We present Spitzer Space Telescope observations of the extrasolar planet HD 189733b primary transit , obtained simultaneously at 3.6 and 5.8 \mu m with the Infrared Array Camera .
The system parameters , including planetary radius , stellar radius , and impact parameter are derived from fits to the transit light curves at both wavelengths .
We measure two consistent planet-to-star radius ratios , ( R _ { p } / R _ { \star } ) _ { 3.6 \mu \mathrm { m } } = 0.1560 \pm 0.0008 \textrm { ( stat ) } \pm 0.00 % 02 \textrm { ( syst ) } and ( R _ { p } / R _ { \star } ) _ { 5.8 \mu \mathrm { m } } = 0.1541 \pm 0.0009 \textrm { ( stat ) } \pm 0.00 % 09 \textrm { ( syst ) } , which include both the random and systematic errors in the transit baseline .
Although planet radii are determined at 1 % -accuracy , if all uncertainties are taken into account the resulting error bars are still too large to allow for the detection of atmospheric constituants like water vapour .
This illustrates the need to observe multiple transits with the longest possible out-of-transit baseline , in order to achieve the precision required by transmission spectroscopy of giant extrasolar planets .