Anne Boucher, an iREx student at the Université de Montréal, submitted her doctoral thesis at the end of 2021. She summarises the research project she carried out as part of her Ph.D here.
During my Ph.D., I was interested in gas giant exoplanets that are very close to their star. These planets are called hot Jupiters or hot sub-Saturns. To study their atmosphere, I used the method of transmission spectroscopy, which works as follows: when the planet transits (or passes in front of its star), a part of the star’s light passes through the planet’s atmosphere and leaves its imprint, which we can capture by taking the star’s spectrum with our telescopes. This allows us to know more about the gasses that are present in the atmosphere of these planets, and gives us a lot of information about how they formed and evolved. I mainly used data from SPIRou, an instrument installed at the Canada-France-Hawai’i Telescope that takes very detailed, high-resolution spectra.
To write and test the computer codes that would allow me to analyse the data and extract the relevant information, I first observed one of the most studied exoplanets, the hot Jupiter HD 189733 b. On this planet, we detected water vapor and saw that it probably has strong winds and few clouds. Since these results are consistent with what others have found before us, it confirmed that our method works.
Next, I studied WASP-127 b, a very puffy exoplanet (less massive, but much larger in size than Saturn) that orbits a very old star. Recently, another team of astronomers studied this planet with the Hubble Space Telescope (HST) and the Spitzer Space Telescope, but they were unable to determine the amount of carbon monoxide (CO) this planet had, which limited their ability to determine how it formed. With the SPIRou data, we have not only confirmed the presence of water in the planet’s atmosphere, but also learned that there is very little CO. This is rather surprising, because, if this planet had formed as expected and the gasses in its atmosphere were in equilibrium, it would have a lot of CO. The only way to explain these results is to use more complex formation scenarios that more realistically account for how the environment within which planets form varies over time.
Thanks to this work, we have developed our expertise with high resolution near-infrared transit spectroscopy at the Université de Montréal, notably with SPIRou, allowing us to explore the atmospheres of hot Jupiters and sub-Saturns. The study of WASP-127b, the first to combine high and low resolution transit data, allowed us to obtain much more precise constraints on atmospheric parameters such as chemical composition and cloud altitude. This method is proving to be a very powerful tool for studying exoplanet atmospheres and will be even more so with the revolutionary capabilities of the James Webb Space Telescope.