 28, 2011 4:18 AM CDT
Updated Oct 28, 2011 5:29 AM CDT
An artist's impression of Eris, the distant planetoid that led to Pluto's demotion from planet status.   (©Kanijoman)

(Newser) – It's been six years since distant Eris was discovered, a small planetoid that led to Pluto losing its planetary status. But since it lies three times further from the sun than Pluto, Eris was very difficult to observe and little was known about it—until now. Last year Eris passed in front of a star, creating a "stellar occultation" that allowed astronomers to turn their telescopes on it and gather large amounts of data. Twenty-six sites tried, but only three telescopes from two locations caught the event.

So what is Eris like? It's smaller than astronomers thought—at 1,445 miles in diameter, it is about the same size as Pluto, or about 20% smaller than expected, reports the Los Angeles Times. And because it is also 27% heavier than Pluto, that means it is also denser, with more rock and less ice; it might even be more like Mercury than Pluto. Eris it is also much brighter than astronomers thought. In fact, it is one of the brightest objects in our solar system, maybe from a 1-millimeter-thick layer of methane-and-nitrogen frost. "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," says one astronomer. You can read an abstract of the full report in Nature.