Astrometry nabs its first exoplanet: method provides more precision for mass measurements.

Researchers for half a century have tried--and failed--to use the motion of a star moving across the sky to discover planets that lie beyond the solar system. Now a team has finally used the method, known as astrometry, to find one.

The planet, about six times the mass of Jupiter, orbits the star VB 10, some 20 light-years from Earth, report Steven Pravdo and Stuart ShaMan of NASA's Jet Propulsion Laboratory in Pasadena, Calif., in an upcoming Astrophysical Journal.

The traditional method of identifying extrasolar planets, 350 of which are now known, tracks the velocity of a parent star along the line of sight to Earth, rather than across the sky. Because a planet pulls its star slightly to and fro, the star's line-of-sight motion speeds up and slows down periodically, revealed by shifts in the color of starlight. This technique, known as the wobble or Doppler shift method, reveals only the minimum mass of a planet. In contrast, astrometry pins down a planet's exact mass.

Researchers have previously used astrometry to measure the precise mass of planets found with the wobble method, but this is the first time the technique has been used to discover an extrasolar planet, says Man Boss of the Carnegie Institution for Science in Washington, D.C.