One of the most prominent and visual features of weather phenomena on Earth are water clouds which govern a large part of Earth’s climate. In Lee et al. (2016), the first of a series of papers, we investigated what the atmosphere of the hot Jupiter exoplanet HD 189733b may look like, in particular what the properties of clouds that maybe present in the atmosphere are. We used a computer simulation of the atmosphere of the hot Jupiter exoplanet HD 189733b including, for the first time, a microphysical description of 3D cloud particle behaviour in the atmosphere.
Simulating a cloudy exoplanet atmosphere.
A major tool for the exploration of the properties of Earth, Solar System and Exoplanet atmospheres are the use of Radiative-Hydrodynamic (RHD) or Global Circulation Models (GCM). These models simulate the temperature, pressure and other important properties of an atmosphere by evolving the equations of geophysical fluid dynamics, representing the large scale motions or “winds” of the atmosphere. The motions of the atmosphere depend on numerous factors such as the rotation rate, which affects the fluid flows primarily through the Coriolis force; pressure gradients, causing flows of gas. These models also take into account the heating from an external source onto the atmosphere, primarily radiation from the host star of the exoplanet. For hot Jupiter atmospheres, the intense effect of heating on dayside of the planet has large consequences on the specific weather properties, driven primarily from the large temperature differences between dayside and nightside regions of the planet.
In our study we used the RHD model from Dobbs-Dixon & Agol (2013) of the hot Jupiter HD 189733b and coupled it to our microphysical based cloud formation model, the details of which can be found in our numerous other blog posts (here and here). We found that our simulated cloud structures varied significantly in latitude, longitude and depth of the atmosphere. Cloud particles were more numerous of the nightside of the planet compared to the dayside regions, primarily due to the fast winds flowing from the dayside to the nightside slowing down as they approached the western terminator region. This creates a “traffic jam” effect, where cloud particles pile up on the western terminator nightside (Fig. 1).
Cloud particle sizes were also very different from the dayside to the nightside. On the dayside, the hotter temperatures evaporate the more volatile Magnesium containing materials MgSiO3 and Mg2SiO4, while these materials are stable on the nightside of the planet. This leads to different particles sizes, smaller particles of ~nm sizes on much of the dayside and ~μm sized particles on the nightside (Fig. 2).
Latitudinal differences in cloud particle size are also seen, with the hot equatorial super-sonic jet containing no Mg containing materials, and only the very stable TiO2 and SiO2 minerals. Higher latitudes, where it is cooler, contained ~90%+ Mg bearing materials. In our simulations we see bands of different materials dependent on the atmospheric properties, similar to how the bands of different materials on Jupiter colour different bands.
Overall, our results show that we can expect very cloudy, dynamic weather on HD 189733b, with cloud particles evaporating and condensing constantly as they are pushed along with the winds from dayside to nightside. However, unlike the temperate conditions on Earth, these cloud particles would be travelling at super-sonic speeds, dwarfing by magnitudes the most violent hail storms found on Earth.
In part II of our weather report, we will look at simulating the observable properties of our RHD modelling results and comparing our modelling efforts to Hubble Space Telescope and Spitzer Space Telescope data of HD 189733b.
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