Eris and Pluto: Does Size Matter?

It's been nearly a year since dwarf planet Eris slipped directly in front of a 17th-magnitude star in north-central Cetus, and I've been curious about the outcome ever since.

Star occulted by Eris
Images taken every 4½ seconds clearly show the brief disappearance of a star behind the dwarf planet Eris on November 6, 2010 (Universal Time).
Emmanuël Jehin & others / TRAPPIST Observatory
This occultation, successfully recorded by observers in South America, had the potential to measure the diameter of Eris with greater accuracy than can be achieved by any other method. (Example: now near aphelion in its orbit, some 9 billion miles away, Eris covers only 1.3 pixels when viewed by the Hubble Space Telescope.)

So I mentally held my breath as the observing team meticulously analyzed the occultation's results. Would Eris remain the reigning king of the Kuiper Belt, up to a third bigger than Pluto, as infrared and radio measurements had indicated? Or would it turn out to be a bit smaller than Pluto, as a quick look at the event's results indicated last year?

The answer, published today in Nature, is a statistical tie! Eris has a diameter of 1,445 miles (2,326 km), with an uncertainty of just 0.5%. Pluto, as best we can tell, has a diameter somewhere between 1,430 and 1,490 miles.

(In the latter's case, occultations of stars can't reduce the uncertainty — in fact, several events have been recorded over the past two decades — because Pluto's tenuous atmosphere muddies the exact timing of when the star winks out and reappears. We won't know Pluto's girth for sure until the New Horizons spacecraft zips by in July 2015.)

Track of Eris's 2010 occultation
When Eris occulted a star in November 2010, only three telescopes at two sites (green dots) witnessed the event. Another observatory recorded a near-miss (blue), and several others were clouded out (red). Dashed lines show the occultation's predicted track. and solid lines show the actual track, which reflect Eris's smaller-than-expected diameter.
B. Sicardy & others / Nature
My hat's off to Bruno Sicardy (Paris Observatory) and his team for a masterful analysis with relatively little to work with. Despite having dozens of observing teams watching that night across South America, southern Europe, and the Canary Islands, only three actually recorded the star's disappearing act. The resulting two chords across the disk of Eris are the bare minimum needed to divine a diameter — assuming that it's got a spherical shape.

One small complication arose with the images recorded by Sebastian Saravia, Alain Maury, and Caisey Harlingten using Harlingten's 20-inch (50-cm) telescope at San Pedro de Atacama, Chile. The star's disappearance just happened to come during a brief gap between successive frames, which yielded two possible timings 1.2 seconds apart. That, in turn, led to some ambiguity regarding Eris's true shape. Sicardy explains that it is either very nearly spherical (the most probable situation) — or it's a fast-spinning ellipsoid that presented a just-so orientation during the occultation, which is just too far-fetched.

In any case, there's more at stake than knowing whether Eris is larger than Pluto or vice versa. Both have satellites, so their masses can be deduced — and in this category Eris wins handily. Despite the two bodies' nearly identical sizes, they are far from "twins" (as the headline of this ESO press release erroneously suggests).

Eris, Pluto, and Triton compared
Even though Eris and Pluto are nearly identical in size, they are hardly twins. Eris is significantly denser and therefore must contain a higher rock-to-ice ratio. Compositionally, Pluto might have more in common with Neptune's giant moon Triton.
Michael Brown (Caltech)
"The only plausible way for Pluto and Eris to be essentially the same size but for Eris to be 27% more massive is if Eris contains substantially more rock in its interior than Pluto," explains Mike Brown (Caltech), who led the team that discovered Eris in 2005. "In fact, the amount of extra rock that Eris contains is about equal to the mass of the entire asteroid belt put together. That counts as a pretty big difference."

But wait — there's more! Given its 19th-magnitude brightness, the new, smaller-than-before Eris must have an incredibly reflective surface that's literally as white as snow. Astronomers already knew Eris was covered with frozen methane, but the occultation results push the reflectivity to an even greater extreme: 96%. This isn't normal! Over time an icy surface should darken either due to exposure to space radiation or to meteoritic impacts.

Eris illustration
A portrayal of the distant dwarf planet Eris, which is almost exactly the same size as Pluto — and thus smaller than many astronomers previously thought. A coating of frozen methane on the surface makes Eris extremely reflective.
ESO / L. Cal?ßada
The only logical explanation is that Eris generated a thin methane atmosphere when it last reached perihelion (back in the late 1600s). Then, as Eris edged farther outward and the feeble sunlight dimmed further, those wisps completely froze onto the surface, creating a frosty crust that's at most just a few inches thick. If there's anything left of the atmosphere, the occultation didn't detect it. Sicardy's team reports an upper limit of about 1 nanobar — one-billionth the pressure of Earth's atmosphere and about 10,000 times more tenuous than Pluto’s.

By the way, the occultation results touched off a testy Twitter exchange today between Brown (@plutokiller), author of How I Killed Pluto and Why It Had It Coming, and Alan Stern (@AlanStern), principal investigator of the New Horizons mission. Check it out!

4 thoughts on “Eris and Pluto: Does Size Matter?

  1. Lucky

    We should go & live there! I am willing to go one way.That saves 10 billion$dollars right there.Ai our probably on there way there now!I just need food,classical music,Binocs,telescopes,radio telescopes,Seti equipment,computers & communications gear,warm Habs for me & my guests,spare spacesuits,Emergency rations & extend & enhance my life a hundred or so years past 100!No kidding! I will supply my own toothbrush if there is a budget problem,& longjohns!A porta potty would be a plus. I could recycle my diapers!

  2. TedAHunt

    Mr. Beatty, may I first agree with ‘Louise’ in complementing your work? Your articles are well compiled, interesting, and you do the reader a courtesy in linking to your sources for further reading. That said, I wonder if S&T might want to reconsider letting any and all reader comments stand as they are? Do you even read them? I refer at the moment to the one by ‘Lucky’ above, but there are many of similar caliber elsewhere. I submit that such addenda are inconsistent with and reflect poorly upon both the quality of your writing and (I hope) the quality of Sky & Tel’s general readership.

    Earlier (in my college years) I had the privilege of working with a number of professionals in astronomy education, including Robert C. Victor, who wrote for S&T. While we were certainly a tribe of eccentric characters, none of us were idiots. I archive many of your articles in their ‘print’ versions, saving the entire page in .mhtml format. I’d be delighted to not drag the wisdom of ‘Lucky’ et al. in with the rest.

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