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Astronomers Determine Mass and Density of Distant Dwarf Planet Eris

Astronomers at the California Institute of Technology are learning more about Eris, the rocky object orbiting at the edge of our solar system that was for a time considered a possible 10th planet. We're learning a lot more about Eris from the orbit of its moon.

Eris is way out there in the neighborhood of Pluto. You remember Pluto? It used to be a planet until astronomers demoted it last year, to a new category they created, dwarf planet. The discovery of Eris, which is slightly bigger than Pluto, was actually a big factor in the new definition of a planet, which doesn't include Pluto, no matter what you learned in school. That's the thing about science: things change.

The discovery of Eris was announced in 2005 by Prof. Michael Brown of the California Institute of Technology, Caltech. Last year, Brown and his team measured the diameter of Eris, and they also discovered a moon circling it. And that was the key to the latest announcement, of the mass of Eris, which can be very accurately calculated based on the orbit of that moon.

"So you follow the moon around the body [Eris]," Brown told VOA, "so if we follow Dysnomia - the name of the moon - around Eris over its [the moon's] 16-day orbit, and very precisely looked at its position every time, and by seeing those positions and seeing how fast it moves around there, you can figure out exactly how much it [Eris] weighs."

And it works out to 16.6 billion billion metric tons, or about 27 percent more than Pluto. With the size of Eris, which had previously been measured, and now the mass, the Caltech astronomers could compute its density, which in turn provides a major clue to what the dwarf planet is made of.

"[It] turns out to have a density very similar to that of Pluto," Brown explained. "And Pluto, we have known for a long time, has a density consistent with it being about half ice and half rock. And it's sort of like the Earth. You know, the Earth has an iron core and a rocky outside, and we think that Pluto - and now Eris also - has a rocky core and then ice on the outside."

By comparison, Earth is about twice as dense.

The calculation of Eris's density depended on knowing its moon's orbit, which in turn depended on getting clear pictures of where the moon was. Even as recently as a year ago, Brown says that couldn't be done from an earth-based telescope. The atmospheric distortion was just too great. But new technology has vastly improved the ability of telescopes on Earth to get clear views of distant objects in space.

The key is something called "laser guide star adaptive optics." You need a telescope with adaptive optics - a mirror that can change shape slightly to compensate for distortion - and a laser that projects a dot of light - a simulated "star" - on a thin layer of the atmosphere, about 90 kilometers up.

"And of course, you know what the laser 'star' is supposed to look like, but the laser 'star' is also distorted by all the turbulence of the atmosphere, so you have a very fast computer and very fast changing mirror that can change that distorted star to look back to what it's supposed to look like. When you're looking at, now, Eris, and you turn on the laser beam and turn on the system, Eris goes from being this big, smeared-out ball to suddenly - and it really is almost instantaneously - it suddenly just pops right in to a tiny little spot, and then you can see the moon right next to it," Brown explained.

That was at the Keck Observatory in Hawaii. Brown's team also used observations from the Hubble Space Telescope.

They're not finished with Eris. Take the matter of its orbit. Unlike most of the objects circling the Sun, Eris has a very elliptical orbit. At its farthest, it's three times farther from the Sun than it is at the closest point in its orbit.

"And when it's closest to the sun, it has an atmosphere that's floating around it," the Caltech astronomer said. "But when it's farthest away from the Sun, which is where it is now, that atmosphere freezes out onto the surface. And so right now there's no atmosphere at all, we think. And we see this very thin layer of frost that's on the surface. So understanding how these frosts develop, understanding where these atmospheres come from, these are the sorts of things we're going to be studying in great detail over the next couple of years."

Astronomer Michael Brown from the California Institute of Technology. His paper on the density of Eris was published in Friday's edition of the journal "Science."