Alien planet. Silhouette of the alien planet HD 209458b as it passes in front of its parent star (HD 209458). This transit was detected by astronomers from Earth as a dip in the star's brightness of 1.7% at the time predicted from orbital calculations. This was the first direct evidence of a planet orbiting another star. The presence of the planet was detected by observing the wobble its orbit has on the star's motion through space. The planet is around 200 times the mass of Earth, but orbits only 6.5 million kilometres from its star. One orbit takes just 3.5 days. HD 209458 is a Sun-like star which lies 153 light years from Earth.