Dwarf Planet's Claim to Fame Is Unshaken by Lingering Mysteries
Artist's rendering of Eris, announced in July 2005 by Mike Brown of Caltech. It is more massive than Pluto. The sun is in the background.
Credit: NASA/JPL/Caltech

Astronomers revealed this month that the dwarf planet Eris, near the edge of our solar system, has a surface similar to Pluto's ? the latest revelation about an icy world that, even five years after its discovery, holds a pivotal place in the planetary debate.

Eris is larger than Pluto and three times farther from the sun. Astronomers discovered the small world in January 2005 (and later spotted its moon Dysnomia), setting in motion events that would ultimately lead to the demotion of Pluto from full-blown planet and to the rise of the "dwarf planet" category.

Five years later, researchers agree it was an epoch-making find ? even if some of them still don't see eye to eye about Pluto. [Artist's rendition of Eris]

"In some ways, what it did was really open up the entire outer solar system," Eris' discoverer, astronomer Mike Brown of the California Institute of Technology, has told SPACE.com.

Dwarf planet debate

Eris changed planetary science by creating room for a third category of planets, distinct from the four terrestrial planets and the four gas giants. They are called trans-Neptunian dwarf planets, or plutoids, and researchers expect to find many more of them in coming years. 

"It's becoming common wisdom that dwarf planets are the most common type in the solar system," said Alan Stern, a planetary scientist at Southwest Research Institute in San Antonio.

Stern recalls writing a paper in 1991 suggesting hundreds of Pluto-scale worlds inhabited the outer reaches of the solar system. "It's completely panned out, and I'm very happy about that," he said.

The corroborating evidence would take nearly 15 years to obtain, however.

Starting in 2000, Brown, working with Chad Trujillo and David Rabinowitz, began a detailed survey of the northern sky, searching for moving objects that would qualify as new planets.

Five years into the search, the researchers had turned up nothing.

"You condition yourself to knowing what to expect," Brown said. "When it showed up on my computer screen, it took me about 30 seconds to know it was larger than Pluto. I almost fell out of my chair."

Planet or not?

After NASA's official announcement of the discovery of a "10th planet," the name that caught on for it was "Xena." That proved to be a placeholder until the International Astronomical Union accepted Eris as the official name in September of 2006.

In the five years since the discovery, Brown said, researchers trained nearly every terrestrial and orbiting telescope on Eris to flesh out a basic picture of the dwarf planet. Images from the Keck telescopes in Hawaii revealed it had a moon.

The Hubble Space Telescope pegged the dwarf planet's size at approximately 1,500 miles (2,400 kilometers) across, while measurements made at the Gemini North Telescope in Hawaii indicated the presence of methane ice on its surface.

But the most intense scrutiny may have centered around Eris' effect on the classification of Pluto. The possibility of discovering an 11th planet ? and a 12th and a 13th and so on ? made Pluto's status a pressing matter for astronomers.

On Aug. 24, 2006, the IAU resolved the conundrum by designating Eris and Pluto dwarf planets, based on the fact they hadn't cleared their orbital zones of other rocky objects. The number of full planets in the solar system was thus reduced to eight.

As an astronomer, Brown agreed with the decision.

"It's the right thing to have happened," he said. "I'm happy the solar system is finally organized correctly."

Pluto planet brouhaha

But although the reclassification was faithful to the idea that Eris and Pluto represented a distinct type of planet, it frustrated researchers who had dedicated their careers to studying Pluto.

The IAU's decision was "laughable," said Stern, principal investigator for the mission of the New Horizons spacecraft to Pluto.

"I think most people know a planet when they see one, and I guarantee when people see the images from the Pluto system, they're going to recognize it as a planet."

Stern and Brown can both agree that New Horizons will add a new level of understanding to the dwarf planets of the Kuiper Belt beyond Neptune. The probe, due to reach Pluto in the summer of 2015, is expected to shed light on Pluto's surface, interior and atmosphere.

Further direct studies of Eris will have to wait until the next generation of telescopes comes online, Brown said.

Better telescopes, better views

Between the planned 30-meter telescope, the European Extremely Large Telescope and the ALMA radio telescope in South America, astronomers will be able to better map the surface of Eris and gauge its temperature, and new infrared telescopes will allow more-precise measurements of Eris's surface composition.

Brown said he would love to be able to stick around for 290 more years, for the time when Eris will approach as close to the sun as Pluto is now.

"Two hundred ninety years is probably a pretty long time to wait," he said, "but hopefully it'll be a pretty spectacular sight when it comes."