Ariel is an ESA space telescope planned for launch in 2029. It is aimed at observing ∼1,000 exoplanets using transit spectroscopy at infrared wavelengths between 0.5-7.8μm, to characterise the chemical composition and thermal structure of their atmospheres. To detect minute flux variations and confidently extract the exoplanet signal, Ariel requires good instrumental flux stability. Moreover, the instrumental response must be monitored through time, and instrumental effects and stellar noise must be estimated and minimised to get as close as possible to the photon noise limit. To do this, Ariel must be calibrated in-flight by observing a set of stellar calibrators, stars whose spectral energy distribution is assumed to be well-studied and constant. This project is aimed at building the stellar calibrator catalogue for Ariel and looking for possible correlations between stellar properties and flux stability. This can also serve as a base study for future space missions other than Ariel. A starting candidate sample mainly including G dwarfs that were observed by TESS is filtered with a three-step procedure using the Lomb-Scargle periodogram and the reduced chi-squared statistic. Assuming an Ariel 3σ flux stability requirement of 100ppm over 6 hours, the defined Ariel Catalogue comprises 581 stars. These are fairly homogeneously distributed over the sky, without periodic variability, and without a large flux excess dispersion. It is found that when selecting the correct effective temperature ranges (5, 000 < Teff < 6, 300K), the probability of finding stable stars is almost constant at 30%. Only the dimmer stars seem to have a higher probability of passing the selection, which is possibly due to the higher noise in the data which makes variability more difficult to identify. Note, however, that the catalogue defined in this study still comprises calibrator candidates. The noise in TESS light curves is probably too high to effectively analyse the stability within the current requirement. So, follow-up observations of these stars with a lower noise are recommended, for instance with PLATO, to greenlight them as final Ariel calibrators.

Planet formation is a topic that still has many unanswered questions, particularly regarding the formation of wide orbit giant planets. Detecting more of these types of planets can aid understanding of how they form by giving examples of what kind of planets exist. Direct imaging is uniquely well suited to detecting these kinds of planets, which is why several direct imaging surveys have been launched with this goal. Including the Young Suns Exoplanet Survey, which is looking specifically for wide orbit giant planets around young solar analogues. The survey makes use of the IRDIS instrument on the VLT, which images in parallel with the IFS. By analyzing the IRDIS data, the researchers have already detected three planets in two systems. However, the IFS data has not yet been analyzed. That was the goal for this thesis: to contribute to YSES and the broader scientific community by analyzing the IFS data to search for potential companions. In total, 41 observations of 37 different star systems were analyzed. The data was pre- and post-processed (with SDI), and candidate companions were identified and examined. Eight candidate companions were found in five systems. Of those candidates, all were determined to be background stars and not related to the host. Two were bright single stars. One system had two M-dwarf candidate companions that were in a binary system together. The last two systems each had two candidate companions, and in both cases it was revealed that these candidates form a triple system of their own, with one of the candidates being an unresolved binary. One of these triple systems was previously thought to be an equal mass binary with the host star, however this thesis has proved that this is not the case. All of the systems analyzed also had detection limits computed. This gives a good idea for what size of objects could have been seen if they had been in the images. For most systems, the limit after SDI is around 5-10 Jupiter masses at 0.3"-0.5". This leaves room for planetary mass companions in these systems which could be found in follow-up observations. The two background triple systems should also have follow-up observations performed, as this configuration is somewhat unusual and warrants further study.