Search For Extrasolar Planets Hits Home

7 August 2000

MANCHESTER, ENGLAND: 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-meter 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 Kürster, Michael Endl, and Sebastian Els, European Southern Observatory; Geoffrey Marcy, University of California, Berkeley; and Paul Butler, Carnegie Institution of Washington.

Contact: Dr. William Cochran
(512) 471-6474