That means that its light will undergo a Doppler shift, and that can be detected.

这意味着它的光会经历多普勒频移，并且可以被探测到。

Another way is to measure the Doppler shift in a star's light due to reflexive motion as the planet orbits.

另一种方法是测量恒星光线的多普勒频移，这种频移是由行星绕轨道运行时的自反运动引起的。

If we know the planets' mass from the star's Doppler shift, we can use the planet's size to calculate its density.

如果我们通过恒星的多普勒频移知道行星的质量，我们就可以用行星的大小来计算它的密度。