Planets Orbiting other Stars

One may well ask how such extrasolar planets can ever be detected, since they lie within the blinding light of their parent stars. The methodology is simple, but intriguing. As planets circle their parent stars, they induce a gravitational tug of war, and very small changes in velocity of the star can be measured, using a spectrograph. It was in this way that astronomers Udry and collaborators discovered a planet whose mass is only fi ve times that of the Earth, in orbit about a star known as Gliese 581. The orbital period is just under thirteen days. The parent star, approximately twenty light years distant, has only one percent of the Sun's total luminosity, but the “Earth-sized” planet orbits the star at only seven percent of the Earth—Sun distance.