How do clouds look like on alien worlds? Graham Lee and Christiane Helling in collaboration with Ian Dobbs-Dixon (New York University Abu Dhabi) and Diana Juncher (University Copenhagen) took the first steps in modelling the formation of clouds for the giant gas planet HD 189733b, a member of a class of exoplanets commonly called ‘hot Jupiters’.
Observations (e.g. Pont et al. 2013; Sing et al. 2015, ) suggest that many hot Jupiters contain a large dust cloud component in their atmosphere because they obscure the absorption signatures of the atmospheric gas underneath the cloud layers. These clouds are made of mineral compounds such as TiO2[s], MgSiO3[s], SiO[s], Al2O3[s], Fe[s] (‘s’ meaning solid particles) (see previous blog post), and not of water like on Earth.
Inspired by previous research which proved clouds exist in brown dwarf atmosphere (see our post on DRIFT-PHOENIX Atmosphere Models) we set out to investigate if the same family of clouds could reside in hot Jupiter atmospheres.
We applied our cloud formation model to a 3D radiative-hydrodynamic simulation (RHD) of HD 189733b (Dobbs-Dixon & Agol 2013), to prove that the temperature and pressure conditions on these planetary atmospheres are suitable for cloud formation. We took temperature, density and pressure data in 1D “slices” of the 3D simulation as input for our cloud formation model (Figure 1). This is like using an atmospheric probe to sample the local conditions of the atmosphere during descent. The combination of a sophisticated 3D RHD atmospheric model and our 1D cloud formation model allowed us to create cloud “maps” of the HD 189733b atmosphere.
Our results show how cloud properties change between different regions of the planet. First we noticed that the size of cloud particles changes with the location on the globe. Grains found on the dayside generally grow faster and larger than those on the nightside. However, because of the lower temperatures on the nightside, more grains form on the nightside. This leads to an cloud structure where numerous small grains reside on the nightside while larger (but less abundant) grains reside on the dayside face of the planet.
We converted the cloud properties across the globe into a map of global cloud properties: Figure 2 depicts the mean particle size at an atmospheric pressure of 10-2 bar across the globe of HD 189733b, where the difference between nightside and dayside is most apparent.
With our simulations, we show that the maximum reflectivity of mineral clouds correspond to the 8 micron Spitzer global flux maximum observed by Knutson et al. (2007). Our results therefore suggest that clouds can significantly contribute to the infrared flux from these planets by scattering photons back into space.
We further found that the clouds on the hot Jupiter HD 189733b reflected more efficiently in blue than red spectral range. This suggests that the clouds on this planet will appear midnight blue in colour if viewed with human eyes. Figure 3 shows an RGB colour estimation for HD 189733b clouds by interpolating the light scattering result. Evans et al. (2013) presented observations with the Hubble Space Telescope suggesting a bluish appearance of HD 189733b which our work now supports on the basis of detailed cloud formation modelling.
Could these clouds sparkle? Mineral cloud particles are thought to form crystalline structures as they travel through the atmosphere (Helling & Ritmeijer 2009). This means that the mineral particles that form the clouds on HD 189733b are likely to “sparkle” similar to gemstones on Earth, such as sapphire.
For more details check out the original paper on ADS:
The LEAP Group can be found here:
And finally, don’t forget to like us on Facebook: