Michael Radica

University of Montréal

Michael is a doctoral candidate at the University of Montréal who is primarily interested in understanding the compositions of the atmospheres of exoplanets, and what we can then learn about the physical and chemical process that govern the formation and evolution of planets and atmospheres. He also spends time developing tools to analyze data from the most state-of-the-art instruments on telescopes like JWST, to which he always gives whimsical names. Primarily an observer, Michael uses observational techniques like transmission spectroscopy to learn about exoplanet atmospheres. When a planet passes in front of its host star as seen from our telescopes, a phenomenon called a planetary transit, atoms and molecules in the transiting planet’s atmosphere will absorb starlight at specific wavelengths — which differ from molecule to molecule. By observing a transit spectroscopically to measure the amount of starlight the planet blocks as a function of wavelength, we can create something called a transit spectrum and detect the fingerprints of molecules in the planet’s atmosphere.

Below is an example of a such a transit spectrum of the giant exoplanet WASP-96b, taken with the Single Object Slitless Spectroscopy (SOSS) mode of the Canadian NIRISS instrument on JWST, from Michael’s recent paper published in MNRAS. The top panel shows the transmission spectrum itself in grey points, with some atmosphere models over plotted. The middle panel shows residuals to the best fitting model, and the bottom panel breaks up the best fitting model to better visualize the molecules making up WASP-96b’s atmosphere. From the bottom panel, we can clearly see that the many broad bumps in the spectrum are all caused by absorption from water vapour.

This does not mean that WASP-96b’s atmosphere is only made of water, however. As a gaseous planet, the vast majority of WASP-96b’s atmosphere is H and He. Furthermore, it likely contains a number of carbon species like CO2 or CO — however the absorption signatures of these molecules are located at longer wavelengths than can be observed with NIRISS. Michael is thus the PI of a JWST Cycle 2 program to probe the atmosphere of WASP-96b at longer wavelengths using NIRSpec and detect these carbon species. The transit spectrum also shows that the atmosphere of WASP-96b has no clouds, which is puzzling since theory suggests it should be entirely cloudy!