My Thesis in 400 Words: Stefan Pelletier

Stefan Pelletier, a doctoral student at iREx, completed his PhD at the Université de Montréal this fall. Here he summarizes his doctoral research project.

Illustration of the exoplanet WASP-76b, a “hot Jupiter” in close proximity to its host star. Intense heat is vaporizing rock-forming elements like magnesium, calcium, and iron in the planet’s atmosphere, offering valuable insights into the formation of our Solar System. Credit: International Gemini Observatory/NOIRLab/NSF/AURA/J. da Silva/Spaceengine/M. Zamani

The present-day atmospheric composition of a planet can help us understand its formation and evolution history. Indeed, for decades scientists have been studying Jupiter and Saturn to gain insights into how they formed. Today, we have the capability of studying the composition of distant worlds and place the Solar System planets in a wider context for the first time in history. Throughout my PhD, I had the opportunity to use different telescopes to study giant gas planets outside of our Solar System with the goal of measuring the composition of their atmospheres and using this knowledge to better inform us about how planets are created.

As part of my thesis, I demonstrated the capabilities of the SPIRou instrument on the Canada-France-Hawaii Telescope for characterizing the atmospheres of exoplanets. By observing the infrared light emitted by the hot giant exoplanet τ Boo b, I found that it has an atmosphere that is enriched by about the same amount of carbon as Jupiter, but is otherwise much drier than expected. This likely indicates that τ Boo b formed in a water-poor region much further away from its star than where it is today.

iREx PhD student Stefan Pelletier speaks at a meeting for the astronomical community. Credit: Stefan Pelletier

In another project, I used the MAROON-X instrument on the Gemini-North telescope in Hawaii to observe the ultra-hot giant exoplanet WASP-76b as it passed in front of its host star. From this, I was able to find many elements in its atmosphere and measure their quantities. I found that WASP-76b has a composition that is almost identical to the star around which it orbits for many, but not all elements. From these findings I was able to learn about the chemistry and composition of clouds on this ultra-hot giant exoplanet.

Finally, I also observed the ultra-hot planet WASP-121b using both the CRIRES⁺ and ESPRESSO instruments mounted on the European Very Large Telescope in Chile. From analyzing the combined visible and infrared light captured by these observations, I was able to find both ice-forming and rock-forming elements. By comparing these, I was able to determine that the atmosphere of WASP-121b is more enriched in ices than rocks. Crucially, these are constraints that are presently impossible to obtain on Jupiter and Saturn, and thus provide a rare instance where studying an exoplanet hundreds of light years away from Earth allows us to gain insight about how the giant planets in our own Solar System may have formed.

More information

Stefan completed his PhD at the Université de Montréal between 20XX and 2023, under the supervision of Professor Björn Benneke of iREx. His thesis will be available soon.