Unveiling the Nature of Faraway Worlds: Intensive characterisation of K2 transiting exoplanets

A new branch of astrophysics was born with the discovery of the first planet orbiting a star other than the Sun. Since then, astronomers have developed techniques to detect and characterise exoplanets. The two more successful techniques are the transit and radial velocity methods. They allow us to measure the planetary radius and minimum mass, respectively. By combining transit photometry with RV measurements, it is possible to measure the true mass and determine the planetary mean density. This allows us to study planets’ internal structure and composition and gives us important hints as to their formation and evolution.

In its extended K2 mission, Kepler is surveying different stellar fields located along the ecliptic, performing 80-day-long continuous observations of 10 000–20 000 stars per campaign. The K2 mission is an unique opportunity to gain knowledge of transiting exoplanets. K2 is targeting a number of bright stars (V < 13 mag) higher than the original Kepler mission. This is a definitive advantage for any RV follow-up observations. The opportunity for exoplanetary science is terrific! We present an intensive high-precision radial velocity follow-up of bright K2 stars (V < 13 mag) hosting transiting planets. We developed numerical tools, which allowed us to simultaneously model radial velocity measurements and transit light curves in order to derive planetary masses and radii.

We derived the masses, radii, and bulk densities of the exoplanets transiting the stars K2-19, K2-98, K2-139, K2-141, K2-111, HD 3167, GJ 9827 observed by K2, and π Mensae observed by TESS. We were able to infer the internal structure, composition, dynamical evolution, tidal interaction, architecture, and the existence/absence of atmospheres of the characterised exoplanets. Some of the planets here presented join the small group of short-period planets known to have rocky terrestrial compositions. The densities of the remaining objects are indicative of planets with a composition comprising a solid core surrounded by a thick atmospheric envelope. Because of the brightness of the host stars, most of the systems here presented are highly suitable for a wide range of further studies to characterise the planetary atmosphere and dynamical properties. This thesis contributes to the exoplanet science by itself. It adds new well-characterised planets to the relatively small sample of super-Earths and Neptunes whose masses and radii are both well-determined. These results will contribute to solve many unanswered questions about the nature of faraway worlds.