Aeronomy of extra-solar giant planets at small orbital distances
Introduction
Radial velocity observations and measurements of the dimming of light during transit events show that HD209458b is a Jupiter-like planet orbiting a Sun-like star Charbonneau et al., 2000, Charbonneau et al., 2002, Henry et al., 2000. Spectroscopic observations during the transit have detected the NaI D lines and the H Lyα line in absorption Charbonneau et al., 2002, Vidal-Madjar et al., 2003, providing the first opportunity to observationally constrain the structure of an Extra-solar Giant Planet (EGP) atmosphere. The spectroscopic detections to date probe different regions of the atmosphere. The apparent size of HD209458b, when viewed in the NaI lines, is only slightly larger than the apparent size of the planet at nearby wavelengths, suggesting that the observations probe a region in the troposphere of the planet, not too far from the cloud tops (Charbonneau et al., 2002). The apparent size of HD209458b in the H Lyα line is several planetary radii, implying that these observations probe the upper-most regions of an extended atmosphere (Vidal-Madjar et al., 2003). Numerous studies of the lower atmospheres of EGPs have been carried out (cf. Sudarsky et al., 2003, and references therein); the first comprehensive investigation of the physical state of EGP upper atmospheres is presented below.
The surprisingly large extent of the H cloud detected by Vidal-Madjar et al. (2003) implies that the scale height of the upper atmosphere is a significant fraction of a planetary radius. Yet, the skin temperature of an EGP at 0.05 AU is expected to be approximately 750 K Goukenleuque et al., 2000, Seager et al., 2000, Sudarsky et al., 2003 and the gravitational acceleration of HD209458b is ∼900 cm s−2, implying a scale height of ∼700 km, a value far too small to explain the observed H cloud. Thus, the existence of an extended H cloud implies that the upper atmosphere of HD209458b is at a much higher temperature than the lower atmosphere. Moreover, as on Jupiter, H2, not H, is the thermodynamically stable form of hydrogen at the temperatures and pressure of EGP atmospheres considered to date Goukenleuque et al., 2000, Sudarsky et al., 2003. The existence of an extended H cloud implies either that the upper atmosphere is much hotter than the skin temperature, or that H is produced at a rapid rate by non-equilibrium processes. This paper investigates whether absorption of stellar EUV radiation in the upper atmosphere of an EGP can lead to the conditions implied by Vidal-Madjar et al.'s measurements. To this end, physical models of EGP upper atmospheric structure are constructed using techniques developed in studies of Solar-System aeronomy. An improved understanding of the upper atmospheric structure of EGPs may also aid in their detection and characterization, by, for example, predicting other emission or absorption features or identifying preferred wavelength bands for extra-solar planet searches. Finally, the escape rate of the atmosphere is determined by conditions in the thermosphere; thus, the evolution of extra-solar planets may depend on their aeronomy Mayor and Queloz, 1995, Burrows and Lunine, 1995, Guillot et al., 1996, Vidal-Madjar et al., 2003, Liang et al., 2003, Lammer et al., 2003.
Our investigation of the aeronomy of extra-solar planets is grounded in studies of Jupiter. Unfortunately, our understanding of the structure of Jupiter's thermosphere and ionosphere is not as robust as we would like. Measurements of the peak electron density in the jovian ionosphere are often a factor of 10 less and occur at an altitude several scale heights above that predicted by photochemical models (McConnell and Majeed, 1987). Measurements by the Galileo spacecraft indicate a complex situation with large temporal and spatial variations that have yet to be adequately interpreted (Majeed et al., 1999). Suggestions for the discrepancies include a non-thermal H2 vibrational distribution Majeed et al., 1991, Cravens, 1987 and thermospheric winds coupled with specific magnetic field geometries McConnell and Majeed, 1987, Matcheva et al., 2001, but these have yet to be verified and the problem is still without a clear resolution (Yelle and Miller, 2004). Also, the thermospheric temperature on Jupiter is much higher than predicted by aeronomical models based on solar energy input and the heat source has yet to be unambiguously identified (Yelle and Miller, 2004). The high temperatures may be due to dissipation of buoyancy or acoustic waves Young et al., 1997, Matcheva and Strobel, 1999, Hickey et al., 2000, Schubert et al., 2003 or precipitation of energetic ions from the jovian magnetosphere (Waite et al., 1997), or meridional transport of energy deposited in the auroral zones (Achilleos et al., 1998), or some combination of all three processes. In any case, our understanding of these processes is insufficient to support confident predictions about the thermospheric temperature of a gas-giant planet at large orbital distances.
Despite these difficulties, it is not unreasonable to apply aeronomical models to EGPs. Several factors suggest that the ionospheres of EGPs may be more easily understood than that of Jupiter. The thermospheric temperature of EGPs should be higher than that of Jupiter. This shortens chemical time constants, reducing the importance of transport of ions through diffusion. Also, higher temperatures should help equilibrate H2 vibrational levels and make any non-thermal distribution less important. In addition, there is reason to hope that the thermospheric temperature on an EGP can be more easily understood than on Jupiter. Although it is possible that wave heating or magnetospheric interactions dominate, it is reasonable to suppose that the primary energy source is the tremendous amount of stellar EUV radiation deposited in the EGP thermospheres. These conjectures need to be tested through observations, but they seem sufficient to justify a first look at the aeronomy of extra-solar giant planets.
Section snippets
Model description
Calculations of the upper atmospheric structure of an EGP must be very general. In Solar-System studies one can usually assume that the major atmospheric constituent is unaffected by chemistry, but, with the large amounts of energy deposited in their atmosphere, this may not be a safe assumption for EGPs. At the outset, it is not clear if the dominant constituent is H2 or H, created from dissociation of H2, or H+, created by photoionization of H and H2. Thus, chemical calculations must be
Results
A reference model is described first in order to focus the discussion and faciliate an in-depth examination of the relevant physical balances prevailing in the atmosphere. The reference model used here adopts parameters similar to those of HD209458b, specifically a mass of 0.6MJ, a radius at 200 dyne cm−2 equal to 1.4RJ and a star-planet distance of 0.05 AU. After discussion of the reference model, trends with changes in semi-major axes are considered in order to examine the range of plausible
Discussion
Upper atmospheric structure and escape rates for close in EGPs have also been considered by Liang et al., 2003, Lammer et al., 2003. Liang et al. (2003) consider the chemistry of an EGP upper atmosphere, but not the thermal profile. Instead, they adopt the thermal profile of Seager et al. (2000). The Seager et al. (2000) models do not include heating by solar EUV and are therefore not appropriate for the thermosphere. In fact, the temperature profile calculated by Seager et al. decreases
Acknowledgements
This research has been supported by grants NAG5-12699 to the University of Arizona. The author thanks Dr. J. Moses for advice on reaction rates and Drs. L. Young, C. Griffith, J. Harrington, D. Hunten, W. Hubbard, and J. Lunine for helpful discussions.
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