There’s a size war being waged at the outskirts of our solar system, and it looks like Pluto may come out as the reigning champ.

Ever since its discovery in 2003, the dwarf planet Eris — located in the Kuiper Belt of frozen worlds far beyond the orbit of Pluto — has been a key player in a controversy that has literally rocked the world of planetary science and astronomy outreach.

ANALYSIS: Eris and Pluto Find Common Ground

First spotted during a survey of the outer solar system by Mike Brown, a professor of planetary astronomy at the California Institute of Technology, Chad Trujillo of the Gemini Observatory, and David Rabinowitz of Yale University, Eris was initially believed by the team to be a bit larger than Pluto, our erstwhile ninth planet.

This caused no small confusion among some astronomers as to what exactly to call Eris. Is it also a planet? Is it a Kuiper Belt Object? Is it a Plutoid? Or something else entirely? And, if the latter indeed be the case, what does that make the supposedly-smaller Pluto?


ANALYSIS: Pluto Might be Bigger, But Eris is More Massive

All of this new speculation, along with some prodding by team leader and self-proclaimed “Pluto-killer” Mike Brown, led the International Astronomical Union (IAU) to revamp their official definition of what constitutes an honest-to-goodness “planet”… a definition which, unfortunately, no longer matched Pluto. And so, in 2006, Pluto was given the newly-minted classification of “dwarf planet”, along with its distant neighbor Eris, and was thus removed from the exclusive club of “real” planets.

Although perhaps only a semantic change this decision by the IAU was nevertheless met with a lot of opposition, both from astronomers and the public alike.

ANALYSIS: Pluto, Sponsored By McDonalds

After 76 years of planet status Pluto had found its way into the hearts of several generations of even the most casual fans of astronomy — not to mention that the new planet definition created by the IAU was not necessarily a unanimous decision across the field. Still, the ruling stands and as of this day our solar system “officially” contains eight planets.

We’ll keep you informed if anything changes, of course.

Meanwhile, beyond Pluto (which, really, couldn’t care less about what it’s called) another world called Eris orbits the sun in a distant region populated by ancient frozen bodies of ice and rock. This is the Kuiper Belt, located billions of miles away, with Pluto and Eris being the best known members thereof. Very similar in size, scientists weren’t sure which was larger… until now.

New data presented during a meeting of the American Astronomical Society Division for Planetary Sciences and the European Planetary Science Congress show that Eris may in fact be smaller than Pluto — or at least pretty much the same size.

SCIENCE CHANNEL: Space School: Pluto

Researcher Bruno Sicardy of the Paris Observatory and the University of Pierre and Marie Curie in France and colleagues came to this conclusion after observation of an important astronomical event called an occultation.

On Nov. 6, 2010, Eris passed briefly in front of a star, giving Sicardy’s team the opportunity to accurately measure its size. What they came up with was a diameter of 2,326 km (1,445 miles)… slightly smaller than Pluto’s last-estimated size of 2,338 km (1,452 miles)! Although very close, this latest finding puts Pluto ahead of Eris in the ongoing Width War.

Still, although Pluto may be the larger dwarf planet, Eris is undeniably more massive — and brighter. Its higher density indicates that it is mostly composed of rock covered by a shell of ice… a shell that makes it also very reflective, putting it alongside Saturn’s moons Enceladus and Tethys as far as albedo is concerned.

But this reflectance may also be variable.

It’s possible that Eris’ icy mantle may come and go as it progresses along its 557-year-long orbit of the sun — an active process that has recently been observed on Pluto as well.

BIG PIC: Pluto: A Complex and Changing World

Regardless of size and density — and how we classify them! — these fascinating worlds at the edges of our solar system have a lot in common, yet still hold their own unique secrets as well. Undoubtedly we will be in for some amazing discoveries when NASA’s New Horizons spacecraft passes by Pluto in 2015, giving us more insight into the curious worlds of the Kuiper Belt.

The team’s data will be published in the journal Nature.

Image credit: NASA, ESA and M. Brown (California Institute of Technology)