Koskinen et al. (2010a) inferred a mean temperature of 8250 K in the thermosphere of HD209458b with p0 = 1 μbar (the M7 model). Taken together with our results based on solar XUV fluxes, this implies a relatively high heating efficiency. Alternatively, with ηnet = 0.5 it may imply that the XUV flux of HD209458b is 5–10 times higher than the corresponding solar flux. This type of an enhancement is not impossible. The activity level of the star depends on its rotation rate, and the rotation rate of HD209458 may be twice as fast as the rotation rate of the Sun (Silva-Valio, 2008). However, the uncertainty of the observed H Lyman α transit depths accommodates a range of temperatures, and thus we are unable to derive firm constraints on the heating rates from the observations. In general, though, the pressure averaged temperature provides a useful connection between observations and temperatures predicted by models that can be exploited to constrain heating rates.
thermosphere of HD209458b
mean temperature
8250 K
K
p0
1 μbar
μbar
0.5
HD209458b
XUV flux
5–10 times
times
HD209458
rotation rate
twice