The radial velocity method for detecting an exoplanet is useful for ground-based telescopes finding massive exoplanets that are close to their host star.

The radial velocity method is based on the assumption that the host star and exoplanet are orbiting one another and that the star’s orbit is perturbed by the gravitational pull of the exoplanet. The method for detecting the perturbation is spectroscopy, as shown in the following figure:

Image credit: European Southern Observatory / CC BY

The figure demonstrates the relationship between the wavelength of light we see from earth as the exoplanet orbits the host star. The idea is to search for periodic changes in the host star’s spectrum, thereby eliminating the need to constantly monitor the host star as is done using the transit method.

Despite its advantages, this method has some drawbacks.

  • It cannot be used to determine the mass of the exoplanet
  • If the orbital plane of the exoplanet is face-on then the star’s movement will be perpendicular to our line of sight and so there won’t be any change in the star’s spectrum
  • The detected planets are not candidates for hosting life because the exoplanets detected using this method are usually multiples larger than Jupiter
  • Measurements are affected by stellar features that cause the surface of the star to be hotter and cooler

Even though there are some drawbacks, this method has been used to detect a large number of exoplanets, furthering our understanding of them.