Radial velocity

Stars with planets aren't stationary

We often picture our Solar System with the Sun in the middle, completely stationary, while all the planets move around it. However, this isn’t true – in reality, the planets and the Sun orbit their common centre of mass. This is most easily pictured by considering just one planet orbiting a star.

In this situation, the centre of mass will lie somewhere between the centre of the star and the centre of the planet. As the star is always more massive that the planet, the centre of mass will always lie closer to the centre of the star (this point often lies inside the star itself but displaced slightly from its centre). This means that the circular or elliptical path the star follows will be shorter than that of the planet and so the star will move slower too. The important thing to note though, is that the star does move.

The star's movement towards and away from us causes its light to be Doppler shifted. If these small variations of the star's light change in a regular pattern, then that may be evidence of a planetary companion!