Carbon monoxide (CO) is the most abundant C-bearing molecule in the universe. It is the second most common molecule in dense interstellar clouds, after H₂. It is found in comets and the atmospheres of several planets. It has also been detected in brown dwarfs such as Gliese 229b and Epsilon Indi B. Since 1952, it has been known that CO is present in the atmosphere of our own sun, residing in the chromosphere. At least four of the isotopomers have been detected CO and some of its isotopomers have also been detected in protoplanetary disks.

CO is observed in the interstellar media by means of its characteristic rotational spectrum. It is used as a tracer for detecting molecular hydrogen and is consequently used extensively for mapping molecular clouds in our own galaxy and in extragalactic sources, where its ¹³CO, C¹⁷O, and C¹⁸O isotopomers have also been detected.