Astronomers¹ at the California Institute of Technology (Caltech) have discovered that the dwarf² planet 2007 OR10 -- nicknamed Snow White -- is an icy world, with about half its surface covered in water ice that once flowed from ancient, slush-spewing volcanoes. The new findings also suggest that the red-tinged dwarf planet may be covered in a thin layer of methane³（甲烷） , the remnants of an atmosphere that's slowly being lost into space. "You get to see this nice picture of what once was an active little world with water volcanoes and an atmosphere, and it's now just frozen, dead, with an atmosphere that's slowly slipping away," says Mike Brown, the Richard and Barbara Rosenberg Professor and professor of planetary astronomy, who is the lead author on a paper to be published in the Astrophysical Journal Letters describing the findings. The paper is now in press.

Snow White -- which was discovered in 2007 as part of the PhD thesis of Brown's former graduate student Meg Schwamb -- orbits the sun at the edge of the solar system and is about half the size of Pluto⁴, making it the fifth largest dwarf planet. At the time, Brown had guessed incorrectly that it was an icy body that had broken off from another dwarf planet named Haumea; he nicknamed it Snow White for its presumed white color.

Soon, however, follow-up observations revealed that Snow White is actually one of the reddest objects in the solar system. A few other dwarf planets at the edge of the solar system are also red. These distant dwarf planets are themselves part of a larger group of icy bodies called Kuiper Belt Objects (KBOs). As far as the researchers could tell, Snow White, though relatively⁵ large, was unremarkable -- just one out of more than 400 potential dwarf planets that are among hundreds of thousands of KBOs.

"With all of the dwarf planets that are this big, there's something interesting about them -- they always tell us something," Brown says. "This one frustrated⁶ us for years because we didn't know what it was telling us." At that time, the Near Infrared⁷ Camera (NIRC) at the Keck Observatory⁸ -- which Caltech professor of physics Tom Soifer and chief instrument scientist Keith Matthews helped design in the 1990s -- was the best instrument astronomers had to study KBOs, according to Brown. But NIRC had just been retired⁹, so no one could observe 2007 OR10 in detail. "It kind of languished¹⁰（失去活力） ," he says.

Meanwhile, Adam Burgasser, a former graduate student of Brown's and now a professor at UC San Diego, was helping¹¹ to design a new instrument called the Folded-port Infrared Echellette (FIRE). Last fall, Brown, Burgasser, and postdoctoral scholar Wesley Fraser used this instrument with the 6.5-meter Magellan Baade Telescope in Chile to take a closer look at 2007 OR10.

As expected, Snow White was red. But to their surprise, the spectrum¹² revealed that the surface was covered in water ice. "That was a big shock," Brown says. "Water ice is not red." Although ice is common in the outer solar system, it's almost always white.