Astronomers Measure Mass of Largest Dwarf Planet (Eris)

June 14, 2007

Ray Villard
Space Telescope Science Institute, Baltimore
1-410-338-4514; villard@xxxxxxxxx

Robert Tindol
California Institute of Technology, Pasadena, Calif.
1-626-395-3631; tindol@xxxxxxxxxxx

Laura K. Kinoshita
W.M. Keck Observatory, Kamuela, Hawaii
1-808-881-3827; newsletter@xxxxxxxxxxxxxxx

Mike Brown
California Institute of Technology, Pasadena, Calif.
1-626-395-8423; mbrown@xxxxxxxxxxxxxxx

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.

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