Press Release

McDonald Observatory Astronomers Discover Neptune-sized Planet with Hobby-Eberly Telescope

By SpaceRef Editor
August 31, 2004
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McDonald Observatory Astronomers Discover Neptune-sized Planet with Hobby-Eberly Telescope

A team of astronomers led by Barbara McArthur, and including Michael
Endl, William Cochran and Fritz Benedict, of The University of Texas at
Austin’s McDonald Observatory has used the Hobby-Eberly Telescope
(HET) and its High Resolution Spectrograph to discover a very small
planet orbiting a nearby star known as rho-1 Cancri (also called
55 Cancri).

31 August 2004: Detection of a Neptune-mass planet in the rho Cancri system using the Hobby-Eberly Telescope

Astronomers already knew that three planets, with periods of 14.6, 44
and 4520 days, orbit rho-1 Cancri, a star about the same size as the Sun.
This new planet is closer to the star than the other three. Named
rho-1 Cancri e, it orbits the star every 2.8 days at a distance of only
0.038 Astronomical Units (3,534,000 miles). This makes the system the
first known extrasolar four-planet system. The research has been accepted
for publication in Astrophysical Journal Letters.

The highlight of the work is the fact that the newly discovered planet,
rho-1 Cancri e, has a minimum mass of only 14 Earth masses, and a most
likely mass of just 18 Earth masses — about the mass of Neptune. It is
the lowest mass planet that has been discovered. Most of the 120 or so
known extrasolar planets are Jupiter-sized. (At about 300 times the mass
of Earth, Jupiter is the most massive planet in our solar system.) This
discovery with the HET indicates that with improved instrumentation
astronomers are coming closer to finding extrasolar Earths.

McArthur’s quest began when she decided to reanalyze archival Hubble
Space Telescope (HST) data to look for the motion of the star on the sky
as it orbits the center of gravity of the star and its planets –a
techniqu known as “astrometry.” Thanks to the exquisite sharpness of
HST’s Fine Guidance Sensors, the anticipated motion of the star was
detected. Thomas Harrison, an astronomer from New Mexico State
University, provided supporting observations of stars in the HST field of

As she and colleague Benedict have done with other stars, McArthur
decided to collect “radial velocities” of rho-1 Cancri to complement the
Hubble data. Radial velocity observations involve measurements of changes
in a star’s velocity toward and away from Earth — its wobble. Astronomers
from the California and Carnegie Planet Search team and Geneva
Observatory contributed radial velocity data. McArthur, seeking highly
precise, intensive data, then collaborated with McDonald Observatory
astronomers Endl and Cochran to make radial velocity observations of
rho1 Cancri with the Hobby-Eberly Telescope in West Texas.

“These data were taken very quickly,” McArthur said. “In 180 days, we
got over 100 observations of the star. This is amazing access to a high
precision instrument,” she said, referring to the HET’s queue-scheduling

Astronomers do not travel to the observatory to operate the telescope
themselves. Rather, a resident astronomer at McDonald Observatory has
a list of all HET research projects and selects the ones best suited to
any given night’s weather conditions and Moon phase. This way, many
targets for different research projects can be observed each night, and
any particular target can be observed dozens of night in a row.
McArthur said getting the data over a short period of time helps reduce
introduction of errors that can build up over months or years due to
changes in a telescope, its instrumentation or even the star system
under study.

Combining the new HET data with earlier data from the other planet search
teams and with archival data from NASA’s Hubble Space Telescope, McArthur
was able to determine the size, shape and orientation of the orbit of the
outer planet (rho-1 Cancri d). Next, McArthur was able to model the inner
planets, revealing a fourth planet in this system. On the assumption that
rho-1 Cancri’s planets all lie in the same plane, as planets do in our
solar system, the mass of the newly discovered planet was found to be a
mere 18 times that of the Earth, equal to the mass of Neptune. Because of
the HST astrometry measurements, the mass of this planet is a true mass,
not the lower limit usually reported by radial velocity-alone techniques.

“This discovery is a leap forward into a new domain of extrasolar
planets,” Endl said. “Finally, we find planets with masses that
probably mean that they resemble more our Neptune or Uranus, which
consist mostly of a rocky/ice core and a small gaseous envelope.”

“This planet detection demonstrates that the HET is an absolutely
wonderful planet-hunting machine,” Cochran added. “The combination of
the queue-scheduled operation, which allows us to get data when we
need it, and the exquisite spectrograph, which gathers outstanding
data, makes the HET uniquely suited for this task.”

“It’s a remarkable discovery,” said McDonald Observatory Director
David L. Lambert. “It’s taking extrasolar planet discoveries to a
new dimension. This challenges theoretical understanding of how
planets form and evolve around stars like our Sun. I am proud of the

The complete list of authors for this paper include: McArthur, Endl,
Cochran and Benedict, McDonald Observatory, The University of Texas
at Austin; Debra A. Fischer and Geoffrey W. Marcy, Department of
Astronomy, University of California, Berkeley; R. Paul Butler,
Department of Terrestrial Magnetism, Carnegie Institution of
Washington; D. Naef, M. Mayor, D. Queloz, and S. Udry, Observatoire
de Geneve, Sauverny, Switzerland; and Harrison, Department of
Astronomy, New Mexico State University.

The Hobby-Eberly Telescope is a joint project of The University of
Texas at Austin, The Pennsylvania State University, Stanford
University, Ludwig-Maximilians-Universitat Muenchen and Georg-
August-Unversitat Goettingen. The researchers would like to thank
the following people with McDonald Observatory who made this
discovery possible: Phillip J. MacQueen, Robert G. Tull, John Good,
John Booth, Matthew Shetrone, Brian Roman, Stephen Odewahn, Frank
Deglman, Michelle Graver, Michael Soukup, Martin L. Villarreal Jr.

Note to editors:

An illustration to accompany this release is available online at
Additional graphics are available from NASA’s Jet Propulsion
Laboratory at:

SpaceRef staff editor.