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Pluto's little pals

Back when there were nine planets, you could keep them straight with a cute little memory aid: "My very eager mother just served us nine pizzas." But now, at least according to the International Astronomical Union, there are only eight (planets, that is ... not pizzas): Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

Pluto was drummed out of the planet platoon in part because something was finally discovered out on the solar system's edge that was bigger than Pluto: an icy world at first nicknamed Xena, and now dubbed Eris. Does Eris' co-discoverer, Michael Brown of the California Institute of Technology, feel bad about Pluto's comedown? If so, he still has a sense of humor about it all, based on his favorite memory aid for the solar system's current lineup: "Mean, very evil men just shortened up nature."

Brown talked a little bit about Pluto and Eris, and a lot more about the other dwarf planets ringing the edge of our solar system, during the final session of the American Astronomical Society's winter meeting on Wednesday. "They're all bodies that are equally as interesting as Pluto," Brown said.

Here are some of the highlights from the talk:

  • Based on spectral analysis of the faint light reflected by Pluto, astronomers have concluded that the icy world has polar caps of methane and nitrogen. And when spectral readings came in from Eris as well, astronomers found that the two have similar composition. "It looks just like Pluto," Brown said.
  • When Eris was discovered, Brown felt confident that it was larger than Pluto, based on its brightness and distance. He became less confident when he found out that Eris' reflectivity was "absurdly high" - and the current estimate sets Eris' diameter at 1,500 miles (2,400 kilometers), just 4 percent wider than Pluto.
  • Why is Eris so bright? "I think that this whole planet is covered with a thin layer of frost," Brown said. He said Eris was "the only object we know for sure in the solar system that's large enough to maintain an atmosphere, but eccentric enough that that atmosphere comes and goes."
  • Another dwarf planet, called 2005 FY9, could serve as "the Rodney Dangerfield of the larger objects in the outer solar system," Brown said - because it's gotten relatively little respect from planetary scientists. But Brown said it's in an interesting niche of the solar system, because it has methane and even ethane, but no nitrogen. He theorized that 2005 FY9's distance and size allows it to retain hydrocarbons on its surface - but not to have a volatile atmosphere.
  • Brown said his "favorite object in the solar system" was the dwarf planet known as 2003 EL61, nicknamed Santa because it was discovered three days after Christmas. It appears to be shaped like a football, and spins end over end. Santa is also notable because it has two tiny moons, orbiting tightly in different planes. "These are the two most strongly interacting satellites of anything we know in the solar system," Brown said.
  • "But wait ... there's more," Brown said. His team found other dwarflets that were in similar orbits and had a chemical composition similar to that of 2003 EL61.  "These five other objects are actually the remnants of the icy mantle of EL61," Brown said, and apparently were struck off in a cosmic collision. EL61 and its family are in a particular type of orbit that is fated to become more and more eccentric over time. In, say, 2 billion years, EL61 "may well become a comet."

Brown uses such lore to support his point that you don't have to be a major planet to be interesting. Maybe we should just add the dwarf-planet category to our memory aids: Even if mean, very evil men just shortened up nature ... "Don't Panic."