The field of exoplanets has been rapidly growing since the discovery of the first exoplanet orbiting a main sequence star, 51 Pegasi. This planet and the early discoveries afterwards were mainly detected with the radial velocity method. A star that is orbited by a planet is gravitationally affected by it and hence moves around its centre of mass at the orbital period of the planet. This change in its radial velocity can be measured through the Doppler shifts of the star’s spectra. The periodicity of this signal corresponds to the orbital period of the planet, i.e. the time it takes the planet to orbit the star once. From the strength of the signal the mass in terms of the orbital inclination can be derived.

While this method contributed to finding the first exoplanets, the majority of planets to date have been found by the transit method, which measures the periodic dimming of the measured flux of a star as a planet passes in front of it. Through this method, more than 4000 planets have been discovered. Among these discoveries were new planet types, specifically Super-Earths and Sub-Neptunes. These planets are larger than our own Earth but smaller than Neptune. Studies have shown that although these planets do not exist in our own Solar System they are the most common planet types in the Milky Way.

As the transit method requires the planet to pass in front of the star, the orbital inclination can be obtained. Hence, in combination with the radial velocity method, we can determine the mass of the planet. Combining the mass and the radius, which is obtained based on the decrease in flux during the transit, we can obtain the planet’s bulk density. This allows us to get insights into the structure of the planet. For small planets, such as Super-Earths and Sub-Neptunes, we use models that assume the planet is made of multiple components, the iron core, silicate mantle, water and atmosphere. To model a planet’s interior well, the radius and mass need to be measured precisely. The radius is usually obtained from space-based telescopes, such as Kepler in the past, TESS and CHEOPS in the present and PLATO in the future. The mass is obtained by spectrographs such as HARPS-N on the TNG and the upcoming HARPS3 spectrograph mounted on the Isaac Newton Telescope (INT).

As planets and stars form from the same material, we expect to find compositional trends between the two, i.e. stars that are poor in metal are expected to be orbited by planets with less metal and hence a smaller core. This would result in a planet of a low bulk density. Exploring these trends is crucial to improve our understanding of planet formation and evolution. However, characterising small planets to a high precision in mass requires a lot of telescope time and hence not many systems have been studied in detail. Furthermore, there is a lack of planets orbiting metal-poor stars. This makes it challenging to claim a trend as the majority of planet-hosting stars are similar to the Sun in composition. Hence, to test whether these trends and with this formation and evolution theory hold in different formation conditions, detecting and characterising these planets is crucial.

Within my PhD, I target these systems, the ones I characterised are highlighted by the star symbols in the accompanying figure. While we can find multiple planets transiting metal-poor stars, characterising their masses precisely using radial velocities remains challenging as most of these targets are not only faint, but characterising small planets well requires many radial velocity observations. During my ING studentship, I was able to propose for time with the HARPS3 spectrograph on the INT and was awarded observation time through the UK and Dutch Time Allocation Committees.

With these observations, I aim to expand the sample of well-characterised planets around metal-poor stars. My proposal focuses on one Sub-Neptune that orbits a metal-poor star, in parameter space that has not been explored to date. Furthermore, this planet has only two measured transits and hence its orbital period is unknown and likely longer than 30 days which puts this system in a sparsely explored period space. The follow-up of the system with HARPS3 will hence allow us to do both: determine the planet’s orbital period and determine its mass. With its mass determined, we will obtain its bulk density which allows us to other systems of different metallicities.