Radial Velocity
Detecting the tiny motion a planet causes in its star.
star moving toward us
star moving away
A planet's gravity can make its star wobble slightly. That motion shifts the star's light toward blue as it approaches and toward red as it recedes — a rhythm we can measure without ever seeing the planet.
Lines in a star's spectrum shift slightly toward blue and then toward red, back and forth, on a repeating cycle. The star itself is moving a small amount along our line of sight.
- 01An unseen companion is pulling the star around a shared centre of mass.
- 02The cycle length gives the orbital period.
- 03The size of the velocity swing constrains the companion's mass.
- Observation
Periodic Doppler shift of absorption lines in the star's spectrum.
- Measurement
Amplitude of the velocity change and the period of the cycle.
- Physical model
Two bodies orbit a common centre of mass; the more massive one moves less.
- Inference
Orbital period and a lower limit on the companion's mass.
- +Orbital period
- +Minimum mass of the companion
- +Orbital eccentricity
- +Presence of additional companions, from extra periodic signals
What a method cannot settle matters as much as what it can. Each note below is tagged by how firm the statement is.
- INFERENCE
Without knowing the orbital inclination, the method yields a minimum mass, not the true mass. A transit or an astrometric measurement is needed to resolve it.
- OBSERVATION
The method measures motion along the line of sight only. Motion across the sky produces no Doppler shift.
- HYPOTHESIS
Stellar activity — rotating spots, pulsation, convective flows — produces periodic signals of its own that can be mistaken for a planet.
HD 189733 b
The host star's periodic velocity shift constrains the planet's mass; combined with its transit-derived radius, that yields a bulk density.
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