sentences:  The first non-transiting planet to have its mass found this way was Tau Boötis b in 2012 when carbon monoxide was detected in the infra-red part of the spectrum.The graph to the right illustrates the sine curve using Doppler spectroscopy to observe the radial velocity of an imaginary star which is being orbited by a planet in a circular orbit.Observations of a real star would produce a similar graph, although eccentricity in the orbit will distort the curve and complicate the calculations below.