Dominic Samra
Over 4000 exoplanets are now known, most are unlike anything in our Solar system. One example is HAT-P-7b, an Ultra-hot exoplanet with a mass roughly that of Jupiter, orbiting so close to their host that they are 'tidally locked', so that one side always faces the star. We use data from a 3D Global Circulation Model for the temperature, pressure, and wind speed around the planet as input to our cloud formation model. We show that with a temperature difference of nearly 3000 K between the day-side and night-side, HAT-P-7b only forms mineral and metal clouds on the night-side of the planet. Super-rotational winds around the planet allow for some cloud formation at the morning terminator, deep in the atmosphere, but not at the evening terminator. Thus the atmosphere is more opaque at the morning terminator than at the evening terminator. The high temperatures also affect the gas phase on the day-side, with the gas being partially ionised and molecular hydrogen and water both being destroyed. At the evening terminator, the gasses Na, K and CO are less optically thick but TiO is much more optically thick than the morning side. Such asymmetry at the terminators should be observable with upcoming instruments such as the James Webb Space Telescope.
We use data from a 3D Global Circulation Model for the temperature, pressure, and wind speed around the planet as input to our cloud formation model [1]. We show temperatures around a slice through the planet's equator, for pressures from the micro-bar level down to 100 bar. Temperatures for 100 to 1 bar are fairly consistent around the planet, but above this they differ for the day-side and night-side. The day-side reaches 3600 K at the top of the atmosphere, whilst the coldest point on the night-side is only 600 K. As HAT-P-7b is tidally locked, it rotates in the same direction as it orbits, this causes winds to blow around the planet from west to east. Thus the coldest and hottest points in the atmosphere are actually east of where they would be expected. The hot temperatures on the day-side in the upper atmosphere, dissociate molecular hydrogen and water. The gas is also partially ionised in this region meaning electrostatic behaviours affect local atmospheric conditions [2].
From the cloud formation model we see that the day-side has (almost) no clouds at the equator. The only exception is at the ‘morning terminator’ where, deep in the atmosphere, cool gas has been blown over from the night-side by super-rotational winds. Using the ratio of cloud mass density to local gas mass density as a guide to where clouds are located, we find they extend from the top of the atmosphere down to nearly 1 bar for the whole night-side. These clouds are made mostly of silicates (e.g. Mg_2SiO_4) and metal oxides (Al_2O_3).
To observe the atmosphere of HAT-P-7b telescopes measure the star’s brightness as the planet passes in front of it, starlight shines through the planet's atmosphere and is absorbed down to an opaque level (optically thick limit) which is wavelength dependent. Thus the planet appears bigger or smaller depending on the wavelength of light observed. Looking at the opaque pressure levels for clouds and the most important gases at the two terminators (morning and evening), we find clouds only at the morning side, thus the atmosphere is more opaque here. At the evening side, the gasses Na, K and CO are less optically thick but TiO is much more optically thick than the morning side.
The dramatic temperature change between HAT-P-7b's day-/night-side affects where clouds form on the planet. The day-side atmosphere is also partially ionised because of the high temperature. Such differences also affect the terminators, and this asymmetry should be observable with upcoming instruments [1].
Acknowledgements
This poster contains work from a collaboration started at the Cloud Academy conference (2018). Contributing authors are: Alam, M. K.; Corrales, L.; Helling, Ch.; Herbort, O.; Iro, N.; Lew, B.; MacDonald, R. J.; Molaverdikhani, K.; Ohno, K.; Parmentier, V.; Steinrueck, M.; Woitke, P.; Worters, M.
References
[1] Helling, Ch.; Iro, N.; Corrales, L. et al. 2019, A&A, 631, A79
[2] Helling, Ch.; Worters, M.; Samra, D. et al. (in prep)