[meteorite-list] Astronomers Measure Mass of Largest Dwarf Planet (Eris)

[Ron Baalke]

June 14, 2007

Ray Villard
Space Telescope Science Institute, Baltimore
1-410-338-4514; villard at stsci.edu

Robert Tindol
California Institute of Technology, Pasadena, Calif.
1-626-395-3631; tindol at caltech.edu

Laura K. Kinoshita
W.M. Keck Observatory, Kamuela, Hawaii
1-808-881-3827; newsletter at keck.hawaii.edu

Mike Brown
California Institute of Technology, Pasadena, Calif.
1-626-395-8423; mbrown at gps.caltech.edu

RELEASE: STScI-PR07-24

ASTRONOMERS MEASURE MASS OF LARGEST DWARF PLANET

NASA's Hubble Space Telescope has teamed up with the W.M. Keck
Observatory to precisely measure the mass of Eris, the largest member of a 
new class of dwarf planets in our solar system. Eris is 1.27 times the 
mass of Pluto, formerly the largest member of the Kuiper Belt of icy 
objects beyond Neptune.

Hubble observations in 2006 showed that Eris is slightly physically
larger than Pluto. But the mass could only be calculated by observing the 
orbital motion of the moon Dysnomia around Eris. Multiple images of 
Dysnomia's movement along its orbit were taken by Hubble and Keck.

Astronomer Mike Brown of the California Institute of Technology in Pasadena, 
Calif. and colleagues also report in this week's Science Magazine that 
Dysnomia is in a nearly circular16-day orbit. This favors the idea that 
Dysnomia was born out of a collision between Eris and another Kuiper Belt 
object (KBO). A gravitationally captured object would be expected to be in a 
more elliptical orbit.

The satellites of Pluto, as well as the Earth-Moon system are also believed 
to have been born out of a collision process where debris from the smashup 
goes into orbit and coalesces into a satellite.

By comparing the mass and diameter, Brown has calculated a density for Eris 
of 2.3 grams per cubic centimeter. This is very similar to the density of 
Pluto, the large Kuiper Belt object 2003 EL61, and Neptune's moon Triton 
which is likely a captured KBO.  These higher densities imply that these 
bodies are not pure ice but must have a significant rocky composition.

The discovery of Eris in 2005 (originally nicknamed Xena, and officially 
cataloged 2003 UB313) prompted a debate over the planetary status of Pluto 
because astronomers realized they would have to call it the "10th" planet 
if Pluto retained its own planetary status, which was already under debate. 
This led the International Astronomical Union, in 2006, to make a new class 
of solar system object called dwarf planets. These are spherical bodies in 
hydrostatic equilibrium (objects that have sufficient gravity to overcome 
their own rigidity and form a spherical shape) like the planets, but unlike 
the major planets in the solar system, they have not gravitationally 
cleared out the neighborhood of particles and small debris along 
their their orbits.

For images and additional information about Eris and Dysnomia, visit:
http://hubblesite.org/news/2007/24

http://www.keckobservatory.org/news.php

The W.M. Keck Observatory is operated by the California Association for
Research in Astronomy (CARA), a non-profit corporation whose governing board
consists of directors from the California Institute of Technology and the 
University of California.  NASA and the W. M. Keck Foundation each have 
liaisons to the board.

The Hubble Space Telescope is a project of international cooperation
between NASA and the European Space Agency. The Space Telescope Science 
Institute conducts Hubble science operations. The institute is operated 
for NASA by the Association of Universities for Research in Astronomy, Inc., 
Washington.