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January 8, 2004, Thursday

NATIONAL DESK

New Clues Are Detected About Planets Of Other Stars

By KENNETH CHANG (NYT) 481 words
ATLANTA, Jan. 7 -- For the first time, astronomers have detected a magnetic field around a planet around a distant star, offering one of the first clues to the properties of any planet outside the solar system.

Over the past decade, astronomers have found 119 planets around other stars. But because the planets are detected indirectly -- by their gravitational tug on the stars -- almost nothing is known about any of them beyond a lower limit of their masses.

Using the Canada France Hawaii Telescope on Mauna Kea in Hawaii, Evgenya Shkolnik, a graduate student at the University of British Columbia, looked at the star HD179949, 88 light-years away in the constellation Sagittarius. Its planet, nearly the size of Jupiter, falls in the class of ''roasters,'' a large planet that orbits very close to its star, in this case 4 million miles. (The Earth, by contrast, is 93 million miles from the Sun.)

Ms. Shkolnik detected a spot on HD179949 that was 700 degrees warmer than the surrounding areas and circled the star at the same pace as the planet's orbit, once every three days. First seen in 2001, it also appeared in two sets of observations in 2002. It is probably not an intrinsic feature of the star, which takes nine days to rotate.

Instead, the planet appears to possess a magnetic field that interacts with the star's magnetic field.

''The hot spot is slightly ahead of the planet and appears to be moving across the surface of the star,'' Ms. Shkolnik said. ''The best explanation for this is that it's an interaction between the planet of the star.''

The findings were presented Wednesday at a meeting of the American Astronomical Society here and have also been published in The Astrophysical Journal.

''The observations look legitimate to me,'' said Dr. Gibor B. Basri of the University of California at Berkeley, who was not involved with the research. However, the theoretical understanding is ''very insufficient to be able to judge whether how such a thing would happen,'' he said.

The presence of a magnetic field implies metal at the core of the planet. Jupiter, which possesses a strong magnetic field, is believed to contain a core of metallic hydrogen. HD179949's planet may be inducing a hot spot on the star similar to how the magnetic fields of Io and Europa, two moons of Jupiter, induce hot spots on Jupiter.

Others have suspected that ''roasters'' must have strong magnetic fields or that they would have been destroyed by the winds of particles ejected from the star. A magnetic field acts as a shield that diverts electrically charged particles around the planet.

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