This story was updated at 7:45 p.m. ET, Nov. 9.
The dwarf planet Eris — once thought to be the largest body in the solar system beyond Neptune's orbit — may actually be smaller than Pluto, new observations suggest.
Three teams of astronomers watched through telescopes as the icy Eris passed in front of a distant star over the weekend. The length of the occultation — as the event is called — showed that Eris is likely less than 1,454 miles (2,340 kilometers) wide, the magazine Sky & Telescope reported.
This would make Eris a smidge smaller than Pluto, which is about 1,455 miles (2,342 km) wide.
Astronomers still believe Eris to be about 25 percent more massive than Pluto. So if Pluto is a bit bigger, or roughly the same size, Eris must be much denser. It must be made of different stuff, which comes as a big surprise to some astronomers.
"The fact that their densities are so different is totally unexpected," said Mike Brown of Caltech, who discovered Eris in 2005. Brown was not involved in the occultation measurements. "Eris is no longer a Pluto twin. It's an entirely different object." [ POLL: Do You Think Pluto's Planet Status Should Be Revisted? ]
The size revision would allow Pluto to regain its status as the largest body in the Kuiper Belt, the icy ring of objects circling the sun beyond Neptune. Pluto could therefore exact a small measure of revenge, for it was demoted from ninth planet to dwarf planet in 2006 partly due to the discovery of Eris (and later Eris' moon Dysnomia).
The new observation efforts, which involved dozens of astronomers around the world, were coordinated by Bruno Sicardy of the Paris Observatory.
Dueling dwarf planets
Eris has one known moon and a highly elliptical orbit, zooming about 9 billion miles (14.6 billion km) from the sun at its farthest point — making it about twice as distant as Pluto.
Early measurements of its size by Brown and others at the time suggested that Eris was slightly larger than Pluto.
Observations by both the Hubble and Spitzer space telescopes, for example, pegged Eris' width as roughly 1,491 miles (2,400 km). A research team using a Spanish radio telescope calculated Eris to be even bigger, around 1,864 miles (3,000 km) across.
The discovery of a "10th planet" larger than Pluto — and the prospect of finding an 11th planet, and a 12th and so on — ultimately led astronomers to reconsider Pluto's status as a full-fledged planet.
In 2006, the International Astronomical Union officially designated Eris and Pluto "dwarf planets," based on the fact that they hadn't cleared their orbital zones of other rocky objects.
The decision introduced a new category of body, and it officially reduced the number of planets in the solar system to eight. It also set off a controversy that still simmers today; some astronomers agree with the move, while others regard Pluto as a full-fledged planet.
The new observations could help take a bit of the sting out of Pluto's demotion.
In an international effort led by Sicardy, dozens of astronomers around the world aimed their telescopes at Eris on Saturday (Nov. 6). Because the dwarf planet is so small and so far away, witnessing the occultation was no sure thing. It would only be visible from certain spots on Earth's surface.
But three teams of astronomers, using different telescopes set up throughout the Chilean Andes, had success. They watched Eris pass in front of a faraway star in the constellation Cetus and timed how long Eris blocked the star's light.
Such information, if recorded at multiple sites, can reveal with great precision how wide a spherical object is. (Astronomers think both Eris and Pluto are spherical). The size calculations made over the weekend may be more reliable than the earlier figures, according to Brown.
"Most of the ways we have of measuring the sizes of objects in the outer solar system are fraught with difficulties," Brown wrote on his blog Sunday (Nov. 7). "But, precisely timed occultations like these have the potential to provide incredibly precise answers."
Rethinking the outer solar system
If the new measurements are accurate, they make a strong case that Eris and Pluto are very different objects.
While the two dwarf planets appear to have very similar surfaces, their interiors are likely quite disparate. Since Eris is apparently much denser, it probably contains more rock and less ice than Pluto, Brown said.
Why are these two far-flung bodies made of different stuff? One possibility, according to Brown, is that Eris formed much closer to the sun than Pluto did, perhaps in the asteroid belt, then was flung to the solar system's outer reaches later.
Brown regards this as unlikely, though, because Eris contains more mass than the entire asteroid belt put together.
"I don't think this is the answer, but it needs to be thought about," he told SPACE.com.
Another possibility is that Eris and Pluto have had very different histories, with Eris being hammered by more cosmic collisions. There's no obvious reason why this should be the case, Brown said, but it needs to be considered.
However Pluto and Eris came to be so different, astronomers have a lot of new information to mull over.
"My view of the outer solar system right now is different than it was a week ago," Brown said.