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:

https://www.facebook.com/leap2010

Advertisements

Aurora Borealis – The play of colours over St Andrews

During the night of 27 February, 2014 a rare phenomenon took place on the sky of St Andrews. Around 10 pm the lucky ones saw the amazing red and green splendour of the Aurora.

IMGP3755

Red and green lights of the Aurora Borealis with the Castle of St Andrews in the front. (Credits go for Pasquale Galianni, astronomer of the University of St Andrews)

Red and green lights of the Aurora Borealis with the Castle of St Andrews in the front. (Credit: Pasquale Galianni)

Everyone heard of the bright, dancing, colourful lights of the Aurora Borealis. Some have seen it by their own eyes, others have seen breathtaking pictures of it. But what is the Aurora exactly?

The Northern Lights are natural light phenomena, which occur at high latitudes on both hemispheres of the Earth (called Aurora Borealis on the North and Aurora Australis on the South). It is the result of the collisions of charged particles coming from the Sun as solar wind and the upper part of a planet’s atmosphere.

In the upper corona of the Sun the velocity of the thermal motion of the particles become higher than the escape velocity. This results in a continuous material loss from the Sun in the form of solar wind. These charged particles (ions and electrons) hit the Earth’s magnetosphere and tie to it. The magnetosphere accelerates some of these particles towards the Earth’s surface. As they reach the upper atmosphere they collide with atoms and molecules releasing kinetic energy, which we is seen as the lights of the Aurorae. The more active the Sun (which means more solar wind) the more frequent the Aurorae.

The colour of the lights depends on the atom/molecule the energetic particle collides with. The most commonly seen type is the green Aurora. At mid altitudes (~ 100 km), where the concentration of oxygen atoms is fairly high, the collisions between atoms and ions/electrons releases energy at ~560 nm, which is in the green part of the spectrum. At the highest altitudes (up to ~300 km) the oxygen atoms emit around 630 nm (red part of the spectrum). Because of the lower concentration of the atoms in this part of the atmosphere, red Aurorae are seen very rarely and only when the Sun is around its activity maximum. The blue colour is the result of the collision with molecular nitrogen. This takes place at lower altitudes, where the amount of atomic oxygen is reduced.

Aurora Borealis seen from the Observatory of St Andrews. (Credit: Diana Juncher)

Aurora Borealis seen from the Observatory of St Andrews. (Credit: Diana Juncher)

The Aurora is not unique on Earth in the Solar System. Planets like Jupiter or Saturn, which have stronger magnetic fields than Earth exhibit even more spectacular light phenomena. Auroral light was observed on Uranus and Neptune as well.

The Northern Light we saw last Thursday was the result of a very energetic solar flare which was erupted on the 25 Feb (00:25 UTC). NASA’s Solar Dynamics Observatory (SDO) captured the gigantic flare in different wavelengths. The one seen below is a composite image of two wavelengths of extreme ultraviolet light (171 and 304 Angstroms). The flare is classified as X4.9 which means it is one of the most powerful types. As the Coronal Mass Ejection (CME) originated from this flare reached the Earth’s magnetosphere the beautiful dance of lights appeared on our sky.

Solar flare erupted on at 00:25 (UTC) 25 Feb as capured by the SDO. This image is the combination of two wavelengths of extreme ultraviolet light (171 and 304 Angstroms). (Credit: NASA/SDO)

Solar flare erupted on at 00:25 (UTC) 25 Feb as capured by the SDO. This image is the combination of two wavelengths of extreme ultraviolet light (171 and 304 Angstroms). (Credit: NASA/SDO)

And to finish with, here is a nice GIF made from some of Diana’s photos. Thanks to Inna Bozhinova for creating this short “movie” for us!

Aurora Borealis on the 27 Feb. (Credit: Diana Juncher, Inna Bozhinova)

Aurora Borealis on the 27 Feb. Click on the image to see it in a better quality. (Credit: Diana Juncher, Inna Bozhinova)

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

leap-2010.eu