In NASA's exoplanet search, why is the gas CO looked for in atmospheres?

Carbon monoxide (CO) as one of the one major air pollutants doesn't have a direct effect on the global temperature (like methane and carbon dioxide) but it takes part in the formation of lower-atmospheric ozone and urban smog. CO also affects the ability of the atmosphere to cleanse itself of many polluting gases.

So, it is important to measure concentrations of CO via sensors on NASA's satellite.