Fig10_10 Starlight shift

Fig10_10 Starlight shift

Fig. 10.10 . An unseen planet exerts a gravitational force on its visible host star. This force tugs the star in a circular or oval path, which mirrors in miniature the planet’s orbit. As the star moves along this path, it approaches and recedes from Earth, changing the wavelength of the starlight seen from Earth through the Doppler effect. When the planet pulls the star toward us, its light waves pile up in front of it slightly, shortening or “blueshifting” the wavelength we detect. When the planet pulls the star away from us, we detect light waves that are stretched or redshifted. During successive planet orbits, the star’s spectral lines are periodically shortened and lengthened, revealing the presence of the planet orbiting the star, even though we cannot see the planet directly.

Copyright 2010, Professor Kenneth R. Lang, Tufts University