Question

1. In astronomy, The Doppler method works only in cases where
   1. There is some component of motion in which the star has a non-zero radial velocity.
   2. There is an extremely massive planet, several times the mass of Jupiter.
   3. We are looking at the star/planet system from a face-on vantage point.
   4. The Doppler method has not yet been successful at detecting extra solar planets.

Answer 1

Doppler effect is defined as a apparent change in the wavelength or frequency of an object when it is either moving towards or away from the observer. So the radial velocity component in the direction of motion must be non zero. The correct answer is

a. There is some component of motion in which the star has a non-zero radial velocity.