Doppler Spectroscopy

Doppler spectroscopy (also known as the radial-velocity method, or colloquially, the wobble method) is an indirect method for finding extrasolar planets and brown dwarfs from radial-velocity measurements via observation of Doppler shifts in the spectrum of the planet's parent star.

880 extrasolar planets (about 21.0% of the total) were discovered using Doppler spectroscopy, as of February 2020.

And this is the formula that relates the brightness to the radius of the exoplanet.

∆B/B=R^2(exoplanet)/R^2(star)

If you want to learn more about this spectroscopy click here .