My PhD thesis in 400 words: Dylan Keating

Dylan Keating, an iREx student at McGill University, submitted his PhD thesis in the Fall of 2021. He summarizes here the research projects he conducted as part of his PhD degree.

Short-period gas giants, more commonly known as hot Jupiters, are the most well-studied class of exoplanets to date. They are tidally locked into synchronous rotation around their host stars, with permanent daysides that get blasted with stellar irradiation, and permanent nightsides that face the darkness of space. Although they are too far away to observe with the same level of detail as our solar system’s Jupiter, hot Jupiters have the advantage of sheer numbers. Around 100 hot Jupiters have had their atmospheres characterized using infrared observations with the Spitzer and Hubble space telescopes.

Hot Jupiters were predicted and observed to transport heat from day to night via atmospheric winds or waves. The hottest part of the planets atmosphere is thus shifted east of the substellar point. Because of their short orbits (around a day or so), we can observe an entire orbit of a hot Jupiter around its host star. Full-orbit infrared phase curves give us the dayside and nightside temperatures, and in between we see different faces of the planet revealed, which gives us information about how a planet’s atmosphere transports heat. In my thesis work I looked for trends in dayside and nightside temperatures on hot Jupiters, using phase curve observations.

Schematic of clouds on the night side of a hot Jupiter. The underlying atmosphere is hot enough to vaporize rocks, which can condense into clouds. Credit: McGill University.

What I found was surprising. The prevailing hypothesis about heat transport on hot Jupiters was that the amount of stellar irradiation each planet receives should be the main factor controlling heat transport. The hottest part of two planets atmosphere should thus be shifted by the same amount if they receive a similar amount of stellar irradiation. However, in a study where I compared three planets with the same amount of irradiation but different rotation rates and surface gravity strength, the three planets all had different phase curve shifts. This could be evidence of thick dayside clouds that obscure the atmospheric dynamics, or magnetic field effects, both of which were not incorporated in early hot Jupiter models.

Planetary dayside temperatures follow a simple trend: planets that receive more stellar irradiation have hotter dayside temperatures. Nightside temperatures were not predicted to follow any simple trend with irradiation, because there are several different possible processes at play in transporting heat from day to night and their interaction is complex. Despite this, in a study published in Nature Astronomy, we found that although planetary dayside temperatures vary by thousands of degrees from planet to planet, the nightside temperatures of the dozen hot Jupiters we studied were clustered around 1000K, regardless of how much irradiation they receives The most likely explanation is that hot Jupiters all have thick nightside clouds, likely made of silicate or minerals. The upcoming launch of Webb telescope will help verify these hypotheses as well as continue the work of comparative exoplanetology that began in the Spitzer era.

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Dylan worked on his PhD degree at McGill University between 2018 and 2021, under the supervision of Nicolas Cowan from McGill University and René Doyon from Université de Montréal. His thesis will soon be available.