My Thesis in 400 Words – Michael Radica

Michael Radica, a PhD student at IREx, recently completed his PhD degree at the Université de Montréal. Here, he summarizes his research project.

To understand our own Solar System’s place in the context of the wider population of planetary systems, it is first essential to understand the diversity of exoplanets themselves. In the past few years, low-resolution spectroscopy from space has become the tool of choice to probe exoplanet atmospheres and gain insights into the physics and chemistry that govern their formation and evolution. During my PhD, I have contributed to atmosphere studies made with the James Webb Space Telescope (JWST), spanning a wide variety of exoplanets. I also designed widely used tools to analyze observations from the Canadian instrument NIRISS’s SOSS mode — one of the key modes for JWST exoplanet studies.

My thesis contains three studies, starting with one of the first-ever exoplanet observations with JWST NIRISS/SOSS. These transit observations of the hot-Saturn WASP-96 b’s atmosphere show the signature of the water molecule and the potassium atom, in quantities roughly what is found in the Sun. We also observe a feature in the blue part of the spectrum which could indicate a phenomenon similar to what makes the Earth’s sky blue also happens in the atmosphere of this giant planet.

Next is a JWST transmission spectrum of the unique ultra-hot-Neptune LTT 9779 b obtained in the near-infrared with NIRISS in its SOSS mode. These observations take place in what astronomers call the “hot-Neptune desert” because there are very few known exoplanets of this size that are known to orbit close to their star. It is the only known hot-Neptune desert planet to have retained an atmosphere composed of the light gas hydrogen and helium, that are usually lost to space because of the intense radiation of the host star. Our observations allow us to conclude that the planet has a cloudy atmosphere at the transition between the day and night side. The composition of the atmosphere also seems to be rich in heavier elements. We posit that these clouds may be part of a feedback loop which decreases the efficiency of atmosphere loss, and aids LTT 9779 b in retaining its atmosphere.

These works, as well as the ∼20 others to which I have contributed throughout my PhD, demonstrate the unparalleled capabilities of JWST for the characterization of exoplanet atmospheres. Every new observation brings us one step closer to uncovering the origins of the diversity of the exoplanet population, as well as the fundamental differences and similarities between different “classes” of planets. As a result, the next decade will surely be one of the most transformative in the history of exoplanetary science.

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Michael completed his PhD degree between 2019 and 2024, under the supervision of IREx professor David Lafrenière. His thesis is available soon on Papyrus.