This can be obtained by looking at the red/blue-shift of the star as it wobbles back and forth due to the planet's motion. Combine this with the mass of the sta

This can be obtained by looking at the red/blue-shift of the star as it wobbles back and forth due to the planet's motion. Combine this with the mass of the star, and you can figure out the mass of the planet. I forget how you get the mass of the star, but I think it can be pretty well established from its brightness.