Exoplanet detection: the radial velocity method

Exoplanet detection has developed a lot during the last decades. Since the first detection of an exoplanet in 1994, many methods have been put into practice in order to discover exoplanets. Due to the enormous difficulties to observe planets directly, almost every detection of exoplanets has been carried out using indirect detection methods, by observing the effects that the planet has on its star.

One of these methods is radial velocity, also known as Doppler spectroscopy. Among the more than 5,000 planets discovered, almost 1,000 of them have been discovered using this technique.

What does the radial velocity method consist of?

When a planet orbits a star, it has a gravitational influence on the star. This makes the two bodies orbit a common centre of gravity. By turning around this point, from the Earth we observe that the star moves towards and away from us in regular periods of time.

This wobbling on the movement of the star has an effect on the light it emits. When the star moves away from us, the light it emits gets “stretched”, for which it loses energy and situates closer to the red end of the electromagnetic spectrum. However, when the star gets closer to us, the light it emits gets “squeezed”, gaining energy in the process, and situating in the blue end of the spectrum. This phenomenon is illustrated in the following image, and is called the Doppler effect:

The planet makes the star orbit a point in space. When the star moves away from us, its light is redshifted. However, when it moves towards us, the light it emits gets blueshifted.

From the velocity at which moves the star, astronomers are capable of deducing the presence of a planet or system of planets. The more noticeable the Doppler effect, the more massive the planet that orbits the star, and inversely. It is important to point out that if around a star there isn’t any planet we wouldn’t see any effect in its light, because the star would remain motionless.

Until 2012, radial velocity was the more effective method to detect exoplanets, but it has been replaced by exoplanetary transits, also known as the method of transit photometry. However, it is still a widely used method, and many times is used in conjunction with exoplanetary transits in order to confirm the existence of exoplanets or to obtain their mass and size.

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