One of the key discoveries in exoplanet research over the past decade is the abundance of small planets in our Milky Way. Despite their high numbers, our understanding of their atmospheres remains... Show moreOne of the key discoveries in exoplanet research over the past decade is the abundance of small planets in our Milky Way. Despite their high numbers, our understanding of their atmospheres remains limited, and it is unknown if they possess atmospheres at all. Predicting the presence of an atmosphere on small planets is challenging due to factors like atmospheric escape and volcanism. Reliable determination requires direct study of thermal emission, reflected light, or transmission spectrum. With the launch of the JWST in late 2021, we gained unprecedented access to detailed observations of rocky exoplanets, enabling the search for atmospheres composed of carbon dioxide, oxygen, and nitrogen on temperate rocky worlds. My thesis summarizes my work on atmospheric characterization of small, rocky exoplanets using space-based telescopes such as Spitzer, Hubble, and JWST. I have studied a wide temperature range, from lava worlds with atmospheres of outgassed rock vapor at over 2000 Kelvin, to terrestrial planets with temperatures around 400 Kelvin, similar to our inner solar system. I characterized the surfaces and atmospheres of exoplanets like K2-141 b and TRAPPIST-1 c to ultimately learn about their surfaces and the conditions under which rocky planets can retain atmospheres. Show less
More than 5,000 exoplanets have been found over the past couple of decades. These exoplanets show a tremendous diversity, ranging from scorching hot Jupiters, common super-Earths, to widely... Show moreMore than 5,000 exoplanets have been found over the past couple of decades. These exoplanets show a tremendous diversity, ranging from scorching hot Jupiters, common super-Earths, to widely separated super-Jupiters on the planet/brown dwarf boundary. We have now moved into the era of exoplanet atmospheric characterisation. Two crucial techniques for characterizing these exoplanets from the ground are high-contrast imaging and high-resolution spectroscopy. In the first part of the thesis, existing facilities are used to characterize the atmospheres of two of the most accessible types of planets: An ultra-hot Jupiter (WASP-76b) and a young supter-Jupiter (beta Pictoris b). The second part of the thesis develops instrumental concepts that are required to push exoplanet characterization towards smaller and closer-in planets. It shows how we can choose between spectral resolution, bandwidth, and field-of-view in developing an instrument for exoplanet detection, and how one can design an nearly optimal wavefront sensor for adaptive optics. Finally, it is demonstrated how machine learning techniques can help us improve the performance of these adaptive optics systems such that we can reach deeper contrasts for exoplanet imaging and characterization. Show less
With the rapidly growing number of extrasolar planets detected, we have firmly stepped into the era of detailed characterization. Diverse types of exoplanets such as gas giants on close-in orbits ... Show moreWith the rapidly growing number of extrasolar planets detected, we have firmly stepped into the era of detailed characterization. Diverse types of exoplanets such as gas giants on close-in orbits (hot Jupiters) and young massive giants on wide orbits (super Jupiters), with no analogs in the Solar System, pose challenges but also opportunities to our understanding of planet formation and evolution. Exoplanet atmospheres with imprints from their history open an important avenue to retrace the origin and evolution of planets. With high-dispersion spectroscopy, we can resolve atomic and molecular spectral features into unique forests of lines that serve as the fingerprints for identifying different species in planetary atmospheres. In this dissertation, I utilize this technique to explore atmospheric compositions, thermal structures, and dynamics of exoplanet atmospheres. I have discovered minor isotopologues in emission spectra of an exoplanet and a brown dwarf for the first time, pioneering the use of carbon isotopic ratios as potential tracers of planet formation. I have investigated the trend of atomic absorption strengths in a sample of ultra-hot Jupiters, which enables disentangling different dynamic regimes of highly-irradiated exoplanets. These works form the foundation to link spectroscopic observations to planet formation and evolution processes. Show less
Over the last three decades, the discovery of exoplanets has revealed the boundless variety of worlds beyond our own Solar System. Majority of planetary systems contain short-period planets that... Show moreOver the last three decades, the discovery of exoplanets has revealed the boundless variety of worlds beyond our own Solar System. Majority of planetary systems contain short-period planets that are larger than Earth but smaller than Neptune. For rocky planets, the strong irradiation causes the surface to melt, forming dayside oceans of molten silicates. These are known as lava worlds. From a theoretical standpoint, lava worlds are expected to outgas silicate-rich atmospheres, which can be characterised using spectroscopy techniques. Spectroscopy allows astronomers to single out a multitude of chemical species in exoplanets, and with the James Webb Space Telescope (JWST), it is now possible to characterise even rocky planets.To reinforce our understanding of distant worlds it is critical that we can reproduce the observed results using computational models. A variety approaches exist, however due to their flexibility and adaptability, using averaged 1-D models is prefered. The work in this thesis heavily focuses on using 1-D chemistry and radiative-transfer codes to simulate atmospheres of super-Earths and sub-Neptunes, including volatile and silicate-rich compositions. The main goal is to guide observers to potentially detectable species that would help us gain insight into many of the drawn assumptions. The research done indicates a multitude of detectable species such as HCN, CN, CO, SiO, and SiO2. Models also show that silicate atmospheres are plagued with deep temperature inversions, strongly affecting observability. Most of the presented results are especially applicable to low-resolution infrared spectroscopy for observations with JWST. Show less
Sreejith, A.G.; France, K.; Fossati, L.; Koskinen, T.T.; Egan, A.; Cauley, P.W.; ... ; De Almeida Vidotto, A. 2023
Understanding the formation and evolution of planetary systems is one of the most fundamental challenges in astronomy. To directly image and study young exoplanets and the circumstellar disks they... Show moreUnderstanding the formation and evolution of planetary systems is one of the most fundamental challenges in astronomy. To directly image and study young exoplanets and the circumstellar disks they form from, dedicated high-contrast imaging instruments are built. Several of these instruments have polarimetric modes that are particularly powerful to reach the large contrasts required to directly image these objects as well as to characterize them. This thesis aims to improve the polarimetric sensitivity, accuracy, and capabilities of high-contrast imaging polarimeters for the detection and characterization of exoplanets and circumstellar disks. In addition, this thesis presents the first direct detections of linear polarization from self-luminous planetary mass companions. The focus of this thesis is mostly on ground-based high-contrast imaging, in particular with the instrument SPHERE-IRDIS at the Very Large Telescope. This thesis covers many aspects of high-contrast imaging polarimetry, ranging from theoretical work, calibrations, and the development of new observing techniques to actual scientific polarimetric measurements and astrophysical interpretation. Show less