Using Laboratory Methods under Extreme Conditions to Better Understand Refractory Cloud Formation in Exoplanet Atmospheres

The high number of extrasolar planets found in recent years has not only challenged our knowledge of solar system sciences, but also our fundamental understanding of some of the processes that operate on Earth. These recently discovered planets show a large diversity in their masses, temperatures, orbital periods, and other properties. With such a diverse mix of planetary parameters, it is safe to assume that the atmospheric properties are just as varied. evidence suggests the presence of silicate and metal condensates in their atmospheres. This has led to new insights into the physics of cloud formation, and new observational data will help test and validate theoretical models. However, these models are fundamentally limited by the insufficiencies of laboratory data on the properties of atmospheric constituents; new laboratory data is desperately needed to advance state-of-the-art of exoplanet atmospheric models.

A laboratory verification of the condensation and vaporization predictions of refractory materials is critically needed in order to inform and improve atmospheric and spectral models. The stability of minerals identified in the literature as potential cloud candidates, are tested in a thermogravimetric balance. The minerals are pumped under vacuum for twenty-four hours at room temperature and then heated to a predetermined high temperature, dependent on the expected vaporization temperature of that sample. If there is apparent mass loss, then the temperature is lowered at preset durations and mass measurements are taken in similar measured increments. The data is processed by a computer program in order to calculate the mass loss as a function of temperature.

The direct output of these experiments is the actual measured vapor pressures of minerals at high temperatures which are used to predict the temperature regimes for stable cloud formation. These temperature regimes are then used as inputs to atmospheric models that predict cloud condensates. Presently, inputs for models of exoplanet atmospheres depend heavily upon extrapolations from terrestrial pressures and atmospheres. Such extrapolations are clearly of dubious value for understanding the atmospheres of exoplanets and for these tests to be conclusive, the theoretical models need to improve through the inclusion of input data on core atmospheric properties. To wit, there are gaps in our understanding of fundamental processes – gas opacities, optical properties of cloud particles, thermochemical reactions, and cloud and haze chemistry and properties (Fortney et al., 2016).

The field of planetary science has significantly changed our perspective of planetary atmospheres from an Earth-centric point of view to exotic worlds in the Solar System where temperatures are far different, and chemical compositions are found to be significantly different than our own. Exoplanets are now opening our eyes to even more complex and exotic atmospheric regimes. Understanding the cloud formation process requires a knowledge of the local gas temperature and pressure, but also the thermodynamics of the cloud constituent in order to model the location at which that cloud may form. This work will lead to significant improvements in the accuracy of exoplanet atmospheric models by eliminating the need to extrapolate data from terrestrial conditions.

Fortney, J.J., Robinson, T.D., Domagal-Goldman, S., Amundsen, D.S. alid, Brogi, M., Claire, M., Crisp, D., Hebrard, E., Imanaka, H., de Kok, R., and others, 2016, The need for laboratory work to aid in the understanding of exoplanetary atmospheres: arXiv preprint arXiv:1602.06305,.