Abstract
Atmospheres regulate the planetary heat loss and therefore influence planetary thermal evolution. Uncertainty in a giant planet’s thermal state contributes to the uncertainty in the inferred abundance of heavy elements it contains. Within an analytic atmosphere model, we here investigate the influence that different cloud opacities and cloud depths can have on the metallicity of irradiated extrasolar gas giants, which is inferred from interior models. In this work, the link between inferred metallicity and assumed cloud properties is the thermal profile of atmosphere and interior. Therefore, we perform coupled atmosphere, interior, and evolution calculations. The atmosphere model includes clouds in a much simplified manner; it includes long-wave absorption but neglects shortwave scattering. Within that model, we show that optically thick, high clouds have negligible influence, whereas deep-seated, optically very thick clouds can lead to warmer deep tropospheres and therefore higher bulk heavy element mass estimates. For the young hot Jupiter WASP-10b, we find a possible enhancement in inferred metallicity of up to 10% due to possible silicate clouds at ∼0.3 bar. For WASP-39b, whose observationally derived metallicity is higher than predicted by cloudless models, we find an enhancement by at most 50%. However, further work on cloud properties and their self-consistent coupling to the atmospheric structure is needed in order to reduce uncertainties in the choice of model parameter values, in particular of cloud opacities.
Highlights
Metallicity and core mass of giant planets contain information on protostellar disks and on the process of planet formation
We find that the high cloud deck at 0.001 bar would heat the upper atmosphere so much that only silicates could condense out at such low pressures
In their 3D global circulation models, a high-metallicity atmosphere was clearly required to explain the spectra while optically thick, uniform clouds would not much influence the fit
Summary
Metallicity and core mass of giant planets contain information on protostellar disks and on the process of planet formation. Wakeford et al (2018) [2] used transmission spectra to determine the metallicity in the atmosphere of the Saturn-mass planet WASP-39b. They retrieved a high value of ∼100–200× solar. This is higher than the prediction from core accretion formation and higher than the upper limit of 55× solar for the atmospheric metallicity as inferred from structure models for this planet [3]
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