If someone were to ask you to name the ninth largest object that directly orbits our sun, odds are you would respond with Pluto. After all, even if you can’t call Pluto a planet anymore, it’s still the ninth biggest rock in local solar neighborhood, right? Actually, no. The ninth biggest rock directly orbiting the sun is Eris, a dwarf planet named for the Greek goddess of discord. Considering how much disagreement was engendered by the discovery of Eris and her sibling dwarf planets, there’s probably no better mythological namesake for this local celestial object.

You see, lots of folks are still bent out of shape that Pluto was demoted from actual planet to dwarf planet. Fan though I am of that most famous of trans-Neptunian objects, Pluto’s claim to planethood was shaky at best. Besides being outsized by Eris, Pluto is actually smaller than seven moons in the local solar system. Its orbit is rather eccentric, slipping occasionally closer to the sun than Neptune before swinging further away again. In fact, Pluto was in many respects considered a planet simply because it was found during Percival Lowell’s attempts to locate the mythical Planet X beyond Neptune, and because initial estimates of its size were far larger than Pluto’s true mass. Put another way, Pluto earned planet status mostly because it was found at the right time and by the right people, who made the right mass-estimate mistakes.

Still, some argue that because Pluto was considered a planet for so long — more than 75 years — it should be grandfathered into the planet club. That notion breaks down when you cite the Ceres precedent. Asteroid Ceres was listed as a planet for more than 50 years, mostly because no one knew what else to call it. Ceres led directly to the coining of the term asteroid to describe sub-planetary rocks of significant size, a title it inherited once it was stricken from the roster of planets. Because of Eris, Pluto went through the same it’s-not-really-a-planet-so-let’s-invent-a-new-category process, giving us dwarf planets.

So, if Eris stirred up such an astronomical hornet’s nest, why haven’t you heard of it? Actually, you probably have. For a long while, Eris was listed by its minor planet name, 2003 UB313. That catalogue number was a little cumbersome, so the discovery team nicknamed it Xena, after the famous fictional warrior princess portrayed on television by Lucy Lawless. (Yes, many astronomers are sci-fi/fantasy geeks. Duh.) The Xena title got a lot of media play, as did the nickname Lila. Funny thing is, nobody ever actually called 2003 UB313 Lila, despite media reports to the contrary. It was all just a rather comical misunderstanding bred by ignorance of — believe it or not — the Internet.


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What rather comical misunderstanding of the Internet led to media outlets referring to the dwarf planet Eris as Lila, despite the fact that neither the discovering astronomers nor the International Astronomical Union ever called the celestial object by that name?

To be fair, it was an honest mistake on the part of the media, though one that never should have made it past the fact-checking stage. Astronomer Mike Brown was part of the discovery team that first identified 2003 UB313 in 2005 (the photo was taken in 2003; it just took two years to analyze that photo well enough to locate the celestial object in the picture). Round about 2005, Brown’s daughter Lilah was born. Thus, the Web page maintained by the Eris discovery team was located under the URL www.gps.caltech.edu/~mbrown/planetlila/. The address was named for Brown’s little girl, the planetoid was not.

Brown and his cohorts thought the formal naming of 2003 UB313 would happen rather quickly, especially since they considered the object the long-sought tenth planet of our solar system. (Brown wanted to name it Persephone, just like many famous sci-fi scribes.) Instead, 2003 UB313 touched off a lengthy debate about the definition of a planet, which delayed the formal christening of Eris until 2006. By then, the final naming of 2003 UB313 was lost in the Pluto-demotion hullabaloo.

So what, exactly, is a dwarf planet? Well, designating a dwarf planet is mostly a diagnosis of exclusion. To be called a planet, a celestial object has to meet three criteria:

  1. A planet must have sufficient mass that it has been rounded by its own gravity.
  2. A planet must not be a satellite of another celestial object, save that of its star.
  3. A planet must have cleared its orbital zone — its “neighborhood” — of other celestial objects, either by collision, capturing them as satellites, or otherwise establishing gravitational dominance over those objects.

A dwarf planet meets the first two criteria but not the third. A dwarf planet is round, it directly orbits its sun, but it doesn’t “own” its orbital zone, likely because it isn’t sufficiently massive. Pluto and Eris are thus dwarf planets. If that seems like a harsh gig for Pluto — or it just isn’t complicated enough for you — fear not. The International Astronomical Union has created a special subcategory of dwarf planets just to make Pluto lovers feel better: plutoids.

Plutoids are any dwarf planets that are also trans-Neptunian objects, which is to say any almost-planets that orbit the sun further out than Neptune. Pluto, being the first of these, defines the category. Eris is also a plutoid, as is the dwarf planet Makemake, named for the Polynesian god of fertility and creation.

That’s not just some auspicious astronomical assignation, it’s a decidedly definitive dose of Geek Trivia.

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