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A stability limit for the atmospheres of giant extrasolar planets

Abstract

Recent observations of the planet HD209458b indicate that it is surrounded by an expanded atmosphere of atomic hydrogen that is escaping hydrodynamically1,2,3. Theoretically, it has been shown that such escape is possible at least inside an orbit of 0.1 au (refs 4 and 5), and also that H3+ ions play a crucial role in cooling the upper atmosphere5,6. Jupiter’s atmosphere is stable7, so somewhere between 5 and 0.1 au there must be a crossover between stability and instability. Here we show that there is a sharp breakdown in atmospheric stability between 0.14 and 0.16 au for a Jupiter-like planet orbiting a solar-type star. These results are in contrast to earlier modelling4,8 that implied much higher thermospheric temperatures and more significant evaporation farther from the star. (We use a three-dimensional, time-dependent coupled thermosphere–ionosphere model6 and properly include cooling by H3+ ions, allowing us to model globally the redistribution of heat and changes in molecular composition.) Between 0.2 and 0.16 au cooling by H3+ ions balances heating by the star, but inside 0.16 au molecular hydrogen dissociates thermally, suppressing the formation of H3+ and effectively shutting down that mode of cooling.

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Figure 1: Temperature and column densities of the dominant ion species versus orbital distance.
Figure 2: Hemispheric projections of the horizontal temperature distribution centred at the ‘dusk’ terminator at an orbital distance of 0.16  au (top) and 0.14  au (bottom).
Figure 3: The total XUV heating and infrared cooling rates at different orbital distances integrated over all pressure levels and both hemispheres.

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Acknowledgements

T.T.K. has been supported by UCL and Perren studentships and A.D.A and S.M. have been supported by the Science and Technology Facilities Council (STFC). This work was partly carried out on the Keter High Performance Computer System, which is managed by the Miracle Astrophysics Project and funded by STFC. SOLAR2000 Research Grade v2.23 irradiances are provided by Space Environment Technologies. We also thank R. Nelson and M. Fogg for advice on giant planet migration rates and N. Achilleos for assistance with global plots.

Author Contributions T.T.K. developed the thermospheric circulation model for extrasolar giant planets based on existing models of gas giants in the Solar System. T.T.K, A.D.A and S.M. directed and carried out the modelling. S.M. calculated the non-LTE emission rates for H3+.

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Correspondence to Tommi T. Koskinen.

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Supplementary Information

The file contains Supplementary Methods, Supplementary Table 1 with Legend and additional references. The details of the model are discussed in Supplementary Methods. In that section the basic features of the neutral thermosphere and ionosphere in our coupled global circulation model are explained. Supplementary Table 1 shows the photochemical reactions that are included in the model and that are similar to those occurring in Jupiter’s upper atmosphere. Supplementary Notes include all the references that are referred to in either Supplementary Methods or Supplementary Table 1. (PDF 102 kb)

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Koskinen, T., Aylward, A. & Miller, S. A stability limit for the atmospheres of giant extrasolar planets. Nature 450, 845–848 (2007). https://doi.org/10.1038/nature06378

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