  We present a re-analysis of archival HST/NICMOS transmission spectroscopy of
three exoplanet systems; HD 189733, GJ-436 and XO-1. Detections of several
molecules, including H20, CH4 and CO2, have been claimed for HD 189733 and
XO-1, but similarly sized features are attributed to systematic noise for
GJ-436. The data consist of time-series grism spectra covering a planetary
transit. After extracting light curves in independent wavelength channels, we
use a linear decorrelation technique account for instrumental systematics
(which is becoming standard in the field), and measure the planet-to-star
radius ratio as a function of wavelength. For HD 189733, the uncertainties in
the transmission spectrum are significantly larger than those previously
reported. We also find the transmission spectrum is considerably altered when
using different out-of-transit orbits to remove the systematics, when some
parameters are left out of the decorrelation procedure, or when we perform the
decorrelation with quadratic functions rather than linear functions. Given that
there is no physical reason to believe the baseline flux should be modelled as
a linear function of any particular set of parameters, we interpret this as
evidence that the linear decorrelation technique is not a robust method to
remove systematic effects from the light curves for each wavelength channel.
For XO-1, the parameters measured to decorrelate the light curves would require
extrapolation to the in-transit orbit to remove the systematics, and we cannot
reproduce the previously reported results. We conclude that the resulting
NICMOS transmission spectra are too dependent on the method used to remove
systematics to be considered robust detections of molecular species in
planetary atmospheres, although the presence of these molecules is not ruled
out.
