Information in the Reflected-light Spectra of Widely Separated Giant Exoplanets

Giant exoplanets located >1 au away from their parent stars have atmospheric environments cold enough for water or ammonia clouds. We have developed a new equilibrium cloud and reflected-light spectrum model, ExoREL, for widely separated giant exoplanets. The model includes the dissolution of amm...

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Bibliographic Details
Published in:The Astrophysical journal 2019-12, Vol.887 (2), p.166
Main Author: Hu, Renyu
Format: Article
Language:English
Subjects:
Ammonia
Astrophysics
Cloud droplets
Cloud formation
Clouds
Direct imaging
Exoplanet atmospheric composition
Exoplanet evolution
Extrasolar gas giants
Extrasolar ice giants
Extrasolar planets
Fluctuations
Heat flux
Heat transfer
Methane
Mixing ratio
Planetary evolution
Signal to noise ratio
Spectroscopy
Spectrum analysis
Stellar systems
Thermal evolution
Uranus
Wavelengths
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Summary:Giant exoplanets located >1 au away from their parent stars have atmospheric environments cold enough for water or ammonia clouds. We have developed a new equilibrium cloud and reflected-light spectrum model, ExoREL, for widely separated giant exoplanets. The model includes the dissolution of ammonia in liquid-water cloud droplets, an effect studied for the first time for exoplanets. While preserving the causal relationship between temperature and cloud condensation, ExoREL is simple and fast to enable efficient exploration of parameter space. Using the model, we find that the mixing ratio of methane and the cloud top pressure of a giant exoplanet can be uniquely determined from a single observation of its reflected-light spectrum at wavelengths less than 1 m if it has a cloud deck deeper than ∼0.3 bar. This measurement is enabled by the weak and strong bands of methane and requires a signal-to-noise ratio of 20. The cloud pressure, once derived, provides information about the internal heat flux of the planet. Importantly, we find that for a low, Uranus-like internal heat flux, the planet can have a deep liquid-water cloud, which will sequester ammonia and prevent the formation of the ammonia cloud that would otherwise be the uppermost cloud layer. This newly identified phenomenon causes a strong sensitivity of the cloud top pressure to the internal heat flux. Reflected-light spectroscopy from future direct-imaging missions should therefore not only measure the atmospheric abundances but also characterize the thermal evolution of giant exoplanets.
ISSN:0004-637X
1538-4357
DOI:10.3847/1538-4357/ab58c7