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Today, nearly 6,000 extrasolar planets have been confirmed to exist in orbit around stars other than our Sun. These so-called exoplanets are objects of great scientific interest as the study of their differences and similarities to the planets of our Solar System can underline several unanswered questions regarding planet formation, astrochemistry, and the evolution of life. As these exoplanets transit in front of their host stars, astronomers may take advantage of stellar light transmitted through the exoplanetary atmosphere and can use spectroscopy to study the chemical composition of the upper layers. 

Our exoplanet observations are taken as a series of exposures to capture the planetary movement across its transit. However, deciding the exact cadence of exposures is not immediately clear as there are advantages and disadvantages to both longer and shorter exposure times. In the first paper of this thesis, we use simulated observations to investigate how to optimise our observing strategy in order to maximise detection significance, and we present recommendations based on our findings.

Once observations have been successfully taken, the data must be reduced before any interpretations can be made. In the second paper of this thesis, we analyse ground-based data of a transiting Neptune-like exoplanet that is particularly challenging to study due to its relatively cool and cloudy atmosphere. Considering that the majority of previous ground-based transmission spectroscopy studies have focused on hotter exoplanets with clearer atmospheres, we discuss how our methods fare when applied to this less-explored regime of exoplanet type. 

Studying exoplanets of increasingly lower masses, smaller radii, and cooler temperatures is crucial as we work towards eventually characterising Earth-like exoplanets – and thereupon, assess habitability and potential for astrobiology on other worlds. In the third paper of this thesis, we use simulated observations to model how different hypothetical climate scenarios might appear on an Earth-like exoplanet from both the ground and space. We discuss whether our current methodology is capable of distinguishing different scenarios from one another, which would be necessary to meaningfully comment on the exoplanet's potential to host life, and we discuss this challenge in the broader context of transmission spectroscopy.