Detection of Planetary Atmospheres

For the detection of extrasolar planetary atmospheres we are at the moment in a state similar to that 200 years ago when the first attempts were made to study atmospheres of planets in our solar system. Generally, the effects of planetary atmospheres are  

The dip in the absorption lines during a planetary transit is very small. Seager and Sasselov, 2000 [302] predict the possibility to observe a relative decrease in flux of about 10 . In order to detect such small variation strong planet spectral features have to be observed and several absorption frequencies (Nal, KI, Hel) are proposed for observations. 

Let us consider the transit of a planet. The stellar flux will be reduced by the amount of the ratio of the area of the planet to the star. If the planet has an atmosphere, then some flux from the star will pass through the optically thin part of the planet s atmosphere. In the case of stellar occultations by giant planets in the solar system, the limb of the giant planets is either defined by the cloud tops or by the 1 bar level. 

Assuming that the radius of the star is / = 1.3 R and the radius of the planet is /Jp = 1.54 R/, where R/ denotes the radius of Jupiter. The ratio of planet-to star area is on the order of 10 to 10 . For the object HD 209458 an estimate for the limb radial depth of 0.01 Rp-0.05 Rp can be assumed. The planet s absorption features are superimposed on the stellar flux. In order to detect these small features, they must be optically thick. In an isothermal atmosphere with gas scale height H, the angle of refraction for a ray passing at planetocentric distance r is given by  

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