In 1995, the first exoplanet, a planet orbiting a star other than the Sun, was discovered. To date, the number of discovered exoplanets has reached over 5,000, revealing that planets are a ubiquitous presence beyond our solar system. By using telescopes to observe the atmospheric spectra of these planets, we can uncover the physical and chemical properties of their atmospheres, such as temperature structures, chemical compositions, and the presence of clouds. These details also serve as clues to understanding the internal structures, origins, and habitability of the planets.
The observable atmospheric properties mentioned above are determined through complex interactions between various physical, chemical, and evolutionary processes of the atmosphere (e.g., thermal balance, chemical reactions, thermal evolution), which depend on a multitude of parameters (e.g., temperature and dominant composition of the atmosphere, gravity, and the ultraviolet irradiation from the central star). Thus, to comprehensively understand the underlying atmospheric processes by unraveling their complex interactions, it is first necessary to systematically observe the physical and chemical properties of diverse atmospheres with different temperatures and dominant compositions. Additionally, constructing atmospheric models that can explain the observed characteristics across a broad parameter space is essential. Therefore, my focus extends beyond exoplanets to include brown dwarfs (intermediate-mass objects between planets and stars) and solar system planets, which occupy a complementary parameter space and are amenable to high-precision observations. By integrating exoplanets, brown dwarfs, and solar system planets, each of which has previously been researched almost independently, within the same framework, I aim to achieve a unified understanding of the atmospheric processes common to these planetary-mass bodies, encompassing their physical, chemical, and evolutionary aspects.