Contact: Dr. William Cochran

Department of Astronomy

RLM 16.342

University of Texas

Austin, TX 78712

Phone: (512) 471-6474

E-mail: wdc@astro.as.utexas.edu

Today, a team of astronomers will announce the discovery of a Jupiter-mass
planet orbiting a star, Epsilon Eridani, 10.5 light-years away, making it
the first such planet found this close to our own solar system. Dr. William
Cochran, of the University of Texas McDonald Observatory, will present the
team’s findings at the symposium on “Planetary Systems in the Universe,” as
part of the 24th General Assembly of the International Astronomical Union,
in Manchester, England.

“Detecting a planet orbiting Epsilon Eridani, a star very similar to our own
Sun and only 3.22 parsecs from Earth, is like finding a planet in our own
backyard — relatively speaking,” said Cochran. “Not only is this planet
nearby, it lies 3.2 AU (478 million kilometers, or 297 million miles) from
its central star — roughly the distance from the Sun to the asteroid belt
in our own solar system.”

The planet of Epsilon Eridani has between 0.8 and 1.6 times the mass of
Jupiter and an orbital period of just under seven years — about 60 percent
the orbital period of Jupiter but longer than that of most other extrasolar
planets discovered recently. In fact, the planet could qualify as the first
“Jupiter analogue” were it not for its highly eccentric (elliptical) orbit
of 0.6. (The orbits of planets in our solar system are more circular.)

To arrive at its discovery, the team studied nearly 20 years of
high-precision radial velocity (RV) measurements of Epsilon Eridani, the
fifth brightest star in the constellation Eridanus, the river. A bright K2 V
star (approximate magnitude of 3.7), Epsilon Eridani is slightly less
massive than our Sun (0.85 solar mass) and slightly cooler (5,180 degrees
Kelvin). The team noted that the star’s high level of chromospheric activity
is consistent with its relatively young age, less than a billion years old.

“We looked very hard at several years worth of spectrophotometric data for
Epsilon Eridani, to make sure that the star’s low RV — 19 meters per second
— was not due to periodic stellar cycles,” stated Dr. Artie Hatzes of the
McDonald Observatory. “Especially helpful were the Ca II H and K S-index
measurements that Dr. Sallie Baliunas, our collaborator at the
Harvard-Smithsonian Center for Astrophysics, had made and analyzed of 100
lower main sequence stars, Epsilon Eridani among them. Earlier data that
showed an RV amplitude higher than what we found had led some researchers to
attribute the RV variations to the star’s activity cycle. After studying the
relative flux in the Ca II H and K emission cores from this star, however,
we concluded that there is no significant periodicity in the Ca II
spectrophotometric measurements that matches the period of the Doppler
variation.”

“We also ruled out the possibility of a stellar companion,” Hatzes added.
“There is just no strong evidence to suggest that Epsilon Eridani is a
binary star.”

The team’s data represent a combination of six independent data sets taken
with four different telescopes and with three different measurement
techniques. In addition to their own observations made with the 2.7-m
telescope at McDonald Observatory, Cochran and Hatzes drew from data
collected by three other planet search groups, including the
Canada-France-Hawaii Observatory and the European Southern Observatory.

Asymmetric, primordial dust rings made up of 1-mm-size particles extend 60
AU from Epsilon Eridani. The irregular shape of this ring may be due to
another, undiscovered planet. “If there is indeed a second planet, the
asymmetry of the disk would suggest that the planet is orbiting just inside
the ring, at a distance of 30 AU — much farther out than the planet we have
found and with a much longer orbital period than the one we’ve discovered,”
according to Hatzes. “Thus, it might also be responsible for the possible
overall slope in our velocity measurements. And where there’s one planet,
there may be more.”

The discovery of the planet around Epsilon Eridani raises the likelihood of
detecting planets with longer orbital periods and multiplanet systems like
our own. “Given its close proximity to Earth, a one-arcsecond separation
between the planet and its central star, and the relatively large degree of
perturbation — about 1.4 milli-arcseconds — of the star from its orbiting
planet mean that we could very likely resolve the true mass of this planet,
using both direct imaging and space-based astrometric measurements with
Hubble Space Telescope,” Cochran noted.

In addition to Cochran and Hatzes, team members consist of Barbara McArthur,
McDonald Observatory; Gordon Walker, University of British Columbia; Alan
Irwin and Stephenson Yang, University of Victoria; Bruce Campbell (formerly
of the Dominion Astrophysical Observatory in Victoria); Sallie Baliunas,
Harvard-Smithsonian Center for Astrophysics; Martin Kuerster, Michael Endl,
and Sebastian Els, European Southern Observatory; Geoffrey Marcy, University
of California, Berkeley; and Paul Butler, Carnegie Institution of
Washington.

NOTE TO EDITORS: Dr. Hatzes can be reached at (817)-732-6492 only from
August 1 to 9. After August 14, he can be reached in Germany at (011)
49-36427-86351.

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