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Emily LakdawallaSeptember 13, 2005

DPS: Updates on 2003 EL61 and 2005 FY9

At a press briefing at DPS, and at some talks given on Thursday of last week, the co-discoverers of the so-called "10th planet" 2003 UB313, Chad Trujillo and David Rabinowitz, along with student Kristina Barkume, gave an update on what is known about this and the other two scattered Kuiper Belt bodies that were announced at the time, 2003 EL61 and 2005 FY9. At the time, I was confused about why they weren't mentioning the Spanish scientists who had reported the discoveries of those last two objects. It now appears that the Spanish scientists had reported the discoveries based upon the data taken by Trujillo, Rabinowitz, and Brown, which is a no-no.

Brown has a commentary on the timeline of events on his website here, and a very detailed timeline of events here. To make a long story short, it appears that the Spanish team only "discovered" the other two objects because they had accessed data on Brown's team's observations through an unprotected server, and used Brown's team's observations to figure out where to aim their own telescopes, and then reported the "discovery" as their own to the Minor Planet Center, without mentioning that Brown's team had seen it first. Yikes.

Anyway, getting back to what they had to say at DPS. Chad Trujillo talked about 2003 UB313, but I didn't hear a lot of new news on that one, unfortunately. There's no name for it, nor any decision on whether it's a planet or not, and no new results from Spitzer yet. But there's much more about the other two bodies.

The coolest results were reported by David Rabinowitz on 2003 EL61. "It may not be as big as Pluto, but it's much more interesting dynamically. It turns out to be rotating once in every 3.9 hours. For an object almost the size of Pluto, that has dramatic consequences. If you think about planets, one thing that defines planets is that they're spherical. But if you spin up a planet to a 4-hour rotation rate it's no longer round. It can't maintain a shape of a flattened sphere. It flattens into a squashed rugby football shape. It's the weirdest shape ever seen for an object this size. Its density is 2.5 times denser than ice and it's very reflective, almost as reflective as pure snow. If you rotate as fast as this object, it has to become a triaxial ellipsoid. It's a squashed Pluto." I asked him how they knew its shape and size. He answered, "When you measure rotation rate, you look at brightness versus time. So you know the amplitude of modulation, which tells you relative dimension of its two longer axes. So you have rotation speed and size of its two longer axes. Then since it's a planet you have a shape that's determined by the forces of gravity. And it actually has a satellite, a very small moon in a circular orbit, and that tells us the mass." On Brown's website I find that he states the mass to be 32% that of Pluto. Its longest dimension is about the same as Pluto's, but its other two axes are smaller. And its moon orbits with a period of 49 days.

Kristina Barkume gave a really nice talk on the spectral properties of the other recently discovered object, 2005 FY9. She made an excellent point, that now that we can get spectral observations on a lot of large objects in the Kuiper Belt, we can effectively test the various ideas proposed for seasonal effects on Pluto, because the different Kuiper Belt Objects (KBOs) span different distances from the Sun. However, I think she also proved that the objects that they're looking at in the Kuiper Belt are really diverse, which may make it very hard to draw generalizations from them. She showed an example spectrum from a small (300-kilometer) object called Huya. She showed that it was a "very flat spectrum, with the barest hint of water ice features. Most KBO spectra look like Huya's. Therefore, when we find that bigger objects have bigger absorption features, it may be an indication that we are seeing planetary processes on the bigger objects." She then showed a spectrum from 2005 FY9, which at roughly 1500 kilometers in diameter is about 2/3 the diameter of Pluto. "We expected to see water ice spectrum, but instead we saw THIS." She pointed to a different spectrum -- not being a spectroscopist, I couldn't read it, but it was clearly different from the Huya spectrum. Barkume said that the spectrum showed signs of methane ice and was similar to but different from Pluto's. She showed that the shape of the spectrum could best be modeled with a surface composed of 60% methane ice with centimeter-sized grains. That's sort of like a methane granite. I don't know how you would get methane crystals that big. It appears that there are a whole lot of strange worlds out there in the Kuiper Belt being explored from Earth. I can't wait for New Horizons to get out there and see a few of them -- but it's going to be a long time before New Horizons gets there!

Read more: dwarf planets beyond Neptune, optical telescopes, conference report

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Emily Lakdawalla

Senior Editor and Planetary Evangelist for The Planetary Society
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