Evidence grows for nearby planetary system

Arrow points to nearby star Epsilon Eridani, located in the constellation Eridanus. North Star Atlas

Although it lies close to Earth, the star Epsilon Eridani could well be overlooked by a casual observer. Skywatchers, however, may want to mark the location of this lone, sunlike star, just west of Orion the hunter.

Astronomers plan to report next week that Epsilon Eridani is the nearest star known to harbor a planet. Together with previous observations, the new find hints that several planets could be orbiting the star. If so, an entire planetary system may lie just 10.5 light-years from Earth.

It’s “like finding a planet in our own backyard,” codiscoverer William D. Cochran of the University of Texas at Austin told Science News.

“This is big news,” says Frank Drake, director of the SETI Institute in Mountain View, Calif., which uses radio telescopes to search for signals from intelligent extraterrestrial broadcasters. Forty years ago, Drake began searching for radio signals from Epsilon Eridani, one of the two sunlike stars closest to our solar system. He found no signs of life, but this star, visible to the naked eye, has remained tantalizing to astronomers.

Cochran and his colleagues, including Artie P. Hatzes of the University of Texas, used a now-standard technique to find evidence of the planet. They monitored the back-and-forth motion of the nearby star along the line of sight to Earth. Studying the star for more than a decade at the McDonald Observatory near Fort Davis, Texas, the team had discerned what appeared to be a wobble from the tug of an unseen companion orbiting the star every 7 years.

The signal was weak, however, and the astronomers weren’t sure. To complicate matters, Epsilon Eridani is a relatively young star prone to outbursts of gas. Researchers worried that if the outbursts were periodic, the variation in light intensity they induce could mimic a shift in light frequency, which astronomers take to indicate the tug of an orbiting planet.

To check, Sallie L. Baliunas of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., and her colleagues analyzed the spectra of light emitted by charged atoms, or ions, in the star’s upper atmosphere. These emissions are known to vary in synch with a star’s outbursts, and Baliunas’ team found no evidence that they fluctuated with a 7-year period.

Assured that they hadn’t been fooled, Cochran and his colleagues combined their data with earlier observations by another team at the Canada-France-Hawaii Telescope atop Hawaii’s Mauna Kea. The signal of an unseen planet “just jumped right out,” says Cochran. Further confirmation, he adds, came in data from two other teams.

Cochran’s team finds that the unseen planet is at least 80 percent as massive as Jupiter and its average orbital distance about 60 percent that of Jupiter’s to the sun. It sets a record for the largest orbit of any known extrasolar planet. Most planets detected by stellar wobble lie much closer to their parent stars, and researchers have dubbed them “hot Jupiters.”

The researchers will present their findings on Aug. 7 at an International Astronomical Union symposium on extrasolar planets in Manchester, England. Reports from other teams are expected to raise the roster of extrasolar planets to more than 50. Astronomers detected the first extrasolar planet orbiting a sunlike star just 5 years ago (SN: 10/21/95, p. 260).

The Epsilon Eridani result means that “we are very close to finding” an extrasolar planet with a mass and orbit similar to those of Jupiter, says theorist Alan P. Boss of the Carnegie Institution of Washington (D.C.). Such a planet would be a dominant force in any solar system, he notes. Its large mass and distant circular orbit would allow more-Earthlike planets to coalesce from the disks of dust and gas that surround infant stars. It would also shield such Earths from space debris.

Unlike Jupiter, however, the newly detected planet has a highly elliptical orbit. Passing closer to its parent star than Mars does to the sun and venturing beyond Jupiter’s orbital distance, the planet’s careening path would prevent material from gathering together to produce an inner, Earthlike planet, Boss says.

Epsilon Eridani, however, could harbor a retinue of planets. Two years ago, Jane Greaves of the Joint Astronomy Center in Hilo, Hawaii, and her colleagues imaged a dusty disk around the star. The disk could be material left over from the formation of planets (SN: 8/8/98, p. 91). It lies about the same distance from its parent star as the Kuiper belt, a reservoir of comets beyond Pluto, lies from the sun. Lumps that Greaves observed in the disk might have been created by the tug of a planet within or near the disk, far from the star.

Combined with the new finding of a much closer planet, the disk “indeed suggests” that Epsilon Eridani may have a planetary system, says Greaves. “It could even be rather similar to ours, with two giants in orbits a bit smaller than Jupiter’s and a bit bigger than Neptune’s,” she says.

The star’s proximity may permit large telescopes outfitted with special optics to obtain pictures of any planets it possesses, Cochran notes.

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