Since molecules were not expected to survive in the hostile high temperature, ionized environment of PN, the first detection of CO in the PN NGC 7027 by Mufson et al. (1975) therefore came as a complete surprise. The CO profile in NGC 7027 is almost 40 km s–1 wide, suggesting that the molecular gas is in an expanding envelope (Fig. 5.1). The amount of the molecular gas inferred from the CO line strength is over 1 and is much higher than the ionized mass usually associated with PN.

The CO profile of NGC 7027 resembles the CO profiles observed in the AGB stars, for example, IRC+10216 (Solomon et al, 1971). By the mid-1970s, CO emission had been detected from many AGB stars (see Section 10.4.3). The amounts of mass observed in the circumstellar envelopes of AGB stars are found to be similar to that observed in NGC 7027. The most likely explanation of the origin of molecular gas in PN is that they are the remnants of the circumstellar envelopes of AGB stars (Kwok, 1982).

The presence of molecules implies that there is more material in PN than what is suggested by the optical images. Vibrational and rotational states of molecules can be excited either collisionally or radiatively, and the observations of these transitions allow new ways to probe the physical structure of PN.