Furthermore, Scientific American wrote

In a paper published online February 21 by Nature, Richardson's group reports that HD 209458 b's spectrum does suggest the presence of clouds of perhaps 10-micron-wide particles of dustlike silicate, or silicon oxide. According to Fortney, "We know silicate clouds affect the spectra of brown dwarfs at similar atmospheric temperatures."

But Fortney notes that the two planets differ in temperature by 200 K, which suggests clouds of the same composition are not present in both. Because the planets are so close to their suns, the star shine may burn off a haze that also obscures light, he says. Another possibility, he adds, is that the atmospheres might have uniform temperatures all the way down, which would flatten out their light spectra because molecules would not transfer energy as easily.