UNDERSTANDING OTHER WORLDS
What is radial velocity?
A star’s subtle motion can betray the gravitational pull of an unseen planet.
A moving star
The Doppler effect shifts spectral lines as a star moves towards or away from Earth. Repeated observations reveal periodic motion that may be caused by a planet.
Minimum mass
Without an independently known orbital inclination, this method usually gives mass multiplied by the sine of the inclination. The planet’s true mass may be higher.
Separating the signals
Stellar spots and activity also alter spectra. Long observing campaigns and independent evidence help distinguish planetary motion from stellar behaviour.