Poor Little Pluto Kicked To The Curb Again

Die-hard Pluto fans still seeking redemption for their demoted planet have added cause for despair this week. New data shows the dwarf planet Eris is 27 percent more massive than Pluto, thereby strengthening the decree last year there are only eight planets in the solar system, and a growing list of dwarf planets.

Mike Brown, who discovered Eris, and his graduate student Emily Schaller, say the data confirms Eris weighs 16.6 billion trillion kilograms. They know this because of the time it takes Eris's moon, Dysnomia, to complete an orbit.

"This was Pluto's last chance to be the biggest thing found so far in the Kuiper belt," says Brown, a professor of planetary astronomy at the California Institute of Technology. "There was a possibility that Pluto and Eris were roughly the same size, but these new results show that it's second place at best for Pluto."

Eris was discovered in 2005 with Palomar Observatory's 48-inch Samuel Oschin Telescope, an instrument specially adapted to do comprehensive searches for objects in the sky, according to Caltech.

When it became apparent that Eris was at least similar in size to Pluto, Brown and others called for the International Astronomical Union to rule on its planetary status. The end result was demotion and re-designation of Pluto and other Kuiper-belt objects as dwarf planets.

Schaller says that the new results, obtained with Hubble Space Telescope and Keck Observatory data, indicate the density of the material Eris made of is about two grams per cubic centimeter. This means Eris very likely is made up of ice and rock, and thus very similar in composition to Pluto. Past results from the Hubble Telescope had already allowed planetary scientists to determine its diameter is more than 1400 miles, also larger than Pluto's.

"Pluto and Eris are essentially twins -- except that Eris is slightly the pudgier of the two," says Brown.

"And a little colder," adds Schaller.

The reason for Eris's blustery surface conditions is its sheer distance from the sun. Currently 97 astronomical units from the sun (an astronomical unit being the distance between the sun and Earth), Eris hovers at temperatures well below 400 degrees Fahrenheit and is pretty dark.

However, things get a little better on Eris now and then. Orbiting the sun on a highly elliptical 560-year journey, Eris sweeps in as close to the sun as 38 astronomical units. But at present it is nearly as far away as it ever gets.

Pluto's own elliptical orbit takes it as far away as 50 astronomical units from the sun during its 250-year revolution. This means that Eris is sometimes much closer to Earth than Pluto, although never closer than Neptune, according to Caltech.

Brown and Schaller are the authors of a paper, "The Mass of Dwarf Planet Eris," appearing in the June 15 issue of the journal Science.

FMI: www.astro.caltech.edu