Star Entangled with Its Giant Planet Experiences Hyperactive Magnetic Cycle

The star Tau Boo’s magnetic cycle is at least 10 times faster than the sun’s
Tau Bootis magnetic field

Karen Teramura/IfA

The relationship between stars and planets is usually rather one-directional—the star rules over its celestial subjects, blasting them with radiation, blessing them with warmth. The puny planets simply take what they get. But sometimes a planet is so massive, and so close to its star, that the smaller object can exert considerable influence on its stellar neighbor.

Such is the case with the planet orbiting the star Tau Boötis—Tau Boo for short. The giant world, six times the mass of Jupiter, was discovered in 1996 circling the bright star some 50 light-years from the sun. Tau Boo b, as the planet is known, passes so close to the star in its orbit—less than one twentieth the distance between Earth and the sun—that it drags the stellar surface along with it, thereby synchronizing the rotation of the star with the orbit of the planet.

The gravitational interaction may drive a hyperactive flipping of the star’s magnetic field as well. New research shows that the magnetic poles of the star reverse on timescales of one Earth year or less, so that the magnetic field cycles back to its original orientation within two years. The researchers say that Tau Boo is only the second star—after the sun—for which a complete magnetic cycle has been documented. But Tau Boo’s progression is far faster than the sun’s corresponding magnetic cycle, which takes 22 years to complete.

The research, which has been submitted for publication in the Monthly Notices of the Royal Astronomical Society, confirms earlier indications of Tau Boo’s rapid magnetic cycling based on preliminary observations spanning just a few years. “When you have only three data points, you could say that it’s possibly a cycle, but it’s good to get more data points to be sure,” says astrophysicist Rim Fares of the University of Saint Andrews in Scotland. She presented the new data this week at the U.K.’s National Astronomy Meeting.

From 2006 to 2011 Fares and her colleagues tracked the magnetic field of Tau Boo and nine other stars using the Canada–France–Hawaii Telescope on Mauna Kea and the Bernard Lyot Telescope in the French Pyrenees. The new observations uphold the suggestion, put forth by Fares and her colleagues in 2009, that the star’s magnetic cycle lasts about two years, but the researchers are not yet able to measure precisely how short the cycle is. “We see the flip after 12 months. Did other flips happen in between? We cannot be absolutely sure of that,” Fares says.

The cause of Tau Boo’s rapid oscillations in its large-scale magnetic field has not yet been identified, but the nearby planet makes a compelling suspect. “This star is interesting because it has a very massive planet,” Fares says. “It has a very massive hot Jupiter that is orbiting very close.” The gravitational pull of the planet may be contributing shear forces to the convective layer of the star, where roiling plasma creates the magnetic field.

Further studies of the magnetic fields of stars, with and without planets, will help clarify the effects that the giant world Tau Boo b has on its host star. Fares and her colleagues are working to gather data on other stars, but as of yet none have shown such a clear-cut cycle of magnetic reversal. “We have observed some flips in polarity, but most of them are quite chaotic,” Fares says. “I think we need to observe much more to be sure that there are cycles on those stars.”

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