Astronomy: Man Bites Dog; Planet heats its star

Observing a sunlike star 90 light-years from Earth, astronomers have found evidence of an extraordinary role reversal. Whereas a star usually heats a planet, a closely orbiting planet appears to be, in this case, heating its star.

ROLE REVERSAL. Illustration shows how a closely orbiting planet might heat its star. S. Erno

Observations suggest that once every 3.09 days, as it whips around a star called HD 179949, the planet generates a hot spot in the star’s atmosphere. Evgenya Shkolnik of the University of British Columbia in Vancouver and her colleagues announced the finding last week at a meeting of the American Astronomical Society in Atlanta.

The hot spot, she says, probably results from a magnetic interaction between the planet and the star. If that’s correct, the findings would constitute the first evidence that a planet beyond the solar system possesses a magnetic field. The presence of a magnetic field, in turn, could provide new information on how and where such planets form.

Spectra of the star taken by another team 4 years ago revealed that an unseen planet, nearly as massive as Jupiter, was tugging on the star from a distance just one-ninth that at which Mercury orbits our sun. Such tightly orbiting planets, nicknamed hot Jupiters, account for about 20 percent of the nearly 120 extrasolar planets identified so far (SN: 10/25/03, p. 270: Available to subscribers at Extrasolar planet gets heavier).

Theorists had predicted that hot Jupiters might heat their stars, either through a gravitational or a magnetic influence. But until now, there had been no data to support that notion.

To look for evidence of the planet’s effect on HD 179949, Shkolnik’s team used the Canada-France-Hawaii Telescope on Hawaii’s Mauna Kea to record ultraviolet light emitted by calcium ions in the star’s chromosphere, a thin layer of gas just above its visible surface. Magnetic storms on sunlike stars such as HD 179949 are known to generate hot spots, which stand out in the ultraviolet light.

The team found a hot spot that was a few percent brighter, on average, than would be expected if the star alone had generated it. Moreover, the observations, taken over more than a year, reveal that the hot spot moves in sync with the planet. The team also found that whenever the planet, according to its calculated orbit, was on the far side of the star, calcium emissions were lowest.

Taken together, this evidence points to a magnetic force exerted by the planet as the most plausible explanation for HD 179949’s hot spot, says Shkolnik. According to one model, the planet’s magnetic field combines with that of the star, increasing magnetic-storm activity and heating a small part of the star’s upper atmosphere.

To generate a magnetic field, a planet must have a liquid metallic core and a rapid rate of rotation. “We don’t know anything about the internal structure of [extrasolar planets], and this could give us some new constraints” on their composition, she says.

Sara Seager of the Carnegie Institution of Washington (D.C.) calls the findings “very exciting and intriguing” but says that she would like proof that the star doesn’t happen to rotate at the same rate that the planet orbits the star. If the star rotation does match the rate of the planet’s orbit, then the star itself, and not the planet, might generate the traveling hot spot.

Shkolnik says that several observational and theoretical arguments indicate that the star rotates more slowly.


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