The pure carbon molecule C₅ was detected toward IRC +10216 (CW Leo) by Bernath et al. in 1989 using the 4-meter telescope at Kitt Peak National Observatory. Since C₅ has no dipole moment and thus no rotational spectrum, it was detected via a particularly intense vibrational mode (see below), which gives rise to a ro-vibrational spectrum that was used to detect the molecule. To date, C₅ has only been detected in the one source. It has been unsuccessfully sought toward ζ Oph by Maier et al. and toward various translucent clouds by Galazutdinov et al.

No vibrational spectrum was available when Bernath et al. set out to search for C₅. Vala et al. characterized it. A number of additional experimental studies have followed. For example, Heath et al. measured the ro-vibrational spectrum (and published back-to-back with the detection), Moazzen-Ahmadi et al. characterized the ¹³C spectroscopy of the intense band, and Boguslavskiy & Maier measured its electronic spectrum.

C₅ or pentatetraenediylidene or, more precisely, 1,2,3,4-pentatetraene-1,5-diylidene, is related to other known astromolecules, particularly C₃, C₅H, and C₅S. C₅ is a classic carbene.