Final answer:
The Doppler method is most effective for detecting massive planets in close orbits around their stars, such as 'hot Jupiters', due to the significant radial velocity changes they cause in the star's motion.
Step-by-step explanation:
The Doppler method of detecting exoplanets is most effective for finding massive planets that are close to their stars. These types of planets, such as 'hot Jupiters,' cause the most significant wobble in their star's motion, leading to the largest Doppler shifts in the star's spectrum.
The strength of the Doppler method lies in its ability to measure these variations in the star's radial velocity, indicating the presence of an orbiting planet. As our capacity to detect smaller Doppler shifts improves and as we lengthen the monitoring periods of stars, this technique will also be able to reveal more distant and less massive planets.
Another aspect to consider is the selection effect, which has been a factor in early detections utilizing the Doppler method. Just as attending only campus events limits one's social circle, the Doppler technique initially favored the detection of large close-in planets. However, as the technique matures, a wider variety of exoplanets can potentially be discovered.