Stars’ wobbles reveal six more planets

Astronomers have found evidence of six additional planets outside the solar system, bringing the tally to more than 40.

The scientists obtained the new results by measuring the velocity of stars with a spectrometer at the European Southern Observatory in La Silla, Chile. The back-and-forth motion, or wobble, of the stars along the line of sight to Earth betrays the gravitational pull of planets too faint to be seen. From the wobble, astronomers can deduce a minimum mass for each planet, although its true mass could be larger.

One object, about 140 light-years from Earth, whips around the star HD 168746 every 6.4 days. Its minimum mass is 80 percent of Saturn’s, making it only the third known planet that could be lighter than Saturn (SN: 4/1/00, p. 220: Available to subscriber at Less Massive than Saturn?).

The planets orbiting two other stars, HD 83443 and HD 108147, each have a minimum mass just slightly greater than Saturn’s. The object circling HD 83443, 141 light-years from Earth, resides just 5.7 million kilometers from its parent star, less than one-tenth Mercury’s average distance from the sun. That gives it the smallest orbit of any planet so far detected.

Correspondingly, it also has the shortest orbit, racing around the star once every 2.986 days. Small irregularities in the wobble of HD 83443 suggest the star may harbor a second planetary companion.

Three other stars appear to have large planets, with minimum masses ranging from 1.07 to 2.96 times that of Jupiter.

Michel Mayor, Didier Queloz, and their colleagues at the Geneva Observatory in Sauverny, Switzerland, announced the findings in May. Since then, they have reported the first spectroscopic detection of a giant extrasolar planet crossing in front of the parent star it tightly orbits. As the planet passes by, it blocks a small area of the stellar disk, slightly reducing the amount of starlight reaching Earth. As it obstructs light from different parts of the rotating star’s surface, the planet’s passage also distorts the spectrum of the starlight. Observing such effects are at present the only way to confirm that the planets exist, since none has been imaged.

Examining the star HD 209458, another team had already detected a dimming of the starlight just at the time that astronomers had predicted its planet would cross the line of sight to Earth (SN: 11/20/99, p. 324: Queloz, Mayor, and their colleagues say they have now found the variation in the star’s spectrum by using a spectrometer at the Haute Provence Observatory in St. Michel, France. They describe their findings in an upcoming Astronomy and Astrophysics.

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