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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?).

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