PhD Defense by Beatriz Campos Estrada

From dusty-tails to cloudy skies: exploring exoplanetary environments through self-consistent modelling


There are now over 5600 known exoplanets, and their characterisation is of primary importance to better understand the still uncertain planet formation processes. However, exoplanet characterisation presents some challenges. We often encounter observational limitations or the existence of complex atmospheric processes which limit our ability to characterise exoplanets. In this thesis, we consider how dust condensation in distinct exoplanetary environments can help us reach a better understanding of the composition of exoplanet’s interiors, surfaces, and atmospheres.

In the first part of this thesis, we investigate the composition of small close-in exoplanets via modelling catastrophically evaporating rocky planets. A catastrophically evaporating planet is a low-mass (similar to Mercury), evaporating, ultra-short-period planet (orbital period of less than 1 day) with a comet-like tail of dust. Catastrophically evaporating planets offer a unique window into constraining the composition of small close-in planets. The dust in the comet-like tail originates from the planet’s molten day-side surface. The light curves of these planets are shaped by the optical properties of the dust. It is then possible to constrain the dust composition, and therefore the planet’s surface composition, via comparing synthetic light curves to the observed ones.

We present a new self-consistent model of the dusty-tails: we physically model the trajectory of the dust grains after they left the gaseous outflow including an on-the-fly calculation of the dust cloud’s optical depth. This is the first time the dust cloud’s optical depth is modelled self-consistently. We investigate two planets in detail, KIC1255 b and K2-22 b. The planet KIC 1255 b presents a trailing tail of dust, whilst K2-22 b presents a leading tail of dust. For both planets, we find the dust is likely composed of  magnesium-iron silicates (olivine and pyroxene), consistent with an Earth-like composition. We constrain the dust to be micron sized (1.25 - 1.75 _m) and the average planetary mass-loss rate to be approximately _3M_Gyr􀀀1. We conclude the origin of the leading tail of dust of K2-22 b is likely 9 10 a combination of the geometry of the outflow and a low radiation pressure  force to stellar gravitational force ratio. In addition to this, the optical depth of the dust cloud is a factor of a few at the vicinity of the planet. The composition constraint we find supports the recently suggested idea that the dusty outflows of these planets go through a greenhouse effect–nuclear winter cycle, which gives origin to the observed transit depth time variability. Magnesium-iron silicates have the necessary visible-to-infrared opacity ratio to give origin to this cycle in the high mass-loss state.

In the second part of the thesis, we explore microphysical cloud formation in substellar atmospheres of self-luminous objects. Clouds present a challenge to atmospheric characterisation as they can often hide spectral features of the gaseous components of atmospheres. However, we have entered an era with JWST where silicate clouds are detectable in the atmospheres of planetary-mass companions. This allows for a unique opportunity to test our understanding of cloud formation, and to help us characterise cloudy atmospheres. To date, no forward model has been able to reproduce the _10 _m silicate absorption feature detected in the emission spectra of these objects.

We compute a new grid of self-consistent 1D cloudy radiative-convective substellar atmospheres: the MSG cloudy grid. The grid accounts for both cloud microphysics and cloud radiative feedback (i.e., self-consistency). We use the MSG model which couples the atmospheric model MARCS, with the equilibrium chemistry model GGchem, and the microphysical cloud formation model DRIFT. To mitigate typical convergence problems, we apply a novel algorithm based on control theory. The grid spans the parameter space of brown dwarfs and directly imaged planets. We investigate the impact of cloud opacities on the resulting cloud radiative feedback on the atmosphere. We compute synthetic atmosphere spectra for each model to explore the observable impact of the cloud microphysics. The impact of choosing different nucleation species (TiO2 or SiO) and the effect of less efficient atmospheric mixing on these spectra are also explored. 

The new MSG cloudy grid, which utilises TiO2 nucleation, produces spectra that appear redder in the near-infrared compared to the known substellar atmospheres. We observe that models incorporating SiO nucleation and those with reduced mixing efficiency exhibit less redness in the near-infrared. Additionally, we find that detached convective zones emerge at effective temperatures of Te_ _ 1600K due to a backwarming effect caused by the clouds. Unfortunately, our grid fails to replicate the silicate features observed in recent JWST data and Spitzer archival observations. We discuss in detail further research that could more accurately represent the effects of convection in cloud-forming regions and propose steps to better capture the silicate cloud feature.

Join Zoom meeting
https://ucph-ku.zoom.us/j/67262381592?pwd=amVtLMlM8kR0aMJWSreF3sTMkcTKnS.1

Meeting ID: 672 6238 1592
Passcode: planet