Abstract

With over 4000 discoveries to date, the field of exoplanets is rapidly expanding. The large numbers of detections allow for population level statistical analyses. As a part of that, the characterisation of planets in their current form is vital to understanding the formation and evolution of exoplanets. Different occurrence rates for planets close to and far from their host stars indicate there may be different formation mechanisms at play. The study of planets at large separations from their host stars is therefore important. Direct imaging is particularly suited to study these planets when compared to other methods. Expanding it to the mid-infrared will allow for the characterisation of cooler and therefore older and smaller planets, but this has not yet been achieved. With the mid-infrared imaging and spectroscopy instrument VISIR and its upgrade NEAR data is obtained of three types of systems: six very young systems with planets that are still expected to be in their formation stage, the young, but fully formed HR 8799 system, and finally four mature systems in the solar neighbourhood. All but one are already known or expected to have planetary mass companions. None of these are detected, but the most stringent mid-infrared flux limits to date are obtained for all of them and for additional companions beyond 1'' in any of the systems. The conversion to mass limits rules out accreting planets with circumplanetary disks beyond 1'' around most of the very young stars, which indicates that planet formation at large radii is rare. The mass limits for companions around the sun-like epsilon Indi A show that it is at the older end of the age range, as a younger planet would have been detected. The achieved sensitivity in the observations also shows that detection of planetary mass companions is within the range of present day instrumentation and that at least four planets are detectable with VISIR and at least sixteen with NEAR in less than 10 hours. There is also a lack of super Earths on very short orbits. Many of these planets undergo high XUV irradiation, causing their atmospheres to puff up and erode. This is an important stage in the planets evolution. The expanded atmosphere also makes them easier to measure in transit and good targets for transmission spectroscopy, as it increases the signal size. The super Earth GJ 1214 b and the hot Jupiter WASP-15 b are such planets. An improved constraint is set on the evaporating helium atmosphere of GJ 1214 b through measurement of the excess transit depth at 10,830A in archival X-SHOOTER data. This also shows that X-SHOOTER has the required stability for these kinds of measurements on more suitable targets. For the hot Jupiter WASP-15 b Gaussian Process modeling is employed to remove correlated noise from the data and construct a transmission spectrum of the atmosphere, although further observations are required to rule out atmospheric models.