The Mysteries of Small Exoplanets: Planetary Systems Investigation via Transit Photometry and Doppler Spectroscopy

The transit and radial velocity methods are two of the most successful techniques for detecting exoplanets. The synergy of both methods gives us access to the orbital geometry and allows us to measure the planetary parameters such as the radius and the mass, leading to the bulk density. The Kepler space mission - NASA’s first space-based survey of transiting planets - has discovered that planets smaller than Neptune are extremely common in our galaxy. Unfortunately, we still know little about their nature, especially in terms of mass and density, which represent the key parameters for studying the planets’ composition and internal structures. Precise mass determinations through ground-based Doppler spectroscopy have only been possible for a few Kepler planets, mainly due to the faintness of the stars observed by the Kepler telescope. Launched in 2018, NASA’s TESS space mission is observing the light curves of thousands of stars searching for shallow transit signals. Targeting mainly bright stars, TESS is a definite advantage for any radial velocity follow-up program. In this work, I describe the detection of exoplanets in photometric and spectroscopic measurements, and how to combine the two datasets to determine the fundamental planetary parameters. In addition, I present the results of high-precision RV follow-up observations carried out with the HARPS and CARMENES spectrographs of bright TESS stars (V≲11) hosting transiting planets. The results of the discovery and characterization of planetary systems presented in this work have been and will be published in peer-reviewed journals. Within this work 6 systems have been investigated providing unprecedented mass and radius determination. This allows us to investigate the internal composition of planets, as well as the formation and evolution history of planetary systems. In particular, the planet GJ 367 b challenges planet formation theories having a ultra high density of 85% that of the Earth. With these well-characterized systems, this thesis provides a benchmark sample of small exoplanets with high-precision planetary parameters and two promising targets for future atmospheric investigations.