HD 209458

▪ star

      seventh-magnitude star 150 light-years away in the constellation Pegasus, the first star having a planet detected by its transit across the star's face. The star, which has physical characteristics similar to those of the Sun, was shown in late 1999 to have a planet by detection of the planet's gravitational effects on the star's motion. Shortly afterward, astronomers independently confirmed the planet's presence by observing that HD 209458 changed in brightness with the same 3.5-day period predicted from the discovery data for the planet's orbit. Although the planet could not be seen directly, its passages between its star and Earth provided important information about its physical properties and atmosphere not otherwise available. The extrasolar planet is about 1.3 times the size of Jupiter but has only two-thirds of Jupiter's mass. It orbits surprisingly close to the star—about 10 stellar radii.

John Donald Fernie

 

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