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Planet from another Galaxy.

Is there any kind of star that can't have planets? The latest unexpected find is a world with at least 1.25 Jupiter masses orbiting the post-red-giant star HIP 13044, a 10th-magnitude yellow giant 2,000 light-years away in Fornax.

A European group found the planet by the radial-velocity wobble it induces in the star. At the inner end of the planet's elliptical, 16.2-day orbit, it comes within a stellar diameter of this large star's surface. The star is on the horizontal branch of the Hertzsprung-Russell diagram, meaning that it has already gone through its first excursion to red-gianthood and has since shrunk and re-heated. The planet must have been farther out during the star's red-giant phase and later worked its way in closer, perhaps by tidal interactions. Otherwise the star would have swallowed it.

The planet is also unique in two other ways. Its star has the lowest metallicity (fraction of elements heavier than helium) of any host star yet known. And this system didn't even originate in our Milky Way. The star is on a high-velocity trajectory that pegs it as member of the "Helmi stream," a moving group that originally belonged to a dwarf galaxy that fell into the Milky Way 6 to 9 billion years ago and has not yet completely dispersed.


Any planets that were closer to HIP 13044 may have been consumed already. "The star is rotating relatively quickly for a horizontal branch star," said Johny Setiawan (Max Planck Institute for Astronomy), who led the research. "One explanation is that HIP 13044 swallowed its inner planets during the red-giant phase, which would make the star spin more quickly."

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Title Annotation:News Notes
Publication:Sky & Telescope
Article Type:Brief article
Geographic Code:1USA
Date:Feb 1, 2011
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