[meteorite-list] The Dwarf Planet Known as Eris is Bigger, More Massive than Pluto, New Data Shows

[Ron Baalke]

Caltech News Release
Embargoed until 11:00 AM  PDT, Thursday, June 14, 2007

The Dwarf Planet Known as Eris is Bigger, More Massive than Pluto, 
New Data Shows

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

According to Mike Brown, the discoverer of Eris, and his graduate 
student Emily Schaller, the data confirms that 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.

When it became apparent that Eris was similar in size if not larger 
than Pluto, Brown and others called for the International 
Astronomical Union to rule on its planetary status. The end result 
was demotion of Pluto and the redesignation of it and other 
Kuiper-belt objects as dwarf planets.

Schaller says that the new results, obtained with Hubble Space 
Telescope and Keck Observatory data, indicate that the density of the 
material making up Eris is about two grams per cubic centimeter. This 
means that Eris very likely is made up of ice and rock, and thus is 
very similar in composition to Pluto. Past results from the Hubble 
Space Telescope had already allowed planetary scientists to determine 
that its diameter is 2,400 kilometers, 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.

Based on spectral data, the researchers think Eris is covered with a 
layer of methane that has seeped from the interior and frozen on the 
surface. As in the case of Pluto, the methane has undergone chemical 
transformations, probably due to the faint solar radiation, causing 
the methane layer to redden. But the methane surface on Eris is 
somewhat more yellowish than the reddish-yellow surface of Pluto, 
perhaps because Eris is farther from the sun.

As for Dysnomia, the tiny satellite remains the only moon discovered 
orbiting Eris so far. Dysnomia is about 150 kilometers in diameter, 
is about 37,000 kilometers from Eris, and has a lunar "month" that 
lasts 16 days.

"But every year is 560 Earth-years," says Brown. "So on Eris they 
have a lot more months in their calendar."

Like the Earth-moon system, Eris-Dysnomia probably formed about 4.5 
billion years ago following a massive collision.

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.

The search for new planets and other bodies in the Kuiper belt is 
funded by Caltech and NASA. For more information on the program, see 
the Samuel Oschin Telescope's website at 
http://www.astro.caltech.edu/palomarnew/sot.html.

For more information on Mike Brown's research, see 
http://www.gps.caltech.edu/~mbrown.

To learn more about Eris, see http://www.planeteris.com.

Contact:
Robert Tindol
tindol at caltech.edu
(626) 395-3631