Radial Velocity Method

The presence of a planet around a star makes it dance, changing the colour of the star as it is observed by astronomers with their telescopes.

In 1995, a team of researchers from the Geneva Observatory, consisting of Michel Mayor and Didier Queloz, discovered the first exoplanet in orbit around a star similar to our Sun. They used the radial velocity method to find the planet 51 Pegasi b, and this discovery earned them the 2019 Nobel Prize in Physics.

Artistic rendition of the planet 51 Pegasi b and its star 51 Pegasi. Credit: NASA/JPL-Caltech

Of stars and planets

It is often said that a planet revolves around its star, but this is not entirely true. If the planet is gravitationally affected by its star, the star is also attracted to its planet. Both the planet and the star therefore revolve around a point called the common centre of mass.

Watch the video below. We see a small planet orbiting a large star. However, if you look closely at the star, you can see that it is also moving! The planet’s gravity makes the star dance.

Credit : NASA

The bigger the planet, the bigger the movement of the star. So a planet like Jupiter will have a bigger effect on the Sun than a planet like Earth.

The movement here is exaggerated compared to the actual motion. The star moves very little, and it’s quite hard to measure.

How do we detect the presence of the planet?

To find the planet, we use the Doppler effect. Christian Doppler is a 19th century physicist who discovered that waves, like sound or light, change frequency according to the speed of the object emitting the waves.

This is exactly what happens with the sound of ambulances! For a person standing still on the sidewalk, the sound wave from the ambulance is compressed and the sound is then higher pitched when the ambulance is getting closer. When the ambulance starts to move away, the sound wave is stretched and the sound becomes lower pitched. Listen to the difference in the video!

The light from a star also moves as a wave, like sound, and this wave can be stretched or compressed depending on the motion of the star. As shown in the animation below, if the star moves away from us, its light will be stretched and shifted towards the color red (or reshifted). If the star is approaching us, its light will be compressed and shifted to blue (or, you guessed it, blueshifted!).

Credit : F. Baron

Thus, the movement of the star causes the light we receive on Earth to be stretched and then compressed, changing the colour of the light we observe. The star’s dance makes it change colour!

iREx and the radial velocity method

At iREx, several researchers use this method to find new planets or confirm the presence of planets found by other methods. This method is one of the detection methods that allows us to measure the mass of an exoplanet, a very important characteristic in the search for Earth-like planets and extraterrestrial life! The SPIRou instrument, built in part at iREx, uses radial velocity to study exoplanets.