Near-IR observations are important for the detection and characterization of exoplanets using the transit technique, either in surveys of large numbers of stars or for follow-up spectroscopic observations of individual planets. In a controlled labora
tory experiment, we imaged $sim 10^4$ critically sampled spots onto an Teledyne Hawaii-2RG (H2RG) detector to emulate an idealized star-field. We obtained time-series photometry of up to $simeq 24$ hr duration for ensembles of $sim 10^3$ pseudo-stars. After rejecting correlated temporal noise caused by various disturbances, we measured a photometric performance of $<$50 ppm-hr$^{-1/2}$ limited only by the incident photon rate. After several hours we achieve a photon-noise limited precision level of $10sim20$ ppm after averaging many independent measurements. We conclude that IR detectors such as the H2RG can make the precision measurements needed to detect the transits of terrestrial planets or detect faint atomic or molecular spectral features in the atmospheres of transiting extrasolar planets.
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 Gibso
n 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.
Using the NICMOS instrument on the Hubble Space Telescope, we have measured the dayside spectrum of HD 209458b between 1.5--2.5 microns. The emergent spectrum is dominated by features due to the presence of methane (CH4) and water vapor (H2O), with s
maller contributions from carbon dioxide (CO2). Combining this near-infrared spectrum with existing mid-infrared measurements shows the existence of a temperature inversion and confirms the interpretation of previous photometry measurements. We find a family of plausible solutions for the molecular abundance and detailed temperature profile. Observationally resolving the ambiguity between abundance and temperature requires either (1) improved wavelength coverage or spectral resolution of the dayside emission spectrum, or (2) a transmission spectrum where abundance determinations are less sensitive to the temperature structure.
We have measured the dayside spectrum of HD 189733b between 1.5 and 2.5 microns using the NICMOS instrument on the Hubble Space Telescope. The emergent spectrum contains significant modulation, which we attribute to the presence of molecular bands se
en in absorption. We find that water (H2O), carbon monoxide (CO), and carbon dioxide (CO2) are needed to explain the observations, and we are able to estimate the mixing ratios for these molecules. We also find temperature decreases with altitude in the ~0.01 < P < ~1 bar region of the dayside near-infrared photosphere and set an upper limit to the dayside abundance of methane (CH4) at these pressures.
