Comparative aeronomy of HD209458b and HD189733b

Transit observations at UV wavelengths probe the hot, extended and highly ionized upper atmospheres of close-in extrasolar planets. HD209458b and HD189733b both orbit their host stars within 0.05 AU and are loosing mass due to intense heating by stellar XUV radiation. However, their surface gravities differ by a factor of ∼2 and while HD209458 is a solar-type star, HD189733 is a K type star. Consequently, the X-ray and EUV flux received by HD189733b is expected to be significantly higher than that received by HD209458b. Therefore, the upper atmospheres of the planets should have different characteristics. Here we discuss the interpretation of the UV transit observations and show how they can be used to obtain information about the thermal structure, composition and escape rates in the upper atmosphere. Based on the results, we highlight the differences and similarities between HD189733b and HD209458b.