Rocky planets that orbit their stars in less than a day are so scorched that their daysides are covered in magma oceans, and the gases boiling off those oceans form thin atmospheres—or, for the smallest planets, comet-like dusty tails that stream behind them as the atmosphere escapes into space. The problem is that current models cannot predict how dust grains condense in these atmospheres, breaking the link between what telescopes observe and what the planet’s interior is actually made of. Without this link, astronomers cannot use the wealth of new data from the James Webb Space Telescope—which is already scheduled to observe more than ten ultra-short period planets, including multiple transits of the dusty tail from K2-22b—to infer planetary composition. This project will build a new dust-formation model that predicts the size and composition of grains forming in these atmospheres, then couple it to simulations of atmospheric escape. If successful, it will allow researchers to read a planet’s interior composition from its escaping atmosphere, and to understand why some ultra-short period planets show unpredictable variability in their brightness. This is fundamental science. It will not change a single switch or pipe on Earth. But understanding how planets form and evolve—including how common low-mass rocky planets are—is the foundation on which any future search for habitable worlds rests.
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Ultra-short period rocky planets (USPs) offer a unique opportunity to study the poorly understood interiors of exoplanets. Traditionally, interior properties are inferred from mass-radius relations, which are plagued by degeneracies and weak constraints (Rogers & Seager 2010; Dorn et al. 2017). However, the intense radiation that USPs receive means the atmospheres of USPs consist of gases evaporated from the magma pools present on the planets’ permanent daysides (e.g. Schaefer & Fegley 2009; Kite et al. 2016). We may therefore probe the interior composition of USPs by studying their atmospheres, or, for very low mass USPs, the comet-like dusty tails produced as their atmospheres escape (Rappaport et al. 2012; Perez-Becker & Chiang 2013). Models are needed to make the crucial link between the observed atmospheres and the planets’ interiors. However, a key ingredient of these models – the condensation of dust grains – is either missing or oversimplified (e.g. Perez-Becker & Chiang 2013; Ito et al. 2015; Zilinskas et al. 2022; Booth et al. 2023; Piette et al. 2023). This breaks the connection between the observed composition and the planets’ interior properties. The problem is particularly critical for the lowest mass USPs, which are probed via the deep, easy to observe, transits of their dusty comet-like tails. Dust condensation should also not be ignored for massive USPs because dust condenses in the flows from their daysides to their nightsides (Castan & Menou 2011; Nguyen et al. 2020). New models are therefore needed to leverage the wealth of JWST data that is becoming available for these systems, with current programs scheduled to observe more than 10 USPs, including multiple transits of the dusty-tail escaping from K2-22b (GO programme 3315). We will solve this problem by developing a new dust formation model that can predict the size and composition of dust grains forming in USP atmospheres. We will couple the model to existing simulations of atmospheric escape from low-mass USPs, using the results to link the properties of dust seen in their comet-like tails to the planet’s interior composition. Next, we will characterize the mass-loss rates from low-mass USPs, needed to understand how common low-mass USPs are. Finally, we will address the cause of variability in USPs, which may be a general feature of USP atmospheres (van Werkhoven et al. 2014; Meier Valdés et al. 2023). Together, these advances will significantly increase our understanding of USP atmospheres, and consequentially their interiors.
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