Image: San Francisco State University

PLANET HUNTER. Geoffrey Marcy and his colleagues have found most of the extrasolar planets discovered in the last two and a half years.

The skies have seemed pretty crowded of late. In the last two and a half years, astronomers have detected 12 new planets outside of our solar system. And last Monday, the heavens got busier still. Geoffrey W. Marcy of San Francisco State University and colleagues announced at an International Astronomical Union symposium in Victoria, British Columbia, that they had found yet another new object circling a star only 15 light-years away from earth.

Two hours after his presentation, Marcy received e-mail from a group led by Xavier Delfosse of the Geneva and Grenoble Observatories revealing that they had confirmed the finding using telescopes at the Haute-Provence Observatory in France and the European Southern Observatory in Chile.

The star in question, Gliese 876, is not only the closest one around which scientists have found companions so far, it is the lightest and faintest, too, having only one third the mass and one fortieth the luminosity of our sun. Thus, the researchers speculate that planetary systems may in fact be far more common than previously thought and many might be quite different from our own.

Finding them won't be easy. Planets only faintly reflect light, and stars, fueled by nuclear reactions, typically outshine them a billion times over. This latest planet--which is 1.6 times as massive as Jupiter--is no exception. As with other recent discoveries, the investigators could not see it outright but instead noticed telltale wobbles in Gliese 876's position--wobbles that could only be caused by the gravitational pull of a nearby planet.

Image: GEOFFREY MARCY, San Francisco State University

DOPPLER SHIFT. Although most distant planets are not directly visible with telescopes, they exert a gravitational pull on the stars they orbit. As a result, the stars wobble slightly, producing a measurable periodic shift in the wavelength of light they emit.

The team measured Gliese 876's wobbles by monitoring the Doppler shift of its starlight. As the star wobbled back and forth relative to Earth, its light waves too moved back and forth, shifting periodically toward the red and blue ends of the spectrum. From this cycle, they calculated that the planet takes only 61 days to spin around Gliese 876 and that it does so very closely--at an average distance shorter than that between the sun and Mercury and one fifth of that between the sun and Earth.

Marcy and his colleagues--R. Paul Butler of the Anglo-Australian Observatory, Steven S. Vogt of the University of California at Santa Cruz and Debra Fischer of SFSU--began studying 400 nearby stars, including Gliese 876, last year using powerful spectrometers at the Keck Observatory on top of Mauna Kea in Hawaii last year. They collected the crucial data on June 18 and 19. Earlier they had observed Gliese 876 using the Lick Observatory. And since 1987, the SFSU program has monitored 107 stars.

Before the Doppler technique revealed its first planet around the star 51 Pegasi in 1995, astronomers could look only to our own solar system for theories on how planets form. From that evidence, they surmised that planets emerged from flat, spinning disks of gas and dust that bulged out from central stars. But were this model entirely accurate, planets would not arise as close to a star as most of the recent finds, and they would follow nearly circular--not elliptical--orbits. The lesson is that our world may be an exception to celestial rule.

Image: Scientific American
ECCENTRIC ORBITS. Many of the extrasolar planets newly found by the Doppler technique, including these four around the star 51 Pegasi, follow highly elliptical orbits that pass quite close to their parent stars--characteristics that are forcing astronomers to rethink theories about how planets form.