On the nature of extremely low-density exoplanets: Haze or Ring?

speaker: Kazumasa Ohno

Observational efforts in the last decade have discovered more than 5000 exoplanets. The most enigmatic exoplanet within the current sample may be extremely low-density (<0.1 g/cm^3) exoplanets called super-puffs, which are Jupiter-sized planets with masses lower than Neptune. Planetary interior structure model indicated that the extremely low bulk density could be explained by the presence of massive atmosphere, whereas it remains mystery why such low-mass planets could acquire massive atmospheres and why they could retain the atmospheres until today. Recent studies have suggested is that the observed radii of super-puffs are rather overestimated somehow, such as due to atmospheric dust or presence of circumplanetary ring. Here, we discuss the feasibility and observational implication for each scenario. We first use a microphysical model of grain growth in exoplanetary atmospheres to study how the atmospheric dust affects the transit observation of super-puffs. We found that the intense atmospheric escape suggested for super-puffs can bring the dust to upper atmosphere to enhance the observable radius if the dust is formed at relatively upper atmospheres, indicating that photochemically produced particles may be responsible for the anomalously large radius of some of super-puffs. Next, for the circumplanetary ring hypothesis, we have developed a method to include the ring in the calculations of atmospheric transmission spectra for arbitrary viewing orientation. We showed that the presence of ring tends to explain both anomalously large radius and flat atmospheric spectrum, in agreement with the current observation of super-puffs. I will discuss the condition with which each process can work to explain the anomalously large observed radius, and how we can distinguish the two hypothesizes from future observations by JWST.


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