One of the most exciting areas of astronomical research today is the study of exoplanetary systems, engaging the imagination not only of the astronomical community but also of the general public. Since the Nobel-winning discovery of a giant planet around the Sun-like star 51 Pegasi (Mayor & Queloz, 1995), about 4000 extra-solar planets have been detected, changing the view of planetary science and placing our solar system into a much broader context. Identifying habitable Earth-like exoplanets and searching for biomarkers in their atmospheres is among the main objectives of this century’s astronomy, motivating ambitious space missions and extremely large telescopes in the ground. Small, rocky planets orbiting M dwarfs are the only candidates whose atmospheric characterization is feasible with available technology. Besides, they will be the best, if not the only, candidates to detect life signatures with the next generation of ground- and space-based instruments.

This thesis focuses on the discovery and characterization of small planets around M dwarfs. M dwarfs constitute 70% of the stars in our Galaxy and given their small size and mass they offer several advantages for the detection and characterization of exoplanets using the two most successful and popular methods, namely the transit and radial velocity techniques. The main goal of this work is to tackle the so-called “M-dwarf opportunity”: to find and study the best planet candidates for atmospheric characterization using upcoming facilities based on the synergy between space-based transit searches around bright stars and ground-based high-resolution spectrographs.

In this context, seven new planets in four planetary systems are detected and characterized as part of this thesis. Chapter 2 describes the discovery of two super-Earth planets orbiting the mid-type M dwarfs GJ 3779 and GJ 1265, monitored with the CARMENES instrument as part of its radial velocity search for exoplanets around low-mass stars. The planets share very similar properties and, despite photometric searches with ground- and space-based telescopes, they do not transit their parent stars. The results are detailed in Luque et al. (2018). Chapter 3 presents the discovery of a three-planet system orbiting the mid-type M dwarf GJ 357. The innermost, transiting planet in the system was detected by the TESS satellite and confirmed with archival and new radial velocity observations from several instruments. It is a hot, Earth-like density planet optimal for atmospheric studies with the upcoming JWST and the closest transiting planet to the Sun orbiting a single M dwarf. The analysis of the radial velocity data revealed two additional super-Earths in longer orbits, being the outermost located within the habitable zone of the host star, i.e., in the orbital range where liquid water can be stable at the planetary surface. The results are detailed in Luque et al. (2019b). Chapter 4 reports the discovery of two transiting sub-Neptune planets orbiting the early-type M dwarf TOI-776. The precise mass and radius determination of the system using HARPS and TESS, respectively, shows that both planets must have retained a substantial atmosphere. Combined with the brightness of their host, they are straightforward targets for atmospheric characterization with the JWST, which will allow to precisely determine their internal composition and study the formation and evolution of the system. The results are detailed in Luque et al. (2021).

The number of precisely characterized small planets around M dwarfs has tripled during the course of this thesis. The new additions did not only allow to find suitable candidates for atmospheric studies, particularly with the JWST, but also to understand the composition, origin, and evolution of these planets in a demographic sense. The techniques developed and the results gathered in this thesis will contribute to a deeper understanding of the most frequent kind of planets in the Universe, the rocky worlds in orbit around red dwarfs, and of their potential to host habitable conditions on their surfaces.