* Astronomy

An extrasolar planet has been caught dragging the outer layers of its host star around with it as it orbits. The newly discovered “tail wagging the dog” phenomenon may be common among massive planets in close orbit around their stars.
Many celestial bodies begin their lives spinning quickly but decelerate over time because of the gravitational tugs of nearby objects. The Moon, for example, has slowed to the point that it rotates on its axis in the same amount of time it takes to orbit Earth.
But smaller bodies rarely dictate their hosts' rotation, unless the two objects are closely spaced and are relatively similar in mass - such as Pluto and its moon, Charon. Now, a team of astronomers has observed this "lock-step" phenomenon in a planet-star pair for the first time.
The team used Canada's MOST - Microvariability and Oscillations of Stars - satellite to search for reflected light from a planet first detected by the wobble it induces in its host star, tau Bootis A. The planet appears to be at least 3.87 times the mass of Jupiter and orbits eight times closer to its star than Mercury does from the Sun. It has one of the shortest known periods, only 3.31 days, and orbits at a close distance of 0.046 astronomical units from the star (6.9 million kilometres) in a nearly circular orbit, making it one of the several "hot Jupiter's known.



The team noticed the star's own brightness changes every 3.3 days - the same orbital period as the planet.
"The star is keeping pace with the planet." - Jaymie Matthews, MOST's leader at the University of British Columbia in Vancouver, Canada. He attributes the effect to the mass and proximity of the planet, which allow it to gravitationally yank on the star's outer atmosphere.
Tau Bootis, at magnitude 4.50, has been classed both as an F6 sub giant and an F7 dwarf. At a distance of 51 light years, it shines with 3.1 times the solar luminosity. Warmer than our Sun, the surface temperature fall around 6340 Kelvin. The star's higher luminosity and temperature show it to have a radius 1.46 times solar and a mass of 1.3 times that of the Sun. The star is most likely a core-hydrogen-fusing dwarf, but one with some age to it. Tau Bootis is a slight variable with a magnitude range of under 10 percent, which would make it unusual among stars with planets. More unusual, Tau Boo has a distant companion, a red class M dwarf of 0.4 solar masses that orbits far outside the planetary system at an average distance of 100 or 240 astronomical units with a revolutionary period 750 or 2600 years. Like most stars that have planets, this one is also metal rich, the iron abundance (relative to hydrogen) 1.7 times solar.

The discovery bolsters another recent finding of a star that heats up in synch with a closely orbiting planet - an effect that may be caused by the entanglement of the two objects' magnetic fields.
"We now seem to be finding evidence that planets interact with their stars to a degree we didn't think possible." - Guillermo Torres, astronomer at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, US.
Indeed, over about a month of observations, the MOST team saw brightness changes on the order of a few tenths of a percent suggesting the star is magnetically active, with constantly varying "starspots" similar to those on the Sun. That activity, says Matthews, along with the rapid rotation of its outer layers, suggests the star is young - perhaps a few hundred million years old - and "rambunctious".
But Matthews says more study is needed to verify its age. "What we really don't know is if tau Bootis A is a young, fast-spinning star and this has simply fine-tuned it, or whether it's a much older star and this process has rejuvenated it and spun up its outer layers."
The answer to this question could shed light on how long it takes for gas giant planets - which probably form at greater distances from such stars - to arrive at their tight orbits.

Matthews says MOST is the only telescope in existence that could have made this discovery. The suitcase-sized telescope orbits above Earth's distorting atmosphere, which gives it about 100 times the precision of ground-based telescopes. And its orbit means it can observe a single object for about two months without interruption.
"We like to think of it as putting stars under a stakeout. You need this continuous coverage to get the complete story."
He presented the results on Monday at a meeting of the Canadian Astronomical Society in Montreal.

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-- Edited by Blobrana at 21:08, 2005-05-18