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"A planet orbiting a giant red star has been discovered by an astronomy team led by Penn State's Alex Wolszczan, who in 1992 discovered the first planets ever found outside our solar system. The new discovery is helping astronomers to understand what will happen to the planets in our solar system when our Sun becomes a red-giant star, expanding so much that its surface will reach as far as Earth's orbit."
The star is 2 times more massive and 10 times larger than the Sun. The new planet circles the giant star every 360 days and is located about 300 light years from Earth, in the constellation Perseus. A paper describing the discovery will be published in a November 2007 issue of the Astrophysical Journal
The method the astronomers use to discover planets is to observe candidate stars, repeatedly measuring their space velocity using the Doppler effect - the changes in the star's light spectrum that result from it being pulled alternately toward and away from Earth by the gravity of an orbiting planet. "When we detect a significant difference in a star's velocity over a month or two, we then start observing that star more frequently," Wolszczan says. "In this paper, the velocity of the star changed by about 50 meters per second, (about 100 miles per hour), between our first and second observations, so we observed that star more frequently and we found a clearly repeatable effect, indicating the presence of a planet."
A star and its orbiting planet move around the center-of-mass of the whole system, so the star alternately approaches and recedes from Earth periodically. "When the star gets closer to us, its light becomes a little bit bluer and when it recedes from us, its light becomes redder, and we can measure that effect to deduce the presence of planets," Wolszczan explains.