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Title: Acceleration of Cooling of Ice Giants by Condensation in Early Atmospheres

Abstract

The present infrared brightness of a planet originates partly from the accretion energy that the planet gained during its formation and hence provides important constraints to the planet formation process. A planet cools down from a hot initial state to the present state by losing energy through radiative emission from its atmosphere. Thus, the atmospheric properties affect the planetary cooling rate. Previous theories of giant planet cooling assume that the atmospheric composition is unchanged throughout the evolution. Planet formation theories, however, suggest that the atmospheres especially of ice giants are rich in heavy elements in the early stages. These heavy elements include condensable species such as H{sub 2}O, NH{sub 3}, and CH{sub 4}, which are expected to have a great impact on atmospheric temperature and thus on radiative emission through latent heat release. In this study we investigate the effect of such condensation on the planetary emission flux and quantify the impact on the cooling timescale. We then demonstrate that the latent heat of these species keeps the atmosphere hot and thus the emission flux high for billions of years, resulting in an acceleration of the cooling of ice giants. This sheds light on the long-standing problem that Uranus is much less bright thanmore » theoretically predicted and is different in brightness from Neptune in spite of the similarity in mass and radius. We also find that young ice giants with highly enriched atmospheres are much brighter in the mid-infrared than ice giants with non-enriched atmospheres. This provides important implications for future direct imaging of extrasolar ice giants.« less

Authors:
;  [1]
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  1. Department of Earth and Planetary Science, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
Publication Date:
OSTI Identifier:
22663591
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astronomical Journal (Online); Journal Volume: 153; Journal Issue: 6; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; ACCELERATION; AMMONIA; BRIGHTNESS; COOLING; EMISSION; EVOLUTION; ICE; LIMITING VALUES; MASS; METHANE; NEPTUNE PLANET; SATELLITE ATMOSPHERES; SATELLITES; URANUS PLANET; VISIBLE RADIATION

Citation Formats

Kurosaki, Kenji, and Ikoma, Masahiro, E-mail: kurosaki.k@nagoya-u.jp, E-mail: ikoma@eps.s.u-tokyo.ac.jp. Acceleration of Cooling of Ice Giants by Condensation in Early Atmospheres. United States: N. p., 2017. Web. doi:10.3847/1538-3881/AA6FAF.
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Kurosaki, Kenji, & Ikoma, Masahiro, E-mail: kurosaki.k@nagoya-u.jp, E-mail: ikoma@eps.s.u-tokyo.ac.jp. Acceleration of Cooling of Ice Giants by Condensation in Early Atmospheres. United States. doi:10.3847/1538-3881/AA6FAF.
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Kurosaki, Kenji, and Ikoma, Masahiro, E-mail: kurosaki.k@nagoya-u.jp, E-mail: ikoma@eps.s.u-tokyo.ac.jp. Thu . "Acceleration of Cooling of Ice Giants by Condensation in Early Atmospheres". United States. doi:10.3847/1538-3881/AA6FAF.
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@article{osti_22663591,
title = {Acceleration of Cooling of Ice Giants by Condensation in Early Atmospheres},
author = {Kurosaki, Kenji and Ikoma, Masahiro, E-mail: kurosaki.k@nagoya-u.jp, E-mail: ikoma@eps.s.u-tokyo.ac.jp},
abstractNote = {The present infrared brightness of a planet originates partly from the accretion energy that the planet gained during its formation and hence provides important constraints to the planet formation process. A planet cools down from a hot initial state to the present state by losing energy through radiative emission from its atmosphere. Thus, the atmospheric properties affect the planetary cooling rate. Previous theories of giant planet cooling assume that the atmospheric composition is unchanged throughout the evolution. Planet formation theories, however, suggest that the atmospheres especially of ice giants are rich in heavy elements in the early stages. These heavy elements include condensable species such as H{sub 2}O, NH{sub 3}, and CH{sub 4}, which are expected to have a great impact on atmospheric temperature and thus on radiative emission through latent heat release. In this study we investigate the effect of such condensation on the planetary emission flux and quantify the impact on the cooling timescale. We then demonstrate that the latent heat of these species keeps the atmosphere hot and thus the emission flux high for billions of years, resulting in an acceleration of the cooling of ice giants. This sheds light on the long-standing problem that Uranus is much less bright than theoretically predicted and is different in brightness from Neptune in spite of the similarity in mass and radius. We also find that young ice giants with highly enriched atmospheres are much brighter in the mid-infrared than ice giants with non-enriched atmospheres. This provides important implications for future direct imaging of extrasolar ice giants.},
doi = {10.3847/1538-3881/AA6FAF},
journal = {Astronomical Journal (Online)},
number = 6,
volume = 153,
place = {United States},
year = {Thu Jun 01 00:00:00 EDT 2017},
month = {Thu Jun 01 00:00:00 EDT 2017}
}
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Journal Article:
DOI:  10.3847/1538-3881/AA6FAF
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