Visible and infrared observations have been able to probe the atmosphere of HD209458b at lower altitudes. In fact, HD209458b was the first EGP to have its atmosphere detected by transmission spectroscopy. The first detection was achieved by Charbonneau et al. (2002) who observed a deeper in-transit absorption in the Na D 589.3 nm resonance doublet compared to the adjacent wavelength bands. This detection was based on four transits observed with the Space Telescope Imaging Spectrograph (STIS) onboard the Hubble Space Telescope (HST) (Brown et al., 2001). The same data were later reanalyzed by Sing et al. (2008a,b) who combined them with other observations (Knutson et al., 2007) and created a transmission spectrum of HD209458b at wavelengths of 300–800 nm. They argued that the abundance of sodium in the atmosphere is depleted above the 3 mbar level either by condensation into Na2S clouds or ionization. We note that the detection of Si2+ in the thermosphere (Linsky et al., 2010) constrains cloud formation mechanisms in the upper atmosphere and implies that the depletion of Na at low pressures is probably due to ionization (see Section 3.3).
Na D
resonance
589.3 nm
nm
transits
four
transmission spectrum of HD209458b
wavelengths
300–800 nm
nm
level
3 mbar
mbar