Final answer:
For astronomers to use the Doppler technique to determine a planet's mass, they need to measure the wobble in the star's velocity caused by the planet's gravity. This is done using high-resolution stellar spectroscopy to calculate the mass of the planet.
Step-by-step explanation:
In order for astronomers to use the Doppler technique to determine a planet's mass, it is essential that the planet's gravity causes the star to wobble, changing its radial velocity by a small but detectable amount. This wobble in the star's velocity is measured using high-resolution stellar spectroscopy, which allows astronomers to calculate the mass of the planet. The distance of the star does not matter, as long as it is bright enough for high-quality spectra to be taken.