The magnetic link between star and planet.

Astronomers say that they have for the first time directly measured the magnetic field of a star known to host a giant planet.

Although the magnetic field of the star, called Tan Bootes, is only a few times as strong as that of the sun, it probably wields enormous influence on the planet, say the researchers. That's because the orb whips around Tau Bootes at just one-twentieth the distance that Earth circles the sun.

To measure the field, Claude Catala of the Observatory of Paris and his colleagues examined the polarization of light from the star using a device on the Canada-France-Hawaii Telescope on Hawaii's Manna Kea. Light waves are composed of electric and magnetic fields that oscillate in specific directions. The extent to which light coming from the star is polarized indicates the strength of a magnetic field along a particular direction.

The astronomers also determined that the equator of Tau Bootes rotates once every 3 days, while the star's poles rotate about 20 percent slower than that. The difference in rotation probably generates the star's magnetic field.

Astronomers would have expected the planet's orbital axis to align with the star's rotation axis. But the Catala team found that the planet moves in sync with material residing at about latitude 45[degrees] on the star's surface. This arrangement suggests that the magnetic field of the star interacts with the planet in a complex fashion.

Catala and his colleagues describe their findings online and in an upcoming Monthly Notices of the Royal Astronomical Society.--R.C.