A weather report for the hot Jupiter HD 189733b I.

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.

The number density of cloud particles at 1 mbar pressure in the HD 189733b simulation. A build up of material at the equator near 270 degrees longitude is seen. Arrows show the direction and strength of the atmospheric flow. The dynamics is dominated by the strong super sonic equatorial jet which efficiently transports cloud particles around the globe.

Figure 1. The number density of cloud particles at 1 mbar pressure in the HD 189733b simulation. A build up of material at the equator near 270 degrees longitude is seen. Arrows show the direction and strength of the atmospheric flow. The dynamics is dominated by the strong super sonic equatorial jet which efficiently transports cloud particles around the globe (Lee et al. 2016).

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).

The average cloud particle size in micron at 10 mbar pressure in the HD 189733b simulation. A large difference in cloud particle sizes between dayside and nightside as well as latitude is apparent.

Figure 2. The average cloud particle size in micron at 10 mbar pressure in the HD 189733b simulation. A large difference in cloud particle sizes between dayside and nightside as well as latitude is apparent (Lee et al. 2016).

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.

A collage of atmospheric properties at the equator of the simulated planet. The radial numbers denote the atmospheric pressure in log_10 bar. Numbers on the outside of the ring denote longitude. Top row: Gas temperature, cloud particle number density, cloud particle mean radius. Middle: TiO2 abundance, SiO abundance, SiO2 abundance. Bottom: MgSiO3 abundance, Mg2SiO4 abundance. Clear differences in the cloud properties can be seen in longitude and depth. For example, the dayside of the planet contains a large region of TiO2, while the nightside has more silicate materials present (Lee et al. 2016).

Figure 3. A collage of atmospheric properties at the equator of the simulated planet. The radial numbers denote the atmospheric pressure in log_10 bar. Numbers on the outside of the ring denote longitude. Top row: Gas temperature, cloud particle number density, cloud particle mean radius. Middle: TiO2 abundance, SiO abundance, SiO2 abundance. Bottom: MgSiO3 abundance, Mg2SiO4 abundance. Clear differences in the cloud properties can be seen in longitude and depth. For example, the dayside of the planet contains a large region of TiO2, while the nightside has more silicate materials present (Lee et al. 2016).

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.

For more details check out the original paper on ADS:

G. Lee, I. Dobbs-Dixon, Ch. Helling, K. Bognar, P. Woitke, 2016, A&A, 594, A48

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Mapping the sparkling clouds of the extrasolar planet HD 189733b

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.

Left: Illustration of the sample trajectories (black points) taken from the 3D radiative-hydrodynamic (RHD) model atmosphere of HD 189733b (Dobbs-Dixon & Agol 2013). Right: Input temperature and pressure profile for the cloud formation model at the equator of the 3D RHD model.

Left: Illustration of the sample trajectories (black points) taken from the 3D radiative-hydrodynamic (RHD) model atmosphere of HD 189733b (Dobbs-Dixon & Agol 2013).
Right: Input temperature and pressure profile for the cloud formation model at the equator of the 3D RHD model. The dayside profiles are φ= 0°, 45°, 315°, nightside profiles φ = 135°, 180°, 225° and day-night terminator regions φ = 90°, 270°.

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.

RGB scale and colour estimate of the cloud particles on the dayside face of the planet. Hubble Space Telescope observations found the planet to be a deep blue colour.

RGB scale and colour estimate of the cloud particles on the dayside face of the planet. Hubble Space Telescope observations found the planet to be a deep blue colour.

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:

Lee, G., Helling, Ch., Dobbs-Dixon, I., Juncher, D. 2015, A&A, 580, 12L

The LEAP Group can be found here:

http://leap2010.wp.st-andrews.ac.uk/

And finally, don’t forget to like us on Facebook:

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