  Spectral features corresponding to methane and water opacity were reported
based on spectroscopic observations of HD 189733b with Hubble/NICMOS. Recently,
these data, and other NICMOS exoplanet spectroscopy measurements, have been
reexamined in Gibson et al. 2010, who claim that the features in the
transmission spectra are due to uncorrected systematic errors and not molecular
opacities. We examine the methods used by the Gibson team and show that,
contrary to their claim, their results for the transmission spectrum of HD
189733b are in fact in agreement with the original results. In the case of HD
189733b, the most significant problem with the Gibson approach is a poorly
determined instrument model, which causes (1) an increase in the formal
uncertainty and (2) instability in the minimization process; although Gibson et
al. do recover the correct spectrum, they cannot identify it due to the
problems caused by a poorly determined instrument model. In the case of XO-1b,
the Gibson method is fundamentally flawed because they omit the most important
parameters from the instrument model. For HD 189733b, the Gibson team did not
omit these parameters, which explains why they are able to reproduce previous
results in this case, although with poor SNR.
