We report a 4.8 \sigma detection of water absorption features in the day side spectrum of the hot Jupiter HD 189733 b .
We used high-resolution ( R \sim 100 ~ { } 000 ) spectra taken at 3.2 ~ { } \mu m with CRIRES on the VLT to trace the radial-velocity shift of the water features in the planet ’ s day side atmosphere during 5 h of its 2.2 d orbit as it approached secondary eclipse .
Despite considerable telluric contamination in this wavelength regime , we detect the signal within our uncertainties at the expected combination of systemic velocity ( V _ { sys } = -3 ^ { +5 } _ { -6 } km s ^ { -1 } ) and planet orbital velocity ( K _ { p } = 154 ^ { +14 } _ { -10 } km s ^ { -1 } ) , and determine a H _ { 2 } O line contrast ratio of ( 1.3 \pm 0.2 ) \times 10 ^ { -3 } with respect to the stellar continuum .
We find no evidence of significant absorption or emission from other carbon-bearing molecules , such as methane , although we do note a marginal increase in the significance of our detection to 5.1 \sigma with the inclusion of carbon dioxide in our template spectrum .
This result demonstrates that ground-based , high-resolution spectroscopy is suited to finding not just simple molecules like CO , but also to more complex molecules like H _ { 2 } O even in highly telluric contaminated regions of the Earth ’ s transmission spectrum .
It is a powerful tool that can be used for conducting an immediate census of the carbon- and oxygen-bearing molecules in the atmospheres of giant planets , and will potentially allow the formation and migration history of these planets to be constrained by the measurement of their atmospheric C/O ratios .