The radial velocity method (RV method, wobble method, or Doppler spectroscopy) is a method of detecting extra-solar planets, and also used to confirm and gather more data on planets detected by other methods, such as the transit method. It requires a spectrometer capable of detecting the shift of spectral lines due to changes in radial velocity (RV) of the star, that follow a pattern suggesting the existence of a body orbiting the star. The same method has been used for binary stars for some time, and in the 1990s when instruments became capable of detecting differences in RV of around 10 meters/second, planets began to be detected. Current state-of-the-art technology can detect velocity differences of 1 meter/second. Stars for which both RV measurements and transit information have been achieved enable an estimate of the planet's density (based on an estimate of the star's mass), offering a strong clue to the planet's constituents.
A larger orbital radius between the star and the body increases the RV differences, but also increase the orbital period. The RV differences are highest, thus most detectable at two points within a period of the orbit, and a long orbital period (e.g., months or years) reduces the efficiency and practicability of catching these. These small radial velocities are far beyond those implied by the spectral resolution of any spectroscope and among the techniques for achieving such detections is the averaging of numerous observations spread over a succession of orbital periods.
With the high resolution spectrography (HRS) instruments necessary to do this, in some cases, lines (such as water lines) from the planet's day side can be detected, analogous to a double-line spectroscopic binary, offering considerably more information, i.e., some of the planet's atmospheric constituents, and RV information on the planet itself.
Much of the HRS for RV comparisons and planet detection has been carried out using visible light, but near infrared (NIR) RV measurement is of current interest, a reason being that most stars are M dwarfs (with black-body spectra peaking in the infrared) and demographics has revealed they do have planets. The HRS necessary to determine deltas of 10 m/s and 1 m/s (measured at different times, e.g., weeks apart) has required many specialized spectrograph features and techniques, and NIR presents its own challenges regarding such spectral resolution and stability, and techniques are being invented and adapted. NIR capability has the side-advantage that it is more likely to pick up data from the planet's day side.