But finding planets is very difficult, even when astronomers think they should be there. Even the largest planets appear as tiny specs, lost in the glare of the nearby parent star. The best current telescopes could not detect Jupiter from Alpha Centauri.
Speed is important because planets gravitationally affect their stars' motion.
With increasing size difference, the centres of gravity move closer to the larger body. Analysing the wobbles provides information about the planets' size, orbit period, and distance from the star. Moreover, they can be found at a variety of distances from their parent stars, ranging from very close in, like the companion to 51 Pegasi, to very far away, like Jupiter relative to the Sun. This technique has some difficulties, since regular changes to the stars' spectra could also appear like radial velocity changes. Discerning the two involves painstaking observations over many years.