The Exoplanet Hunt: the radial velocity method
By Oscar Barragán
The night has finally arrived at the Roque de Los Muchachos Observatory. The blue sky has turned into a deep ocean full of stars which eclipses the beautiful horizon that is scattered with pink clouds. The telescopes are ready to hunt for starlight. At first sight, all the stars seem static in the night sky which is victim to the Earth’s rotation. However, this is a misconception, as all the stars that shine at night are moving within our galaxy, the Milky Way. Our mission for the night is is to detect their subtle movement which may tell us about the existence of faraway worlds.
The motion of the stars manifests itself in two ways. The first one is their movement in the plane of the sky – also known as their proper motion- which slowly re-shapes the constellations. The second one, and the one that we are searching for, is the movement of the stars with respect to us. This receding and approaching velocity of the stars is known as their radial velocity. This stellar motions, however, is so small that is is imperceptible to our naked eyes, meaning that we need to use big telescopes and state-of-the-art instruments in order to detect it.
You may be familiar with the acoustic version of the Doppler effect: the change in sound as a car first moves towards and then away from you. This change in sound is caused by the compression and elongation of the car’s sound waves caused by the motion of the car. In the same vein, light travels as a wave, and the Doppler effect results in an apparent change in color. If a light-emitting astronomical object moves towards us the waves are compressed and appear redder. Conversely, if the object moves away from us the waves are elongated and appear bluer. This effect is extremely small, and thus we have to use specifically designed instruments, known as spectrographs, to measure it. These devices work by dividing starlight into all the colors of the rainbow. The resultant colourful decomposition of light -called a spectrum– is imprinted with strange dark lines which, combined, make up a signature conveying information about the building blocks of the star. This is because the dark lines are a result of the emitted light travelling through the atmosphere of the star which absorbs specific colours depending on its composition. Astronomers have been using this technique to learn about stars for centuries. Additionally, we can look at these dark spectral features to study how the star dances across the sky. The position of the lines, with respect to where we expect them to be if the star were not moving, allows us to measure the Doppler effect and therefore the radial velocity of the star. It is this effect that hints at the presence of exoplanets around stars.
Let’s picture a planet orbiting a star as a gravitational tango where one of the dancers, the planet, isinvisible. By analysing the movements of the visible dancer, we can reconstruct the choreography, the song and even the nature of the hidden companion. We, the planet hunters, search for the periodic changes in the stellar pulsations, fluctuating between red and blue, which can last anywhere from hours to years. These changes indicate perturbations in the stellar velocity, suggesting that there is a planet affecting the galactic dance of one of the stars which illuminates our night sky.
Changes in stellar radial velocity are not only useful to learn about the existence of exoplanets, but can also be used to determine the minimum mass of the planets. This is because the effect of the ‘wobble’ of the star is larger when the difference in mass of the star and the planet is higher. We can, therefore, use the the spectra of a star to understand if a planet is massive like Jupiter, or relatively light like the Earth. The problem with this method is that these changes in velocity are very small. Jupiter, for example, causes the Sun to wobble with a mere velocity of 13 m/s every 10 years, while the Earth does it with an almost insignificant 9 cm/s each year. Hence, we need instruments with extremely high levels of precision and stability if we want to be able detect the effect that exoplanets have on their stars.
We are now in the Telescopio Nazionale Galileo, which hosts one of the best exoplanet hunter instrument in the northern hemisphere: HARPS-N. This spectrograph is a copy of the original HARPS (High Accuracy Radial Velocity Planet Searcher) which is located in the Southern hemisphere, in Chile. Both of these instruments allow us to measure the stellar velocity with a mean precision of 1 m/s, which is approximately equivalent to the speed of a crawling baby. Our mission here is to follow-up exoplanets discovered by the Kepler space telescope, TESS’ predecessor. If we combine our radial velocity measurements with the transits observed by Kepler we are able to obtain the real planet mass (and not just a lower limit). This gives us a first approximation of what the planet is made of, and paves the the first step along the way of testing for habitability. Perhaps the next time we are here we will be measuring the mass of an exoplanet discovered by you via Planet Hunters…
As a matter of symmetry, the end of the night is announced on the horizon with the same colors that we saw at the beginning of the night, 9 hours ago. The vibrant colors mark the time to close to telescope before the Sun is back as a protagonist in the bright blue sky. We leave the telescope in the early hours of the day after having successfully measured the radial velocity of tens of potential planet-hosting stars. Each datum taken this night will help us to decode, step by step, a gravitational choreography, which will tell us about the existence of faraway worlds.